Learners' AlleyDesigned by Khow Willard

Atomic Structure

Sub-atomic Particles

1.1What an atom is made of

An atom is the smallest particle of an element that still has the chemical characteristics of that element.

Particle	Relative charge	Relative mass	Location
pproton	+1	1	nucleus
nneutron	0	1	nucleus
eelectron	−1	1/1836	electron shells

Protons and neutrons are together called nucleons. They are in the nucleus at the centre of the atom.
Atoms are electrically neutral, so the number of protons = the number of electrons.

1.2Proton number and nucleon number

126C

12 nucleon (mass) number 6 proton (atomic) number read as carbon-12

DefinitionProton number: the number of protons in the nucleus of an atom. It tells us which element the atom is from.

DefinitionNucleon number: the total number of protons and neutrons in the nucleus of an atom.

number of neutrons = nucleon number − proton number

Worked exampleTungsten is used to make drill bits. Tungsten-184 is written ¹⁸⁴₇₄W. Deduce the numbers of protons, neutrons and electrons in one atom of tungsten-184.

Protons = proton number = 74 Neutrons = nucleon number − proton number = 184 − 74 = 110 Electrons = protons = 74

12Topic 03 · Atomic Structure

Sub-atomic Particles (continued)

1.3Sub-atomic particles in ions

An ion is a particle formed when an atom or group of atoms gains or loses electrons. The numbers of protons and neutrons stay the same.

Particle	Protons	Neutrons	Electrons
calcium atom, Ca	20	20	20
calcium ion, Ca²⁺	20	20	18
oxygen atom, O	8	8	8
oxide ion, O²⁻	8	8	10

Cation (positive ion)

The atom loses electrons, so it has fewer electrons than protons, e.g. Ca²⁺.

Anion (negative ion)

The atom gains electrons, so it has more electrons than protons, e.g. O²⁻.

Isotopes

2.1Isotopes of an element

DefinitionIsotopes are atoms of the same element with the same proton number but different nucleon numbers. They have different numbers of neutrons.

	hydrogen-1	hydrogen-2	hydrogen-3
Notation	¹₁H	²₁H	³₁H
Protons	1	1	1
Neutrons	0	1	2
Electrons	1	1	1
Nucleon number	1	2	3

Chemical properties

Isotopes have the same number of electrons, so they react in the same way.

Physical properties

Isotopes have different masses, so they may have different densities, melting points and boiling points.

NoteIsotopes are named with their nucleon number, e.g. chlorine-35 and chlorine-37.

Topic 03 · Atomic Structure13

Learners' AlleyDesigned by Khow Willard

Electrons in Shells

3.1Electron shells

Electrons in the shell closest to the nucleus have the lowest energy. Electrons fill the lowest energy shells first.
The first shell holds up to 2 electrons. The second and third shells usually hold up to 8 electrons each.
The outermost shell is the valence shell, and its electrons are valence electrons.
A full valence shell is stable, so atoms of helium and argon are unreactive. An atom with a valence shell that is not full is reactive.

3.2Electronic configuration

The electronic configuration of an atom shows the number of electrons in each shell, starting from the innermost shell, e.g. sodium is 2.8.1.

From the periodic table

The period number is the number of electron shells.
The group number gives the number of valence electrons: Groups 1 and 2 have 1 and 2, and Groups 13 to 18 have 3 to 8. Helium has 2.

3.3Shell diagrams

Li 2.1

Be 2.2

Ne 2.8

Na 2.8.1

14Topic 03 · Atomic Structure

Electrons in Shells (continued)

3.4Configurations of the first 20 elements

Period	Group 1	2	13	14	15	16	17	18
1	H1							He2
2	Li2.1	Be2.2	B2.3	C2.4	N2.5	O2.6	F2.7	Ne2.8
3	Na2.8.1	Mg2.8.2	Al2.8.3	Si2.8.4	P2.8.5	S2.8.6	Cl2.8.7	Ar2.8.8
4	K2.8.8.1	Ca2.8.8.2

Worked exampleHow are the electrons arranged in an atom of potassium (proton number 19)?

Potassium has 19 protons, so it has 19 electrons. Fill the first shell with 2, then the second with 8: 2.8. That is 10 electrons. The remaining 9 electrons: the third shell takes 8, so 1 is left for the fourth shell. Electronic configuration = 2.8.8.1

NoteElectronic configurations are needed only up to calcium (proton number 20).

Exam tipIsotopes of an element have the same number of electrons, so they have the same electronic configuration.

Summary

4.1Atomic structure at a glance

Nucleus

Contains protons and neutrons, together called nucleons.
Protons: relative charge +1, relative mass 1. The proton number identifies the element.
Neutrons: relative charge 0, relative mass 1.

Electron shells

Electrons: relative charge −1, relative mass 1/1836.
Up to 2 electrons in the first shell and usually 8 in the second and third.
Electronic configuration, e.g. oxygen is 2.6.

Isotopes

Atoms of the same element with the same proton number and different nucleon numbers. They have the same chemical properties and different physical properties.

4.2Finding the numbers

To find	Use
Protons	the proton number
Neutrons	nucleon number − proton number
Electrons in an atom	the same number as the protons
Electrons in an ion	protons − charge, e.g. Ca²⁺ has 20 − 2 = 18
Electron shells	the period number
Valence electrons	the group number (Groups 1 and 2), or 3 to 8 for Groups 13 to 18

Topic 03 · Atomic Structure15