Elements and isotopes
Read nuclide notation, distinguish isotopes by proton and neutron counts, and explain why shared electron arrangements give very similar chemistry.
On this page
Isotope questions become straightforward when you read Z and A in a fixed order, then use “neutrons = A-Z”.
Understand the key terms
An element is a pure substance whose atoms have the same number of protons. Its atoms can have different numbers of neutrons: these are isotopes. Isotopes have the same proton number (Z) but different nucleon numbers (A).
Key ideas
- Proton number (Z) identifies the element.
- Nucleon number (A) counts protons + neutrons.
- Neutrons = A - Z.
- For an ion, adjust the electron count from the charge; the nucleus is unchanged.
Build the explanation
- Proton number identifies the element.
- Neutron number distinguishes its isotopes.
- Electron number determines whether the atom is neutral or an ion.
Read nuclide notation
Nuclide notation is written as A_(Z)X where:
- Z = proton number = number of protons
- A = nucleon number = protons + neutrons
So the number of neutrons is A-Z. The element symbol identifies the element too; a charge, if present, is written at the top right. It is separate from A at the top left.
For example, ³⁵₁₇Cl⁻ has nucleon number 35, proton number 17 and charge 1-. The “35” does not give the ion’s charge or electron count.
Why isotope identity depends on the nucleus
Carbon-12 and carbon-14 both have six protons, so both are carbon. Their neutron counts are 12-6 = 6 and 14-6 = 8. Neutral atoms of both have six electrons in the same arrangement, so their chemical properties are very similar.
Their masses differ, so some mass-dependent physical properties can differ. For gases at the same temperature, molecules containing lighter isotopes have a greater average speed. Changing neutron number does not give an atom an electric charge.
Do not confuse a whole-number nucleon number with the Periodic Table’s relative atomic mass, which is an average relative mass for the element’s isotope composition, not a count of nucleons in one atom.
Revise proton number and electron counts in Atoms if needed.
Count electrons in single-atom ions
Nuclide notation tells you the nucleus. The ion charge tells you how the electron count differs from the proton count.
- A 1 + ion has lost one electron: electrons = Z-1.
- A 2 + ion has lost two electrons: electrons = Z-2.
- A 1- ion has gained one electron: electrons = Z + 1.
For example, ²⁴₁₂Mg²⁺ has 12 protons, 12 neutrons and 10 electrons.
Drag protons, neutrons and electrons onto the atom and watch its nuclide notation, name and charge change.
6 protons and 6 neutrons in the nucleus, 6 electrons in shells 2, 4: a carbon-12 atom.
- Nucleon number
- 12
- Charge
- 0
- Ar
- 12.01
- Configuration
- 2,4
- Energy to remove the last electron
- — kJ/mol
- First ionisation energy
- 1086 kJ/mol
Try this
0 of 4 doneBuild an atom of carbon-12, then change it into carbon-14. (not done yet)
Both have 6 protons, so both are carbon. Carbon-14 has 2 more neutrons: atoms of one element with different numbers of neutrons are isotopes.
Make a sodium ion, Na⁺, and a chloride ion, Cl⁻. (not done yet)
An ion has more or fewer electrons than protons. The protons, and so the element, stay the same.
In the mass spectrum of chlorine, make the two peaks the same height. (not done yet)
Ar is the mean mass of the isotopes, weighted by abundance. Real chlorine is 76 % chlorine-35, so its Ar of 35.45 is nearer 35 than 37.
Remove sodium's electrons one at a time until the energy needed jumps. (not done yet)
The second electron comes from a full shell much closer to the nucleus, so it needs far more energy. One electron before the jump puts sodium in Group 1.
Misconceptions to check
- Saying isotopes have different proton numbers (wrong). Same Z = same element.
- Writing neutrons = A + Z (wrong). It is A - Z.
- Using isotope notation without labels (write A_(Z)X clearly, not random superscripts).
- Adding a positive charge to the proton count. Ion formation changes electrons, not the nucleus.
Check your reasoning
“Same Z, different A (different neutrons).”
Worked examples
Modelled example 1
Identifying Isotopes
Problem
Two atoms have these compositions:
- Atom X: 6 protons, 6 neutrons, 6 electrons
- Atom Y: 6 protons, 8 neutrons, 6 electrons
Determine whether X and Y are isotopes and explain.
