Atoms & Atomic Models — Grade 8 Natural Sciences

What is an atom?

Everything around you — your desk, the air you breathe, even you — is made of matter. Matter is made up of incredibly tiny particles called atoms. Atoms are so small that a single full stop on this page contains billions of them.

Key idea: An atom is the smallest unit of an element that still has the chemical properties of that element. You cannot divide an atom further without changing what it is.

Inside an atom

Atoms are not solid balls — they have structure. Every atom has three types of sub-atomic particles:

Protons — positively charged (+), found in the nucleus
Neutrons — no charge (neutral), found in the nucleus
Electrons — negatively charged (−), orbit the nucleus in shells

Atomic number = number of protons = number of electrons (in a neutral atom)

Mass number = protons + neutrons

The nucleus is at the centre and is extremely dense — it contains almost all the atom's mass. The electrons orbit the nucleus in fixed energy levels called electron shells.

Electron shells

Electrons fill shells from the inside out. For the first 20 elements (hydrogen to calcium) the shells fill like this:

Shell 1 (closest to the nucleus): up to 2 electrons
Shell 2: up to 8 electrons
Shell 3: up to 8 electrons, then shell 4 starts
Shell 4: the remaining electrons (1 for potassium, 2 for calcium)

So potassium, with 19 electrons, is 2,8,8,1 and not 2,8,9. Shell 3 can eventually hold up to 18 electrons, but only in heavier atoms beyond calcium.

We write the electron arrangement as a series of numbers, e.g. carbon has 6 electrons arranged as 2,4 (2 in shell 1, 4 in shell 2). This is called the electron configuration.

History of atomic models

~450 BC

Democritus — "Atomos"

Ancient Greek philosopher who suggested matter is made of indivisible particles he called "atomos" (uncuttable). No experiments — just reasoning.

1803

Dalton — Solid Sphere Model

John Dalton proposed atoms as solid, indestructible spheres. Each element has identical atoms; compounds form when atoms combine.

1904

Thomson — Plum Pudding Model

J.J. Thomson discovered the electron and proposed atoms as a ball of positive charge with electrons scattered throughout like plums in pudding.

1911

Rutherford — Nuclear Model

Ernest Rutherford's gold-foil experiment showed most of the atom is empty space, with a tiny dense positive nucleus at the centre.

1913

Bohr — Planetary Model

Niels Bohr added that electrons orbit the nucleus in fixed energy levels (shells). This is the model used in Grade 8 science.

Atom Builder

Select an element from hydrogen (1) to calcium (20) and see how its electrons are arranged in shells. Each blue dot is one electron.

Predict first: How many electrons does a sodium atom have in its outer shell? Then test it: Select sodium (atomic number 11) and count.

Nucleus (red: protons and neutrons) · Electrons (blue dots)

Select Element

Hydrogen

Z = 1

Reactive nonmetal

Atomic number 1

Electron config: 1

Show elements beyond calcium (Z > 20)

Atomic Data

Protons

Neutrons

Electrons

Mass number

Period

Group

Did you know?

Hydrogen is the lightest and most abundant element in the universe. The sun is mostly hydrogen!

Answer these questions in your exercise book. Show all working where required.

Question 1 · (1 mark)

Define the term atom.

Question 2 · (3 marks)

Name the three sub-atomic particles found in an atom and state the charge of each.

Question 3 · (3 marks)

Carbon has an atomic number of 6 and a mass number of 12. Calculate the number of (a) protons, (b) neutrons, and (c) electrons in a neutral carbon atom.

Question 4 · (2 marks)

Write the electron configuration for sodium (Na), which has atomic number 11. How many electrons are in each shell?

Question 5 · (4 marks)

Briefly describe two different atomic models and name the scientist associated with each. Explain what each model got right and what was later shown to be incorrect.

Question 6 · (2 marks)

An atom of oxygen has 8 electrons. Write its electron configuration in the form used in science (e.g. 2,6) and explain which shell the outermost electrons occupy.

Question 7 · (6 marks)

Complete the table below for each neutral atom, using the relationships: protons = atomic number, electrons = protons (in a neutral atom), and neutrons = mass number − atomic number. Show your working for at least two rows.

Element	Atomic number (Z)	Mass number	Protons	Neutrons	Electrons
Nitrogen	7	14	?	?	?
Aluminium	13	27	?	?	?
Chlorine	17	35	?	?	?
Calcium	20	40	?	?	?