Overview
What you’ll learn
- Describe an atom as electrically neutral, with a positive nucleus of protons and neutrons and negative electrons moving around the nucleus.
- State that atoms of an element have a unique number of protons, and that most of the atom’s mass is in the nucleus.
- State that a molecule is a group of two or more atoms chemically combined.
- State the number and type of atoms in a given formula, such as CO₂ or H₂O. Writing new formulae is not required.
- Compare the size of an atom with everyday objects, and recognise that technologies based on atoms raise social and ethical questions.
In 30 seconds
- 1An atom is electrically neutral. The nucleus is positive. The electrons around it are negative. The charges balance.
- 2The nucleus contains protons (positive) and neutrons (no charge). Almost all the mass of the atom is in the nucleus.
- 3Atoms of the same element have the same number of protons. That number is unique to the element.
- 4A molecule is two or more atoms chemically combined. O₂ is a molecule of an element. CO₂ and H₂O are molecules of compounds.
- 5In CO₂ there is 1 carbon atom and 2 oxygen atoms. The small number after a symbol counts that atom. You are not asked to write formulae from scratch.
- 6An atom is vastly smaller than a grain of sand or a cell. Knowledge of atoms has benefits and serious risks, including nuclear weapons.
Notes · 1 of 5
Inside an atom
The atomic model puts almost all the mass in a tiny positive nucleus, with electrons around it.
| Particle | Charge | Where it is |
|---|---|---|
| Proton | Positive | In the nucleus |
| Neutron | None | In the nucleus |
| Electron | Negative | Moving around the nucleus |
A whole atom is electrically neutral because the number of negative electrons equals the number of positive protons. The nucleus is positive because it contains the protons. Neutrons add mass but no charge. Electrons have so little mass that almost all the mass of the atom is in the nucleus.
Notes · 2 of 5
Proton number and elements
What makes an element is the number of protons, not the name on the jar alone.
All atoms of an element have the same number of protons. Carbon atoms have 6 protons. Oxygen atoms have 8. A different proton number means a different element. That number is one reason the Periodic Table can list the elements in order.
If an atom is neutral, the number of electrons equals the proton number. You will not be asked to draw shells. You may be asked which particle decides the element: the proton.
Notes · 3 of 5
Molecules
A molecule is a chemically combined group of atoms. A mixture of atoms in a beaker is not a molecule.
A molecule is two or more atoms that are chemically combined. Oxygen gas in the air is mostly O₂, a molecule of two oxygen atoms. It is still an element, because both atoms are oxygen. Water, H₂O, is a molecule of a compound: two hydrogen atoms and one oxygen atom, chemically combined.
- Atom
- One neutral particle. Example: a single helium atom.
- Molecule of an element
- Two or more atoms of the same element, chemically combined. Example: O₂.
- Molecule of a compound
- Atoms of different elements, chemically combined. Example: CO₂ or H₂O.
Notes · 4 of 5
Reading a formula
The symbol says which atom. The small number says how many of that atom.
| Formula | Atoms in one molecule |
|---|---|
| O₂ | 2 oxygen atoms |
| H₂O | 2 hydrogen atoms and 1 oxygen atom |
| CO₂ | 1 carbon atom and 2 oxygen atoms |
| NH₃ | 1 nitrogen atom and 3 hydrogen atoms |
The small number applies to the symbol just before it. In CO₂ the 2 belongs to oxygen, not to carbon. If there is no number, read it as one. You are given the formula. You are not required to write a new formula from a name, and giant structures such as diamond are not in this chapter.
Notes · 5 of 5
Size, and why the model matters
Atoms are far smaller than anything you handle. Using what we know about them is a choice with consequences.
An atom is much smaller than a cell, and a cell is much smaller than a grain of sand. That is why we use a model. Nobody has held an atom in tweezers. The model is still good enough to count atoms in a formula and to explain that elements differ by proton number.
Knowledge of atoms has been used in medicine and in power stations, and also to build nuclear weapons. The science does not by itself decide the use. A Sec 1 answer only needs the awareness: the same knowledge can carry benefits, risks and ethical questions, and societies have to choose how to use it.
Revise
Key terms
The words examiners expect you to use precisely.
- Atom
- An electrically neutral particle with a positive nucleus and negative electrons moving around the nucleus.
- Nucleus
- The central part of the atom. It contains protons and neutrons and is positively charged.
- Proton
- A positively charged particle in the nucleus. The number of protons decides the element.
- Neutron
- A particle in the nucleus with no charge. It contributes to the mass.
- Electron
- A negatively charged particle moving around the nucleus. It has very little mass.
- Proton number
- The number of protons in an atom. It is the same for every atom of an element.
Show 3 more key termsShow fewer key terms
- Molecule
- A group of two or more atoms that are chemically combined.
- Chemical formula
- Symbols and numbers that show the types and numbers of atoms in a molecule, such as H₂O.
- Element
- A substance whose atoms all have the same proton number.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Saying the nucleus is negative, or that electrons are in the nucleus.
The charges and the places are the opposite of that.
Instead: The nucleus is positive (protons and neutrons). Electrons are negative and move around the nucleus.
Mistake: Saying a neutral atom has more electrons than protons.
The charges would not balance.
Instead: In a neutral atom the number of electrons equals the number of protons.
Show 4 more mistakesShow fewer mistakes
Mistake: Using neutrons to decide the element.
Neutrons affect mass, not which element it is.
Instead: The proton number identifies the element.
Mistake: Reading CO₂ as two carbon atoms.
The 2 follows the O.
Instead: CO₂ is one carbon atom and two oxygen atoms.
Mistake: Calling every group of atoms in a beaker a molecule.
A mixture is not chemically combined.
Instead: A molecule is chemically combined. Iron filings with sulfur powder are a mixture until they react.
Mistake: Thinking an atom is about the size of a cell.
A cell is made of an enormous number of atoms.
Instead: An atom is far smaller than a cell or a grain of sand.
Check yourself
Quick quiz
10 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Describe atomic particles
Which particle in an atom has a negative charge?
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Practise
Topical questions
Exam-style practice in three tiers, from recall to data questions.
- 5Foundation
- 5Standard
- 5Challenging
15 questions worth 41 marks, each with a model answer, mark scheme and examiner tips.
Practise topical questionsExam practice
Practice paper
A 30-mark paper set out like a school paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 8 Practice Paper
30 marks · 40 min · 6 pages
10 multiple-choice questions and structured questions with answer lines. Chapter 8.
Mark scheme
PROChapter 8 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Where this chapter is tested
Each school sets its own scope — check your assessment outline.
Questions
Frequently asked questions
What is an atom in Sec 1 Science?
An atom is electrically neutral. It has a positive nucleus made of protons and neutrons, with negative electrons moving around the nucleus. Most of the mass is in the nucleus.
What is the difference between an atom and a molecule?
An atom is a single neutral particle. A molecule is two or more atoms chemically combined, such as O₂ or H₂O.
How do I read CO₂?
One carbon atom and two oxygen atoms. The small 2 applies to the oxygen symbol just before it. You are not asked to invent formulae.
What decides which element an atom is?
The number of protons. Every atom of an element has the same proton number. Neutrons do not decide the element.
Are atoms and molecules in the Sec 1 EOY?
Yes in the usual G2/G3 scheme. Schools such as Yishun Secondary and Yio Chu Kang Secondary examine Chapters 1–8, including atoms and molecules, in the Sec 1 end-of-year paper. Forces and the later topics are Sec 2.