1.3Unit 1. Structure, Bonding and Introduction to Organic Chemistry

Atomic Structure — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

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Lesson summary

1.3 Atomic Structure — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Describe the structure of the atom, isotopes and mass spectrometry, and calculate relative atomic mass from isotope data.Atom — mostly empty space around a very small, dense, positively charged nucleus containing proton…

Relative masses and charges
Actual charge
Atomic (proton) number, Z
Mass (nucleon) number, A
Neutral atoms
Positive ions

Part 1 of 7Structure of the atom

Structure of the atom

Atom

Atom mostly empty space around a very small, dense, positively charged nucleus containing protons and neutrons.

Electrons

Electrons negatively charged particles found in orbitals in the empty space around the nucleus.

Relative masses and charges

Relative masses and charges actual masses and charges are too small for conventional units, so particles are compared with each other.

Particle	Relative mass	Relative charge
Proton	1	+1
Neutron	1	0
Electron	1/1836 (negligible)	-1

Actual charge
- an electron has a charge of -1.602 x 10^-19 C and a proton +1.602 x 10^-19 C.

Part 2 of 7Atomic number, mass number and ions

Atomic number, mass number and ions

Atomic (proton) number, Z

Atomic (proton) number, Z the number of protons in the nucleus;
- it equals the number of electrons in a neutral atom and fixes which element it is.

Mass (nucleon) number, A

Mass (nucleon) number, A the total number of protons and neutrons, which are together called nucleons.

Neutrons

Neutrons number of neutrons = mass number - atomic number.

Neutral atoms

Neutral atoms have equal numbers of protons and electrons.

Positive ions

Positive ions have lost electrons, so they have fewer electrons than protons.

Negative ions

Negative ions have gained electrons, so they have more electrons than protons.

Ions of one element

Ions of one element all have the same number of protons.
Mg²⁺ has 12 protons, 10 electrons and 12 neutrons (mass number 24).

Part 3 of 7Isotopes

Isotopes

Isotopes atoms of the same element with the same number of protons and electrons but different numbers of neutrons.

Notation

Notation chemical symbol and mass number, as in carbon-12 and carbon-14 .
- (6 and 8 neutrons)

Chemical properties

Chemical properties the same, because the isotopes have the same electron arrangement and electrons determine chemistry.

Physical properties

Physical properties slightly different, because extra neutrons only add mass.
Mass, density, melting point and boiling point differ slightly.

Part 4 of 7The mass spectrometer

Purpose
- determines relative isotopic masses and the relative abundance of each isotope.
- Also used to detect illegal drugs, in forensic science, space exploration and carbon-14 dating.

The sample is injected, then heated and vaporised.

High-energy electrons from an electron gun knock electrons off the atoms or molecules, forming positive ions.

The ions are accelerated by attraction to negatively charged plates.

A few ions pass through a gap into the flight tube, where electromagnets deflect them in a curve.

The ions strike the detector plate, and each strike is recorded as a small current that is amplified.

Varying the electric and magnetic fields brings each ion to the detector, giving the m/z ratio and abundance.

Part 5 of 7Relative atomic mass from isotope data

Relative isotopic mass

Relative isotopic mass the mass of an isotope relative to carbon-12.

Relative atomic mass

A_r = sum of / 100.
- (relative abundance x isotopic mass)
The abundance is given in a table or read from the height of the peaks on a mass spectrum.

Reverse calculations

Reverse calculations the abundance of one isotope can be found if A_r and the other data are given.

Data forms

Data forms expect tables or graphs of data.

Part 6 of 7Mass spectra of molecules

Molecular ion (M+)

Molecular ion (M+) formed when a molecule loses one electron;
- the peak with the highest m/z gives the relative molecular mass.

Mass spectrum

Mass spectrum the pattern of ion abundances is a molecular fingerprint that a computer can match to a database.

M+1 peak

M+1 peak a small peak caused by carbon-13.
Its height increases with the number of carbon atoms, so hexane has a larger M+1 peak than ethane.

Fragments

Fragments ethanol, CH₃CH₂OH, gives CH₃⁺ at 15, CH₃CH₂⁺ at 29, CH₂OH⁺ at 31 (the base peak) and M⁺ at 46.

Distinguishing compounds

Distinguishing compounds propanal has M⁺ at 58 and butanal at 72.

Part 7 of 7Halogen isotope patterns

Chlorine

Chlorine isotopes 35Cl and 37Cl occur in the ratio 3 : 1.
One chlorine atom gives M⁺ and [M+2] peaks in the ratio 3 : 1.
Two chlorine atoms give M⁺, [M+2] and [M+4] in the ratio 9 : 6 : 1.
The ratio comes from probabilities3/4 x 3/4 = 9/16, 2 x 3/4 x 1/4 = 6/16 and 1/4 x 1/4 = 1/16.

Bromine

Bromine isotopes 79Br and 81Br occur in equal abundance.
One bromine atom gives M⁺ and [M+2] peaks in the ratio 1 : 1.
Two bromine atoms give M⁺, [M+2] and [M+4] in the ratio 1 : 2 : 1.

Recognition

Recognition an [M+2] peak, and possibly an [M+4] peak, shows chlorine or bromine in a compound.

Key terms

Term	Meaning
Isotopes	Atoms of the same element with different numbers of neutrons.
Atomic number	The number of protons in the nucleus of an atom.
Mass number	The total number of protons and neutrons in the nucleus.
Relative atomic mass	The weighted mean mass of an atom of an element relative to 1/12 of the mass of a carbon-12 atom.
Molecular ion	The ion formed when a molecule loses one electron, whose m/z equals its relative molecular mass.
Nucleon	A proton or a neutron.
m/z	The mass to charge ratio of an ion in a mass spectrum.

Avoid these mistakes

Treating electrons as having a relative mass of 1 — the relative mass is about 1/1836.

Saying isotopes have different chemical properties — they have the same electron arrangement.

Counting electrons in an ion as equal to the atomic number — subtract the positive charge or add the negative charge.

Reading the molecular ion as the tallest peak — it is the peak with the highest m/z (apart from M+1 and M+2).

Last updated: 12 October 2026