Atomic structure

Study H1 Chemistry particles, isotopes, orbitals and electron configurations, then use ionisation-energy evidence to explain atomic structure.

GCE A-Level H1 Chemistry 2027
5 lessons

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Before you begin

Every property of an element depends on how its atoms are built. This topic starts with the protons, neutrons and electrons in atoms, isotopes and ions, then shows how electrons are arranged in shells, subshells and orbitals. It ends with ionisation energies, the experimental evidence for that arrangement.

Be comfortable with: atoms, elements and isotopes, electron configuration and the periodic table.

Learning goals

Identify and describe protons, neutrons and electrons by their relative charges and masses
Deduce how beams of protons, neutrons and electrons behave in an electric field
Describe how mass and charge are distributed in an atom
Work out the numbers of protons, neutrons and electrons in atoms and ions from proton number, nucleon number and charge
Describe how protons and neutrons make up a nucleus, using proton number and nucleon number
Distinguish isotopes by their different numbers of neutrons
Describe the number and relative energies of the s, p and d orbitals for principal quantum numbers 1 to 3, and the 4s and 4p orbitals
Describe the shapes of s and p orbitals
Write the electronic configuration of an atom or ion from its proton number and charge
Explain the factors that affect the ionisation energies of elements
Deduce electronic configurations from successive ionisation energies
Relate an element's successive ionisation energies to its position in the Periodic Table

Syllabus statements covered

identify and describe protons, neutrons and electrons in terms of their relative charges and relative masses
deduce the behaviour of beams of protons, neutrons and electrons in an electric field
describe the distribution of mass and charges within an atom
deduce the numbers of protons, neutrons and electrons present in both atoms and ions given proton and nucleon numbers (and charge)
describe the contribution of protons and neutrons to atomic nuclei in terms of proton number and nucleon number
distinguish between isotopes on the basis of different numbers of neutrons present
describe the number and relative energies of the s, p and d orbitals for the principal quantum numbers 1, 2 and 3 and also the 4s and 4p orbitals
describe the shapes of s and p orbitals [knowledge of wave functions is not required]
state the electronic configuration of atoms and ions given the proton number (and charge)
explain the factors influencing the ionisation energies of elements (see the Data Booklet) (see also Section 4)
deduce the electronic configurations of elements from successive ionisation energy data
interpret successive ionisation energy data of an element in terms of the position of that element within the Periodic Table

Lessons

Work through them in order.

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Topic reference

Quick reference

Question	First move	What to show
Particles in an ion	protons = Z; neutrons = A-Z	electrons = Z-n for Xⁿ⁺, Z + n for Xⁿ⁻
A beam in an electric field	the sign of the charge gives the direction	the size of the deflection depends on charge ÷ mass
Capacity of a subshell	s has 1 orbital, p has 3, d has 5	two electrons per orbital: 2, 6 and 10
Configuration of an atom	fill 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p	Cr is [Ar]3d⁵ 4s¹ and Cu is [Ar]3d¹⁰ 4s¹
Configuration of a d-block ion	write the atom’s configuration first	remove 4s electrons before 3d
A trend in IE₁	nuclear charge, distance and shielding	how the attraction for the outer electron changes
A dip in IE₁	Group 2 to 13, or Group 15 to 16?	a new p subshell, or a paired p electron
Successive ionisation energies	find the first large jump	the electrons removed before it give the group

Core knowledge to remember

Proton number, Z: the number of protons in the nucleus. Nucleon number, A: the total number of protons and neutrons.
Isotopes: atoms of the same element with the same number of protons but different numbers of neutrons.
Orbital: a region of space around the nucleus with a high probability of finding an electron; it holds at most two electrons, with opposite spins.
Electrons fill the lowest-energy subshell first, and occupy the orbitals of a subshell singly before pairing.
An s orbital is spherical; a p orbital has two lobes, and a p subshell has three, along perpendicular axes.
First ionisation energy: the energy needed to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1 + ions, X(g) → X⁺(g) + e⁻.

Common mistakes

Changing the number of protons when an ion forms.
Defining isotopes without both parts: the same number of protons and different numbers of neutrons.
Describing an orbital as a fixed path for an electron.
Removing 3d electrons before 4s when a transition metal forms an ion.
Leaving “gaseous” or “one mole” out of the definition of ionisation energy.
Saying shielding increases greatly across a period.
Reading the group from the label of the ionisation energy after the jump.