Atomic Structure and the Periodic TableCambridge IGCSE Chemistry: Revision notes

Section 1

What is the basic structure of an atom?

An atom consists of a central nucleus surrounded by electrons in shells (or energy levels). The nucleus contains two types of particles:

Protons – positively charged particles
Neutrons – particles with no charge (neutral)
Electrons – negatively charged particles that orbit the nucleus in shells

The nucleus is extremely small but contains almost all the mass of the atom. Electrons occupy the space around the nucleus and determine the chemical properties of an element.

Key termsatomnucleusprotonsneutronselectronsshells

Think of it like this

Think of an atom like a solar system: the nucleus is the Sun (central, massive), and electrons are planets orbiting in shells (paths) around it.

Section 2

What are the relative charges and masses of subatomic particles?

Particle	Relative Charge	Relative Mass
Proton	+1	1
Neutron	0 (neutral)	1
Electron	–1	1/1836 (negligible)

Key points:

Protons and neutrons have roughly equal mass; electrons are much lighter
The total charge of an atom is zero because the number of protons equals the number of electrons
For calculations, the mass of electrons is usually ignored when determining the mass of an atom

Key termsrelative chargerelative massprotonneutronelectron

Exam tip

Examiners expect you to state charges and masses as exact values: +1, 0, and –1 for charges; and 1, 1, and 1/1836 for masses. Learn these figures.

Common mistake

Students often forget that an electron's mass is negligible and include it when calculating atomic mass; always ignore electron mass unless specifically asked.

Section 3

What are proton number and mass number?

Proton number (atomic number) is the number of protons in the nucleus of an atom. It is represented by the symbol Z.

The proton number defines which element an atom is
All atoms of the same element have the same proton number
The proton number equals the number of electrons in a neutral atom

Mass number (nucleon number) is the total number of protons and neutrons in the nucleus. It is represented by the symbol A.

Formula:

Mass number = Proton number + Number of neutrons

Or: A = Z + N (where N = number of neutrons)

Example: Carbon-12 has 6 protons and 6 neutrons, so its proton number is 6 and its mass number is 12.

Key termsproton numberatomic numbermass numbernucleon number

Example

For sodium-23: proton number = 11, mass number = 23. Therefore, number of neutrons = 23 – 11 = 12. A neutral sodium atom has 11 protons and 11 electrons.

Exam tip

In exam questions, proton number and atomic number mean the same thing; mass number and nucleon number mean the same thing. Use these terms interchangeably.

Section 4

How do you determine the electronic configuration of atoms and ions?

Electronic configuration describes how electrons are arranged in shells around the nucleus.

Rules for filling electron shells:

Electrons fill shells in order from the innermost shell outward
Maximum electrons in each shell: first shell = 2, second shell = 8, third shell = 8 (for elements up to atomic number 20)
Write the configuration as numbers, e.g. 2,8,1

For neutral atoms: Number of electrons = proton number

For positive ions: Subtract the charge from the number of electrons. Electrons are lost from the outermost shell first.

For negative ions: Add the charge to the number of electrons. Electrons are added to the outermost shell.

Examples (proton numbers 1–20):

Atom/Ion	Proton Number	Electrons	Configuration
H	1	1	1
O	8	8	2,6
O²⁻	8	10	2,8
Na	11	11	2,8,1
Na⁺	11	10	2,8
Cl	17	17	2,8,7
Cl⁻	17	18	2,8,8
Ca	20	20	2,8,8,2

Key termselectronic configurationelectron shellsneutral atomsionscationsanions

Example

Magnesium (Mg) has proton number 12. Neutral Mg has 12 electrons: configuration 2,8,2. Mg²⁺ loses 2 electrons from its outer shell, leaving 10 electrons with configuration 2,8.

Common mistake

Students often remove electrons from the wrong shell when forming ions. Always remove from the outermost shell first, and always remove the number specified by the charge.

Exam tip

Check your work: the total of all numbers in a configuration must equal the number of electrons. For ions, compare the electron count to the proton number to confirm the charge is correct.

Section 5

How do the periodic table and electron shells relate to group and period number?

The periodic table is organised into groups (vertical columns) and periods (horizontal rows). The position of an element reveals information about its electrons:

Period number = number of occupied electron shells

Elements in period 1 have 1 electron shell (H, He)
Elements in period 2 have 2 electron shells (Li to Ne)
Elements in period 3 have 3 electron shells (Na to Ar)
Elements in period 4 have 4 electron shells (K to Kr)

Group number = number of electrons in the outermost shell (for Groups I–VII)

Group I elements have 1 outer electron (alkali metals)
Group II elements have 2 outer electrons (alkaline earth metals)
Group III elements have 3 outer electrons
Group VII elements have 7 outer electrons (halogens)

Group VIII (Group 0) – Noble gases:

Have a full outer electron shell (8 electrons, except He which has 2)
This full outer shell makes them very stable and unreactive
Group VIII elements are at the far right of the periodic table

Examples:

Lithium (Li): Period 2, Group I → configuration 2,1
Oxygen (O): Period 2, Group VI → configuration 2,6
Chlorine (Cl): Period 3, Group VII → configuration 2,8,7
Argon (Ar): Period 3, Group VIII → configuration 2,8,8

Key termsperiodic tablegroupperiodouter electron shellnoble gasesfull outer shell

Example

Sodium (Na): It is in period 3 and group I. Period 3 means 3 electron shells (2,8,1). Group I means 1 outer electron. Configuration is 2,8,1 ✓

Exam tip

Use the periodic table position to predict electronic configuration. Period tells you the number of shells; group (I–VII) tells you outer electrons. This method is faster than counting.

Common mistake

Students sometimes confuse group number with the number of shells. Remember: period = shells, group = outer electrons.

Must Know

Atomic structure: Nucleus (protons + neutrons) at centre, surrounded by electrons in shells
Subatomic particles: Proton (+1 charge, mass 1), neutron (0 charge, mass 1), electron (–1 charge, mass ~0)
Proton number (Z) = number of protons = atomic number; defines which element
Mass number (A) = protons + neutrons; A = Z + N
Electronic configuration: First shell max 2, second shell max 8, third shell max 8 (up to atomic number 20)
For ions: Neutral atoms have electrons = protons; remove electrons for positive ions, add for negative ions
Period number = number of electron shells; Group number = number of outer electrons (Groups I–VII)
Noble gases (Group VIII) have full outer shells (8 electrons, except He with 2) and are extremely stable
Always check electronic configurations: total electrons must equal proton number for neutral atoms, or proton number ± charge for ions

That's the notes covered.

Carry on to the next subtopic.