Chapter 8: Model of Matter – Atoms and Molecules (Summary Sheet)
Created by Miss Clarissa Ng | www.clartutors.com
Part A · The Atom
1 What an Atom Is
- An atom, which is the smallest particle of an element, possesses the chemical properties of the element. It is the building block of all matter.
- Every element is made up of one type of atom only — that is what makes an element an element (see Chapter 3).
- Atoms are far too small to see with the naked eye, which is why we study them through models and experiments rather than by looking.
2 Inside the Atom: the Three Subatomic Particles
| Subatomic particle | Charge | Relative mass | Where it is found |
|---|---|---|---|
| Proton | +1 | 1 | In the nucleus, at the centre of the atom |
| Neutron | 0 (no charge) | 1 | In the nucleus, beside the protons |
| Electron | −1 | About 1/1840 | Moving around the nucleus, in electron shells |
- Protons and neutrons sit together in the nucleus and are together called nucleons.
- Because the protons are positive and the neutrons carry no charge, the nucleus is positively charged.
- The electrostatic forces of attraction between the positive nucleus and the negative electrons are what hold the atom together — and the electrons stay in constant motion in their shells.
3 Why an Atom Is Electrically Neutral
- An atom has equal numbers of protons and electrons.
- The positive charges and the negative charges therefore cancel out exactly, so the atom as a whole carries no charge — it is electrically neutral.
- Say it precisely when you answer: the atom is neutral overall; the nucleus is positive and the electrons are negative.
4 Where the Mass Comes From, and What Identifies an Element
- Almost all of an atom's mass is in the nucleus: protons and neutrons each count as relative mass 1, while an electron is about 1/1840 — so small it can be ignored.
- The number of protons is called the proton (atomic) number, and it is unique to each element: every atom of an element has the same number of protons, and no two elements share it.
- So the proton number is the element's identity card: it is what tells oxygen from carbon.
- In a neutral atom, number of protons = number of electrons; the neutrons carry no charge and do not affect this.
Part B · Sizes and Models
5 How Big Is an Atom?
| Object | Width | Atoms across it (roughly) |
|---|---|---|
| An atom | About 0.1 nanometre (10−10 m) | — |
| A strand of human hair | About 0.1 mm | About 100,000 atoms |
| A single drop of water | About 2 mm | About 5,000,000,000,000,000,000,000 atoms (5 × 1021) |
- Even a speck you can barely see holds an unimaginable number of atoms — a drop of water holds more than five thousand quintillion particles.
- Comparing those widths is the point of the outcome: an atom is roughly a hundred-millionth of a millimetre across, so everyday objects such as a hair are a hundred thousand atoms wide.
- That gap in scale is why we cannot draw what we have not seen — every picture of an atom is a model, built from what measurements and experiments show.
6 How We Know: the Atomic Models
- A model is a picture that explains what we cannot see, and it is kept only while it fits the evidence — the same idea as the particle model in Chapter 7.
| Model | What it said the atom was | The evidence behind it |
|---|---|---|
| Dalton, early 1800s | A solid, indivisible sphere | Atoms combine in whole-number ratios of mass — the atom was the smallest possible piece |
| Thomson, 1897 | A ball of positive charge with electrons embedded in it (the plum-pudding model) | The electron had been discovered, so the atom had to contain negative particles |
| Rutherford, 1909–1911 | A tiny, dense, positively charged nucleus with the atom being mostly empty space | In the gold-foil experiment, most alpha particles passed straight through, but a few bounced back |
| Bohr, 1913 | Electrons travel in fixed shells at set distances from the nucleus | Atoms give out and take in light at very specific energies, which fixed paths explain |
| The modern model | A nucleus surrounded by a cloud, where an electron's exact position cannot be pinned down | Later experiments showed the electron behaves as a wave as well as a particle |
- Read the table as a story: each model was the best explanation of the evidence available at the time, and each was revised when new data arrived.
- Rutherford's result is the one to remember: if the atom were a solid ball, nothing would bounce back — so most of the atom must be empty space with the mass in one small spot.
- This is the Value outcome in one sentence: models are constructed, justified by evidence and continuously revised as new phenomena are probed.
Part C · Molecules
7 What a Molecule Is
- A molecule is a group of two or more atoms that are chemically combined.
- Chemically combined means the atoms are bonded together as one particle — not merely mixed, which is what a mixture is (Chapter 3 again).
- Molecules come in two kinds: a molecule of an element (all the atoms the same, e.g. O2) and a molecule of a compound (different atoms, e.g. H2O).
8 Molecules of Elements
- An element is made of one type of atom, and it exists either as single atoms or as molecules.
- Monoatomic elements exist as single atoms: the noble gases (helium, neon, argon) never need a partner.
- Diatomic molecules are two atoms of the same element joined together — the seven to know are H2, N2, O2, F2, Cl2, Br2, I2.
- Polyatomic molecules have several atoms: ozone, O3 (triatomic), and molecules of four or more atoms such as methane, CH4.
- Most non-metals exist as molecules; the formula tells you how many atoms are in one molecule, e.g. Cl2 is two chlorine atoms.
Monoatomic · He
Diatomic · O2
Polyatomic · O3
9 Molecules of Compounds
- A molecule of a compound is a group of two or more different types of atoms chemically combined.
- Compounds made up only of non-metals exist as molecules: carbon dioxide, CO2 and water, H2O are the two to know well.
- One carbon dioxide molecule = one carbon atom + two oxygen atoms. One water molecule = two hydrogen atoms + one oxygen atom.
- The difference from an element molecule is only this: the atoms in a compound molecule are of different types.
