9.04.2 Atomic & Mass Numbers

Syllabus

In an atom, the number of electrons is equal to the number of protons in the nucleus.
Atoms have no overall electrical charge.
The number of protons in an atom of an element is its atomic number.
All atoms of a particular element have the same number of protons.
Atoms of different elements have different numbers of protons.
The sum of the protons and neutrons in an atom is its mass number.
Atoms can be represented as shown in this example:

Students should be able to calculate the numbers of protons, neutrons and electrons in an atom or ion, given its atomic number and mass number.
Students should be able to relate size and scale of atoms to objects in the physical world.
Use SI units and the prefix nano.

Every element on the Periodic Table comes with two numbers

There will be a Periodic Table on the examination so it's not necessary to learn any of the numbers - only what to do with them.

The Atomic Number tells us the number of protons in the nucleus.

So, Helium has 2 protons

Because atoms have no overall charge, there must be equal numbers of positive protons and negative electrons.

So, Helium has 2 electrons as well

But neutrons don't affect the charge, so they don't have to be the same.

The Mass Number tells us the number of "heavy" sub-atomic particles - protons and neutrons.

So Helium also has 2 neutrons because 4 - 2 = 2

Lithium has 7 heavy particles because its Mass Number is 7.

But 3 of them are protons - because its Atomic Number is 3

4 of these heavy particles must be neutrons (because 7-3 = 4)

So, atoms don't always have the same number of protons neutrons and electrons because the neutrons may be different.

And the bigger the atoms the bigger the difference between the number of protons and the number of neutrons is likely to be.

Ions are atoms that have either gained or lost electrons - you'll learn why later.

This means that ions have a charge - atoms are neutral

Since electrons are negative, gaining an extra electron makes an atom into a negative ion.

Losing an electron makes an atom into a positive ion.

So, a Chloride ion ₁₇Cl^- ... must have started with 17 electrons to balance its 17 protons but has gained 1 more and now has 18 electrons.

An Oxide ion ₈O^2- ... must have started with 8 electrons to balance its 8 protons but has gained 2 more and now has 10 electrons.

An Nitride ion ₇N^3- ... must have started with 7 electrons to balance its 7 protons but has gained 3 more and now has 10 electrons.

A Lithium ion ₃Li⁺ ... must have started with 3 electrons to balance its 3 protons but has lost 1 and now has 2 electrons.

A Calcium ion ₂₀Ca²⁺ ... must have started with 20 electrons to balance its 20 protons but has lost 2 and now has 18 electrons.

An Aluminium ion ₁₃Al³⁺ ... must have started with 13 electrons to balance its 13 protons but has lost 3 and now has 10 electrons.

2021

Answers

Electrons = 31

Neutrons = 38

2018

Answers

Protons = 26

Neutrons = 30

Electrons = 26

2017

Answer

Protons - 13

Neutrons - 14

Electrons - 13