“Success is a standard that lowers itself to no one. You raise yourself to its level.” – Quintessential Education
Since our inception, Quintessential has developed a reputation for consistently producing Singapore’s top IB and IGCSE students. We have taught over 1000 IB students to date; all of whom scoring 6s’ and 7s’ in the IB exams. As IB and IGCSE specialists, our teachers dedicate full attention to writing and enhancing our Master Curriculum; built on proven strategies developed by subject experts. Now in our eighth year, we offer a series of IB Masterclasses that offer professional guidance, targeted help and a highly-rated personalised curriculum. Classes extend from topical reviews, IA/EE guidance, question practices, to customised intensive revision.
Small class sizes and premium tutors remain our code of excellence. We dedicate our time to crafting intelligently designed guides and a highly rated teaching curriculum to help you stay ahead of your peers. Each Quintessential teacher is an IB specialist: either a current IB examiner or school teacher with invaluable experience. This year, we are also introducing our junior tutors- IB 45 pointers who are bound for Oxbridge Law, Medicine, and PPE. They are cherry picked and undergo extensive training with us to teach at the highest level.</p5
Since our inception, Quintessential has developed a reputation for consistently producing Singapore’s top IB and IGCSE students. As IB and MYP specialists, our teachers dedicate full attention to writing and enhancing our IB Master curriculum; each curriculum is built on proven strategies developed by subject experts. Now in our eighth year, we offer a series of IB Masterclasses providing professional guidance, targeted help and a highly-rated personalised curriculum. Classes extend from topical reviews, IA/EE guidance, to customised intensive revision.
Small class sizes, premium tutors who have mastered teaching the IB, and customised curriculum remains our code of excellence. We dedicate our time to crafting intelligently designed guides and teaching materials to help you achieve your potential. Each Quintessential teacher is an IB specialist: either a current IB examiner or school teacher with vast experience teaching at top schools. This year, we are also introducing our junior tutors- IB 45 pointers who are bound for Oxbridge Law, Medicine, and PPE. More than just a source of tutorial services, Quintessential encompasses a highly regarded brand of education designed to promote academic excellence.
EXAM QUESTIONS DIFFER FROM
FROM WHAT IS TAUGHT IN CLASS
Schools prioritise imparting content and knowledge over problem solving, internal assessments, and extended essays. This makes lessons dry and lacking of pinpoint help for exam and coursework needs.
LACK EXPOSURE TO EXAM-STYLE QUESTIONS
The IB exams are known to be set on a few iterations of specific question types. Students who lack exposure to question trends and analysis do not fare well in the exams.
LARGE AMOUNT OF TIME DEDICATED
TO PROJECTS AND COURSEWORK
Most students spend an average of 90-120 hours on their Internal Assessments, Extended Essay, TOK, and other coursework, leaving little time and for proper exam preparations.
TIGHT TIMELINE IN SCHOOL
The rigour of the IB curriculum leaves school with little time to provide comprehensive coverage for exams. Holidays are spent catching up on coursework rather than review of syllabus; students should start their own IB preparations before each new term.
INABILITY TO PINPOINT WHAT TO
PRIORITISE FOR EXAMS
Each subject covers a wide range of topics, emphasising breadth over depth. Students learn dynamically but struggle with what to prioritise and practice for exams.
NO ONE TEACHES THE IB LIKE WE DO.
The IB Syllabus, simply taught brilliantly.
PROVEN IB CURRICULUM
Our comprehensive personalised curriculum has been tested and proven across hundreds of students. Masterfully written by our IB subject examiners, the exclusive curriculum stands strong with insider knowledge on what types of questions are tested, how to best score for various essays and projects.
Choose from a large network of cherry-picked senior and junior teachers, all with first-hand experience in the IB.
Senior Tutors: IB examiners and school tutors.
Junior Tutors: IB 45 pointers, Oxbridge medicine, law, and PPE undergraduates.
Our IB resource bank is systematically designed to help tackle all forms of IB questions, catering to each student’s individual learning pace, style, and preferences. Quintessential considers the weaknesses and needs of each individual student in helping them achieve maximum performance in the shortest time.
PROVEN IB CURRICULUM
Masterfully written by our IB subject examiners, the exclusive curriculum stands strong with insider knowledge on what types of questions are tested, how to best score for various essays and projects.
