Atoms & Electron Arrangement

What Is an Atom?

An atom is the smallest particle of an element that still has the chemical properties of that element. Every piece of matter around you — water, air, rock, your own body — is made of atoms. Atoms are unimaginably tiny: about 100 million atoms lined up side by side would span just one centimetre.

Every atom has two distinct regions:

The nucleus — a dense, positively-charged centre containing protons (positive charge) and neutrons (no charge).
The electron cloud — negatively-charged electrons that occupy specific energy levels (shells) around the nucleus.

Key point: The positive charge of the protons in the nucleus attracts the negative electrons and keeps them in orbit — just like gravity keeps planets orbiting the Sun.

Atomic Number & Mass Number

Each element is defined by two fundamental numbers:

Atomic number (Z) = number of protons = number of electrons (neutral atom)

Mass number (A) = protons + neutrons

Neutrons = Mass number − Atomic number (N = A − Z)

For example, sodium (Na) has Z = 11 and A = 23, so it has 11 protons, 11 electrons, and 23 − 11 = 12 neutrons.

Isotopes are atoms of the same element with the same atomic number but different mass numbers — they have extra or fewer neutrons. Carbon-12 and Carbon-14 are both carbon, but C-14 has 2 extra neutrons.

Electron Shells

Electrons do not orbit the nucleus randomly — they occupy energy shells (also called energy levels). Each shell has a maximum number of electrons it can hold:

Shell 1 (closest to nucleus): maximum 2 electrons
Shell 2: maximum 8 electrons
Shell 3: maximum 8 electrons (for elements 1–20)

Electrons always fill the innermost shell first before moving to the next. This arrangement is written as a series of numbers separated by commas — for example, sodium (11 electrons) is written 2,8,1.

Electron arrangement of the first 20 elements follows the 2 → 8 → 8 → 2 filling pattern.

Bohr Diagrams for the First 20 Elements

A Bohr diagram shows the nucleus in the centre (labelled with the number of protons) and circles around it representing each shell, with dots or crosses for the electrons in each shell.

Z	Element	Symbol	Electron Arrangement	Valence e⁻
1	Hydrogen	H	1	1
2	Helium	He	2	2
3	Lithium	Li	2,1	1
4	Beryllium	Be	2,2	2
5	Boron	B	2,3	3
6	Carbon	C	2,4	4
7	Nitrogen	N	2,5	5
8	Oxygen	O	2,6	6
9	Fluorine	F	2,7	7
10	Neon	Ne	2,8	8
11	Sodium	Na	2,8,1	1
12	Magnesium	Mg	2,8,2	2
13	Aluminium	Al	2,8,3	3
14	Silicon	Si	2,8,4	4
15	Phosphorus	P	2,8,5	5
16	Sulfur	S	2,8,6	6
17	Chlorine	Cl	2,8,7	7
18	Argon	Ar	2,8,8	8
19	Potassium	K	2,8,8,1	1
20	Calcium	Ca	2,8,8,2	2

Valence Electrons & Reactivity

The electrons in the outermost shell are called valence electrons. They are the electrons that participate in chemical reactions and bonding. The number of valence electrons is the single most important factor in determining how reactive an element is and what kinds of bonds it forms.

Elements with 1 valence electron (like Na, K) are very reactive — they easily lose that electron.
Elements with 7 valence electrons (like Cl, F) are also very reactive — they easily gain one electron to complete their outer shell.
Elements with a full outer shell (like He, Ne, Ar) are almost completely unreactive — they are the noble gases.

The Octet Rule: Atoms tend to react in ways that give them a full outer shell of 8 electrons (or 2 for the first shell). This stable configuration is called a noble gas configuration.

The Periodic Table & Electron Configuration

The periodic table is arranged so that electron configuration is immediately visible:

Period number = the number of electron shells the element has. Period 1 elements have 1 shell; Period 3 elements have 3 shells.
Group number (Groups 1–8, also called main groups) = the number of valence electrons. All Group 1 elements have 1 valence electron; all Group 7 elements have 7 valence electrons.
Elements in the same group have the same number of valence electrons and therefore similar chemical properties — this is why lithium, sodium, and potassium all react violently with water.

Period 3, Group 7 → 3 shells, 7 valence electrons → that's Chlorine (Cl), electron arrangement 2,8,7. ✓

Noble Gases: The Stable Ones

Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe) and Radon (Rn) make up Group 8 (also called Group 18 or Group 0) — the noble gases. Their outer shells are completely full, which means they have no "drive" to gain, lose, or share electrons. This makes them almost completely chemically inert (unreactive). They exist as single atoms (monatomic) rather than molecules.

Answer the following questions in your notebook or on a separate sheet. Draw all diagrams neatly with a pencil and ruler. Show all calculations clearly.

Question 1

Draw fully labelled Bohr diagrams for the following elements: Lithium (Li), Nitrogen (N), Sodium (Na), Chlorine (Cl), and Calcium (Ca). For each diagram, label the nucleus with the number of protons, and show the correct number of electrons in each shell. (2 marks each — 10 marks total)

Question 2

Complete the table below for each element. Write the number of valence electrons, the group number, and the period number.

Elements: Magnesium (Mg, Z=12) | Fluorine (F, Z=9) | Silicon (Si, Z=14) | Potassium (K, Z=19)

(3 marks per element — 12 marks total)

Question 3

Sodium (Na) and Potassium (K) are both in Group 1 of the periodic table. Explain, using the concept of valence electrons, why elements in the same group have similar chemical properties. Use the electron arrangements of Na (2,8,1) and K (2,8,8,1) in your answer. (4 marks)

Question 4

An element X has an atomic number of 16 and a mass number of 32.
(a) How many protons does element X have?
(b) How many electrons does element X have (assuming it is neutral)?
(c) How many neutrons does element X have? Show your calculation.
(d) Write the electron arrangement of element X.
(e) Identify element X by name and symbol.
(1 mark each — 5 marks)

Question 5

Neon (Ne) is a noble gas. Using your knowledge of electron arrangement and the octet rule, explain why neon is chemically unreactive and does not form compounds with other elements. (3 marks)

Atoms & Electron Arrangement

What Is an Atom?

Atomic Number & Mass Number

Electron Shells

Bohr Diagrams for the First 20 Elements

Valence Electrons & Reactivity

The Periodic Table & Electron Configuration

Noble Gases: The Stable Ones

Interactive Atom Builder — Select an Element