Study the worked solution
Test whether they are the same element
Method
Compare proton numbers.Reason
Proton number defines the element.Working
Both have Z = 6, so both are carbon atoms.Test whether their nucleon numbers differ
Method
Compare neutron counts and hence nucleon numbers.Reason
Isotopes of one element differ in neutron number.Working
For X, A = 6 + 6 = 12; for Y, A = 6 + 8 = 14.State the relationship
Working
X and Y are isotopes of carbon: same Z, different A.
Guided practice 2
Counting Neutrons From Notation
Problem
Read A and Z before subtracting
Hints
Hint 1: identify the two numbers
The upper number is A = 31 and the lower number is Z = 15.
Hint 2: remove the proton contribution
Use neutrons = A-Z.
View solution step by step
Read the notation
Method
Identify nucleon and proton numbers.Reason
The neutron count is not written directly in nuclide notation.
Working
A = 31, Z = 15.Calculate neutron number
Working
Neutrons = A-Z = 31-15 = 16.
Common misconception 3
Do different masses mean different elements?
Learner response
A student says: “Carbon-12 and carbon-14 are different elements because they have different masses.” Correct the element classification, then explain why their chemical properties are very similar.
Separate chemical from physical properties
View solution step by step
Locate the first error
Method
Reject the claim that a difference in mass makes these different elements.
Reason
Both atoms have six protons. A different element would have a different proton number, not just a different neutron number.
Working
Isotopes have the same proton number and, as neutral atoms, the same electron configuration.
Correct the property comparison
Method
State that chemical properties are very similar but some physical properties can differ.
Reason
The electron arrangements are the same, while isotope masses differ because neutron counts differ.
Working
Very similar chemical properties; potentially different mass-dependent physical properties.
Examiner practice 4
Count particles in an ion
Examination question
Read the nucleus before applying the charge
View solution step by step
Read the proton number
1 markMethod
Use the lower number, Z = 13.Reason
Proton number is the number of protons.Working
Protons = 13.Find the neutron count
1 markMethod
Subtract proton number from nucleon number.Reason
A counts protons and neutrons together.Working
Neutrons = 27-13 = 14.Apply the ion charge
1 markMethod
Remove three electrons from the neutral-atom count.
Reason
A 3 + charge means the atom has lost three electrons.
Working
Electrons = 13-3 = 10.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark one mark for each correct particle count.
Challenge 5
Predict Properties of Isotopic Molecules
Properties transfer
One chlorine-gas sample contains molecules made only from chlorine-35 atoms. Another contains molecules made only from chlorine-37 atoms. Compare their chemical behaviour and their diffusion rates at the same temperature.
Use electrons for chemistry and mass for diffusion
Hints
Hint 1: separate the property causes
Chemical behaviour depends on electron configuration; gas diffusion rate at fixed temperature depends on particle mass.
Hint 2: compare the isotope masses
A molecule containing chlorine-35 atoms is lighter than the corresponding molecule containing chlorine-37 atoms.
View solution step by step
Compare chemical behaviour
Method
Predict very similar chemical behaviour.Reason
Chlorine-35 and chlorine-37 atoms have the same proton number and electron configuration.
Working
Both samples have the same valence-electron arrangement and very similar chemical properties.
Compare diffusion rates
Method
Predict faster diffusion for the chlorine-35 sample.
Reason
Its molecules are lighter, so at the same temperature they have a greater average speed.
Working
Chlorine-35 sample: lower molecular mass → faster diffusion.
Try these independently
Mind stretcher 1: Compare isotope ionsExtension
Question: Compare ³⁵₁₇Cl⁻ and ³⁷₁₇Cl⁻. State what is the same and what is different in their proton, neutron and electron counts.
Show Answer
Both ions have 17 protons because they are chlorine. Both have 18 electrons because each has gained one electron to form a 1- ion.
³⁵Cl⁻ has 35-17 = 18 neutrons, while ³⁷Cl⁻ has 37-17 = 20 neutrons.
So the proton and electron counts are the same; the neutron counts differ.
Mind stretcher 2: Hydrogen Isotopes (Read the Notation)Extension
Question: Hydrogen has three isotopes: ¹₁H, ²₁H, and ³₁H. State the number of neutrons in each.
Show Answer
- ¹₁H: neutrons = 1 - 1 = 0
- ²₁H: neutrons = 2 - 1 = 1
- ³₁H: neutrons = 3 - 1 = 2
Practise and check
See what you know across this topic, then go back to anything you got wrong.
Syllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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