Choose from a large network of cherry-picked senior and junior teachers that undergo extensive training to teach at the highest standards.
Our IB resource bank is systematically designed to consider the needs, learning paces and styles of each individual student. They tackle all forms of IB questions to help maximise their potential.
FOR TERM 1: COMMENCING JAN 1, 2018
FOR TERM 2: COMMENCING JAN 1, 2018
$55000monthly (Y11)$57500monthly (Y12)
Our IB Economics programme is designed to help students excel in the subject through developing analytic and evaluative minds, and the ability to present these economic ideas convincingly through essay writing skills.
QE’s classes focus on ensuring that students have an accurate understanding of core economic concepts. Thereafter, our tutors deconstruct each question type systematically and help students to examine and contextualise key economic content in a clear and concise manner. Each class adopts a focused approach to understanding question trends and patterns, with rigorous review of essay writing skills and critical thinking abilities designed for all students.
Our targeted focus on a range of topics encompassing real-world examples ranging from EU monetary policies to Japan’s negative interest rates, and pressing economic issues across the world. This helps to develop strong application skills to prepare for out-of-context questions and analysis.
Our economics curriculum is critically designed and committed to helping students achieve the best possible grade in economics.
Our IB Mathematics programme is designed to build firm foundational understanding of the content for each topic, following which we will focus on problem solving techniques and sharpen each student’s familiarity with the syllabus through practices. We believe that memorising notes are an inefficient approach and we help weaker students to build a strong foundation, and the stronger ones to score well in exams.
We work to enhance understanding and use real-life examples in our teaching. By simplifying core concepts, we hope to not only aid students’ understanding but also application of formulae and methodology. With proper guidance and understanding of techniques, practice would be doubly effective. Our classes have developed simple yet effective problem solving techniques to help students through the process of answering questions.
Classes are also supplemented with advanced materials, intricately designed to help students gain exposure to a widest range of questions.
Our IB Chemistry programme is designed to enable students to achieve excellent academic performance in the subject. To excel in IB chemistry, students are required to attain a comprehensive understanding of the core concepts within the syllabus, and be proficient in the application of these concepts when attempting exam questions.
The primary focus of our chemistry tutors is to ensure that students thoroughly understand the syllabus content. We believe that memorising notes is an inefficient approach to chemistry because students often face problems retaining the knowledge in the long run, and get stuck once questions get complicated. Instead, through comprehending the WHY and the HOW for each chemistry concept, our students find themselves having to memorise less yet knowing more.
After ensuring that students have a solid grounding in content knowledge, our tutors turn their focus onto techniques for answering exam questions. Practices are tailored to each individual student’s weaknesses and needs. These questions are carefully selected to reflect IB exam questions, and assess their competency in the topic.
Classes are also supplemented with advanced materials, intricately designed to help students gain exposure to a widest range of questions.
Our IB Physics programme is designed specially with the needs of our students in mind. Each class places a strong emphasis on building a strong foundation in core Physics principles and concepts. We believe in developing the right foundation concepts so that students can always work out their solutions from first principles. Each class is carefully designed to ensure that students master contextual knowledge and retain them in the long run. Our tutors spend extensive amount of time to craft customised curriculums suited to the needs of students struggling with the subject, by producing notes that break down complicated theories and apply them to real world situations. Students are hence able to identify how Physics concepts relate to daily phenomenon that occur around them, and better appreciate its importance and relevance in real life.
Additionally, our tutors work to improve students’ answering techniques through thorough practices on past IB questions. Trend analysis, key question review, and intensive practices across a broad range of past IB questions are key features of each class. Questions are specifically selected based on the answering skills required, thus exposing students to a wide range of question types.
Each class works on addressing the unique, individual weak points of students and hence provide them with personalised assistance that can best help them to respond to challenging Physics problems.
Our IB Biology programme is designed to help students excel in the subject. The IB Biology syllabus is extremely content heavy, and IB students often struggle to cope with the sheer quantity of information they are expected to know, in order to do while for the course. This is why our tutors focus heavily on techniques to help students compartmentalise and retain their biology knowledge.
Unlike the school classroom where learning takes place mainly through audio processing, we believe that the best way for students to learn and retain information is through kinaesthetic and visual processing. Students are often encouraged to explain biology concepts through drawing instead of narrating. This helps us ensure that the students have comprehensive understanding of the content. This approach to learning biology has been proven to help students in dealing with the heavy content, and hence score better for their exams.