Carbon dioxide · CO2
Water · H2O
Ammonia · NH3
10 Reading a Chemical Formula
| Formula | Name | Types of atoms in one molecule | Total atoms |
|---|---|---|---|
| CO2 | Carbon dioxide | 1 carbon + 2 oxygen | 3 |
| H2O | Water | 2 hydrogen + 1 oxygen | 3 |
| CH4 | Methane | 1 carbon + 4 hydrogen | 5 |
| NH3 | Ammonia | 1 nitrogen + 3 hydrogen | 4 |
| O3 | Ozone | 3 oxygen (a molecule of an element) | 3 |
- The chemical symbol tells you the TYPE of atom; the subscript tells you HOW MANY of that atom.
- A symbol with no subscript means one atom — in CO2 there is one carbon, not none.
- To count the atoms in a molecule, add the subscripts: CO2 has 1 + 2 = 3 atoms.
- You are asked to read formulae and say what is in them — not to work out formulae yourself.
The symbols you need to recognise
| Element | Symbol | Element | Symbol |
|---|---|---|---|
| Hydrogen | H | Sodium | Na |
| Helium | He | Magnesium | Mg |
| Lithium | Li | Aluminium | Al |
| Beryllium | Be | Silicon | Si |
| Boron | B | Phosphorus | P |
| Carbon | C | Sulfur | S |
| Nitrogen | N | Chlorine | Cl |
| Oxygen | O | Argon | Ar |
| Fluorine | F | Potassium | K |
| Neon | Ne | Calcium | Ca |
Part D · Atoms and Molecules Compared
11 Atoms and Molecules Compared
| Atom | Molecule | |
|---|---|---|
| What it is | The smallest particle of an element that has that element's chemical properties | A group of two or more atoms chemically combined |
| How many atoms | One | Two or more |
| Which atoms | Only ever one type | One type (a molecule of an element) or different types (a molecule of a compound) |
| Can it exist on its own? | Yes — noble gases exist as single atoms | It exists as a particle made of its atoms, e.g. Cl2, H2O |
| Examples | He, Ne, Ar, Na, C | O2, Cl2, H2O, CO2 |
- The safe summary sentence: every molecule is made up of atoms, but not every atom is part of a molecule.
- An atom is the smallest particle of an element; a molecule can be the smallest particle of an element or of a compound.
Part E · Atoms and Society
12 What Knowledge of Atoms Made Possible
| Technology | What it gives us | The risks, costs and issues |
|---|---|---|
| Nuclear power stations | Large amounts of electricity with almost no carbon dioxide released | Radioactive waste stays dangerous for thousands of years; accidents (Chernobyl, 1986; Fukushima, 2011) forced people from their homes; building and closing a plant is very expensive |
| Radioisotopes in medicine | Find and treat disease, e.g. cancer treatment and tracers that show up inside the body | Patients and staff are exposed to radiation; the material must be handled and disposed of safely |
| Atomic bombs | Nothing that helps daily life — the knowledge was used as a weapon | Hiroshima and Nagasaki, 1945: tens of thousands died at once and many more died later from radiation; the ethical question of whether any country should hold such weapons |
- The same knowledge of the atom can heal or destroy — nuclear medicine saves lives while a bomb takes them, and the difference is how people choose to use it.
- Every use carries a question of who benefits and who carries the risk: the electricity goes to the whole grid, while the waste, the accidents and the cost land on the places where the plants sit.
- So an answer on this outcome needs two halves: what the technology does for us, and the social, ethical and cost concerns that come with it.
Part F · Exam Tips
13 Exam Tips: What Each Syllabus Pointer Asks For
| The pointer | What earns the mark |
|---|---|
| Describe an atom as an entity that is electrically neutral, made up of a positively charged nucleus (protons and neutrons) with negatively charged electrons moving around the nucleus | Name the three particles, say where each sits, then finish with the balance: equal protons and electrons, so the atom is neutral |
| Recognise that atoms have mass that is mainly contributed by the mass of the nucleus | Say the protons and neutrons carry the mass, and that an electron's relative mass (about 1/1840) is small enough to ignore |
| Show an awareness that the atoms of an element have a unique number of protons | Use the term proton (atomic) number and state that no two elements share it; in a neutral atom it equals the number of electrons |
| Show an understanding that a molecule is a group of two or more atoms that are chemically combined | Use the printed definition, and add that the atoms are bonded, not merely mixed |
| State the numbers and types of atoms, given the chemical symbol of an element or the chemical formula of a compound | Read the symbol as the type and the subscript as the number: CO2 is 1 carbon and 2 oxygen, 3 atoms in all — and never attempt to write a formula yourself |
| Compare the size of an atom with the sizes of everyday objects | Give the scale: an atom is about 0.1 nm across, so a hair is about 100,000 atoms wide and a drop of water holds about 5 × 1021 atoms |
| Compare atoms and molecules | Put both in one sentence: an atom is one particle of an element, while a molecule is two or more atoms bonded together, of the same element or of different elements |
| Show an appreciation of how models are constructed, justified and continuously revised as they are used to probe new phenomena (e.g. the various atomic models) | Name the sequence — Dalton, Thomson, Rutherford, Bohr, the modern model — and tie each change to the evidence that caused it (the gold-foil result for Rutherford) |
| Show an awareness that technologies from the knowledge of atoms have created social and ethical issues, risks and costs (e.g. atomic bombs) | Give one benefit and one concern from the table (nuclear medicine against nuclear weapons), and say the choice of use is the ethical issue |
The two errors to avoid in this topic: saying the atom is charged (it is neutral — the charges balance), and saying the electrons carry the mass (they do not; the nucleus does). Also keep the words apart — an atom of an element is one particle, a molecule of that element is two or more atoms bonded together.