Other than a strong focus on the subject’s content, our tutors also train students in answering questions that are not content based, such as inference and data-based questions. Analysis of experimental data is a key component of the IB biology course, and learning the correct techniques to approach these questions will definitely give our students a competitive advantage.
Our IB Literature programme equips students with the critical skills and literary finesse to achieve excellent academic performances. QE’s curriculum ensures their proficiency in understanding the nuances of literary texts, critically analysing them, and articulating their thoughts coherently.
Firstly, our teaching philosophy includes the inculcation of universal skills needed to excel in literary analysis. An acute sensitivity to literary techniques that underscore prose and poetry is trained through practice, and nurtured by experienced tutors. Our materials are specifically curated based on past IB questions, and feedback on students’ work is given by tutors well versed with the IB marking criteria.
Secondly, QE’s curriculum instills knowledge and skills unique to the Set Text component of the formal examination. Students are taught to identify literary conventions specific to their text types, seamlessly incorporate them into their analyses, and explicate their literary functions. QE’s customised curriculum immerses students in the socio-cultural backgrounds of their texts, and such enhanced understanding adds depth to their written analyses.
Thirdly, students master the English language through our rigorous programme. Regular feedback from tutors on students’ written work elucidates areas for improvement in linguistic proficiency and argumentative flow, ensuring that students write with lucidity.
Our IB Language & Literature programme helps students to achieve excellent academic performance through a three-prong approach. To do well for this subject, both the language and literature component requires the same set of skills – the ability to understand, analyse, and write coherently.
First, our tutors supplement students’ understanding of the text beyond mere literal analysis. We stimulate our student’s interests by framing the social, historical and cultural context of the written work, which is also a crucial component to help our students stand out from others.
Second, we provide our students ample practices in both the language and literature component. Our tutors believe that studying smart triumphs studying hard. Our materials and practices are specifically curated based on past IB questions and the IB marking criteria. By providing our students ample practice pegged at an appropriate IB exams standard, we help our students maximise their academic performance and optimise their studying time.
Last, comprehensive understanding and regular practice is never complete without knowledge of exam techniques. At QE, we help our students to polish up their writing skills by teaching them how to incorporate key phrases and keywords into essays, giving frequent feedback, and exposing them to a broad range of texts and materials.
Our IB Psychology programme critically examines how psychological knowledge is generated, developed and applied. This will allow students to have a greater understanding of themselves and appreciate the diversity of human behaviour.
The holistic approach reflected in the curriculum, which sees biological, cognitive and sociocultural analysis being taught in an integrated way, ensures that students are able to develop an understanding of what all humans share, as well as the immense diversity of influences on human behaviour and mental processes. The ethical concerns raised by the methodology and application of psychological research are also key considerations of our IB psychology course.
Our classes are designed for in-depth analysis, evaluation and consolidation of learning. The overall aim of our course is to give students a deeper understanding of the nature and scope of psychology. Each class is crafted in best ways of delivery that are most relevant to their students’ interests and allow them to evaluate different psychological theories and arguments.
Our students will develop the capacity to identify, analyse critically and evaluate theories, concepts and arguments about the nature and activities of the individual and society and develop an awareness of how psychological research can be applied for the better understanding of human behaviour.
Our TOK classes adopt a focused approach to learning key ideas in various areas of knowledge. Each class is critically designed to enrich with principles of essay writing and critical thinking designed for high-performing students. With targetted focus on knowledge acquisition and critical analysis, our programme features engaging content to explore key insights and ideas to help question various topics in the TOK syllabus.
The QE curriculum is streamlined to prepare students to excel in both methods of assessment: the oral presentation and 1,600 word essay. Students are equipped with the argumentative skills to develop structured analyses, and to articulate their opinions coherently and succinctly. They are also exposed to a broad range of pertinent real life situations, and trained to relate them to knowledge questions. This imbues depth into students’ analyses and ensures their insights remain relevant, well-informed and objective.
Our TOK programme is both academically rigorous and comprehensive with a strong focus on developing a clear framework for students to understand the subjects, different ways of knowing, areas of knowledge, and apply their understanding in a thorough and systematic manner.
Our IGCSE and IB tutors are experienced IB examiners, current or past IGCSE and IB teachers, and top educators who are passionate about giving an uncompromising education and guidance to our students.
In addition, we hold a small pool of carefully selected junior tutors- IB 45 pointers who are heading to Oxbridge and Ivy League universities. They undergo extensive training with us in order to teach at the highest level. Having recently completed the IB with flying colours, our junior tutors are in a unique position to share the very techniques that led to their success with the IB students they mentor.
Jayden is a former NTU President scholar with solid experience teaching IB Maths and Physics. Having taught at Raffles Junior College, Jayden uses teaching methodology and curriculum that has worked consistently at the highest level.
His classes are built on conceptual visualization to help students understand topics and retain knowledge and sharpen intuition in Maths and Physics to handle non-routine problems. He strikes good rapport with students and uses a combination of quantitative and qualitative pedagogical approach to prepare them for questions that require them to write explanations, arguments and proofs and inferences. To date, student feedback are sterling and student scores average highly too (6s and 7s).
Malcolm graduated from National Junior College and later NUS with a Bachelor in Engineering Science with top honours and was on the Dean’s list for outstanding academic achievements. He taught at 2 local schools (covering Physics and Mathematics) before moving on to a well known firm in the private sector, but eventually found himelf going back to teaching as he finds it a joy to be able to connect with students and give them the push they need to excel in their studies. He prefers being a mentor rather than a dictator towards his students, after finding it much more effective to engage with them after he has gotten to know them on a personal level.
You will never find him giving out final answers in class without first providing ample explanation and reasoning as to how he derived the solution. By breaking the process down for his students, he believes that not only would it help with their understanding of both Math or Physics formulae, but it would also facilitate their ability to apply the relevant concepts to other questions.
Willy’s teaching journey started as early as Junior College. Back then he started with his peers, and then moved on to students while serving National Service. To him, sharing knowledge and helping others to excel is a joy. Having started from teaching peers, he understands the difficulties that students encounter, and sees the importance of simplifying complex mathematical theories into easily understood concepts. Rather than teaching answers, he focuses lessons on cultivating techniques to help students build understanding, and provide them with tools to tackle increasingly demanding and creative Physics examination questions.
He believes that each student’s results are dependant not only on their resilience when faced with difficulties, but also on having a good tutor who can provide good explanations and help clear any doubts that the student may have. His students always feel assured of being able to attain the results they set out for, and he, too, always has faith that they will be able to do well in their examinations!
I’m known as Mike, an established chemistry super tutor for over 15 years in IB and A levels syllabus. I have taught at various top Junior Colleges and IB schools and possess a plethora of knowledge and experience. I am passionate about inspiring students to pursue science further and am a firm believer of the importance of science in improving the world. My student reviews consistently place me as one of the best teachers in school, and I am sure to reflect that in my tutoring as well. Most students will find me very approachable, friendly, and happy to share. They see me more as a mentor than their typical teacher. I firmly believe in establishing core concepts effectively and enjoyably. Student rapport and trust is very important to me, and I trust that all my students find my class an enjoyable time.
Hello! I’m Sean, an established tutor who has taught over 600 students to date. A full time educator, I was schooled in Hwa Chong Institution and Raffles Junior College before completing my bachelors degree with top honours at SMU. I moved on to work in the finance sector before deciding to pursue my interest in teaching. My strong passion for teaching has fuelled me for ten years and counting. Students and parents alike find me an approachable and personable tutor, and my economics class will be anything but one way lectures and boring theories on monetary or fiscal policies. I often focus on application skills and evaluation of various content.
I believe that students do not need lengthy textbooks or lecture notes, much less cramming these right before exams. By simplifying concepts, I often challenge students to “Explain an economic concept to a 12-year old” or “Explain a difficult economic concept in 100 words”. My teaching style focuses on building core understanding and foundation. Tons of key questions, analysis of exam trends and practices, burgers, pizzas and post class consultations pepper my lessons. In my free time, I am an aficionado of music and automobiles. I also love languages and arts and am currently learning French and Korean.
I strongly believe in keeping explanations simple, and utilising layman terms during lessons to relate to my students and reaffirm their basic Mathematics knowledge. I enjoy injecting jokes into my teaching to make lessons more interesting and fun. I think that it is important for me to build up a good rapport with my students so that I can better attract and retain their attention. I hope that my cold jokes will allow students to warm up to me!
Through my numerous years of teaching experience for both O level & A level, I feel that the poor understanding of concepts and presence of other school commitments are the two main reasons that have severely hindered students from having enough practice with Math questions. This acute lack of practice is usually a student’s main obstacle from obtaining better grades for Math. Everyone can excel in Mathematics, and excellent grades are not hard to achieve. To do well for Maths, a student needs perseverance, hardwork and proper guidance. As long as you put in the effort, I am confident in providing the necessary guidance and techniques to ace your examinations.
I did my undergraduate studies at NUS, reading Mathematics under a scholarship programme. A lifelong learner and educator, I pursued an education track and did my Postgraduate Diploma in Education (PGDE) at NIE. This led to my next posting as a full time educator, and I am a proud recipient of the Outstanding Contribution Award (Team) in 2015. Students know me for my fun-loving Mathematics lessons, clarity in explanations and my efforts to go the extra mile. Over my years of teaching, I have guided numerous students to outstanding results in the ‘O’ Level examinations.
Drills and practice are only effective when students have a complete understanding of the concept. I believe in emphasising links between different mathematical concepts; regularly dividing my drill packages into different difficulty levels, progressively introducing concepts based on prior knowledge. Differentiated practices will raise students’ confidence in their learning capacity and make sense of Mathematics. Apart from teaching, I also design various assessments to monitor my students’ progress. For instance, students will complete a ‘Check-in’ quiz once a concept is taught.
I constantly observe how my students learn, what interests them and how much they have progressed so as to make informed decisions to better cater to their individual learning needs. If a student loves Maths and a teacher complements it with good guidance, the student will be able to draw out his/her maximum potential. This is why I invest immense effort in getting to know my students, their likes & dislikes, and make them love the subject.
I studied Politics, Philosophy, and Economics at University of Oxford. During the course of my studies at Oxford, I learnt and read widely across various fields and this has greatly shaped my understanding of the world around me. With a strong passion for teaching, I became a full-time educator specialising in curriculum planning. In my GP tuition classes, I help students pin down not only the basics of question-type recognition and writing structure, but also the essential higher-level skills of understanding nuance, signposting arguments and more. This is done through a constant cycle of the four key components of the GP examination, with content knowledge imparted alongside. All these add up to a strong ability to present your GP essays in a convincing and intellectual manner. Above all, I welcome an intellectual conversation on any issue under the sun.
I have been teaching GP since I was reading law at the National University of Singapore. Now a full-time Advocate and Solicitor, I choose to also continually impart my skills to numerous JC students in my GP tuition classes, especially those aspiring to become lawyers in the future. My vocation keeps me updated with various events in family and criminal law, and allows me to share a wide range of perspective and debate on these issues. In today’s information-intensive world, I believe that everyone has access to much knowledge; yet the key to success lies in how one critically evaluates and implements that knowledge for the betterment of both self and society. As your personalised GP tutor, I am able to deliver value beyond that of many others giving GP tuition, by combining both my years of experience as a tutor and also years of practice in the law. Outside of teaching, I am a food lover who likes to try everything. I would encourage all my students to recommend any good eats they have tried recently!
Hi, my students know me as the sprightly and caring Ms Chun. I read Law in the UK and would describe myself as a lawyer/tutor/entrepreneur who is passionate about teaching and equal access to education to fight poverty and the world’s troubles. My interests span across fields of KI, GP and History. I believe that the pen is mightier than the sword and that history is an important collection of life’s many lessons. Epistemology- the study of how we acquire knowledge, is an equally interesting field of study to me. I regularly discuss such topics at large and my GP tuition lessons are peppered with real life experiences and examples. In my free time, I am also involved in a technology startup that pursues equal access to law and a cooperative researching on pedagogies.
My students know me as Miss Sheela. I graduated from NUS as the valedictorian and gold medalist (top in cohort) of my batch. Post-NUS, I pursued my Masters at USC, Santa Cruz and am now a passionate full time educator. To date, I have been teaching for over a decade and counting. My interests span across various fields; I believe in holistic and broad based education, with balanced analysis of real world news, to make for informed global citizens.
ZC teaches Biology and Chemistry at QE. He graduated from Raffles Institution with a perfect score in his A Levels, and consistently ranked amongst the top in his cohort for various subjects. He will be studying Law at Cambridge, Downing College. A dynamic individual, he enjoys a range of activities, from tennis to debate to gaming. Much of his free time is spent reading and learning in the company of good books.
ZC believes that practice makes perfect. He prepares extensively for each class, often using a range of key exam questions and intricately designed curriculum to help students gain maximum exposure to exam content. The high standards of his class leave no stone unturned and work on building strong conceptual understanding to aid application skills and techniques, helping many students improve in a short period of time.
Colin graduated from Anglo-Chinese School (Independent) with outstanding academic results, achieving a perfect score of 45 for the IB Diploma Programme. His diligent work ethic and ardent passion for the sciences has earned him a place to read Medicine at King’s College, University of Cambridge.
Colin channels his keen interest in the sciences into every Chemistry and Biology lesson he teaches. His passion is infectious – his students are always encouraged to participate actively in his classes, and to engage in interesting discussions about what they study. Colin advocates for the thorough understanding of concepts, as opposed to simply memorizing formulae or facts. By inculcating such foundational skills, his students are better able to derive answers from conceptual understanding.
Mr Lim, my teacher, is a very committed and experienced teacher. He puts in a lot of effort to get us to understand and do better and his notes are concise and comprehensive. The teachings are very clear, in depth, and easy to understand and I have improved tremendously under his guidance.
Mr Lim is an engaging and fun tutor who taught me exam skills and techniques, supplementing and reinforcing the knowledge I learnt in school. Under Mr Lim’s guidance and effective teaching methods, I learnt a great deal on application skills and evaluation even though I joined very last minute before the examinations.
The classes were very helpful as they were explained in a very simple and clear manner with huge amount of resources for reviews and drills. These added help gave me good preparation for my exams and made it very easy for me to approach the final IB papers. Thank you!
Thank you for making even the most boring subjects like English and Economics fun. Will always remember the commitment and extra effort all my tutors put in to help me prepare for exams! The practice sessions and reviews really made a big difference in my revision and I really enjoyed my lessons at QE!
Thanks for the awesome guidance for my mathematics class Mr Heng. I learnt so much from you and the classes were very fun and interesting. They made a very dry subject fun and interesting. I feel that I have improved greatly over the past year and I would like to thank you really for your help.
I’m very grateful for the guidance by Mr Lim and Mr Gopalan. They have helped me greatly with getting more practices and this has really given me much familiarity and exposure to the IB questions. The practices made it very easy for me to handle related questions. Thank you Sirs.
Mr Lim often teaches us contexts from both local and international sources to aid us in drawing connections between textbook theories and that of real world. He is also an extremely knowledgable and dedicated tutor to us and I’ve Mr Lim’s thoughtful and patient guidance to thank for my improvement. Thank you so much!
Thanks for the help Sir, the classes were extremely helpful to my understanding and it allowed me to comprehend the mathematics syllabus very clearly. I found the resources and practices critical in my exam preparations and the classes were insightful and filled with interesting examples and analogies.
Thanks so much for helping me improve my Economics at my own pace of lesson. The exam practices and notes, among others, really helped me for the exams and I have improved tremendously over the last year thanks to your guidance!
Our tutors are dedicated to teaching beyond what schools can offer. Each tutor is cherry-picked and teaches a customised curriculum written by past MOE teachers and examiners of the ‘A’ Level, IB and ‘O’ Level syllabus, crafted to address common misconceptions of students and to ensure that students are in touch with the requirements of their examinations.
Moreover, QE’s tutors are committed to ensuring that students are provided with the most premium tutoring services – they are more than willing to spend time going through students’ practices, and re-explain any concepts after classes, if students require clarification.
For new students, we offer a paid trial lesson. This means that:
- You only pay for the trial lesson when you enrol.
- We do not collect your material fee and deposit upfront.
- Only when you decide to continue with lessons, will we itemise the material fee and deposit in your bill.
The joining fees applicable are:
- A one-time material fee of $50.
- 1 month of course fee as deposit for each subject.
- A termly fee for each subject.
We also believe that class sizes should be kept small to facilitate the discussion and discourse between students during lessons. The dynamism and effectiveness of such discussions are ensured by our small class sizes.
Individual classes are conducted on a weekly basis over one-half hour sessions.