1.2 Isotopes

Question 1

Numbers of particles

Answer 1

Atom can be described by its:
- Mass number, the total number of protons and neutrons in the nucleus
- Atomic number, the number of protons in the nucleus

Atoms of a given element have the same number of protons in the nucleus:
- They have the same atomic number
- This number is unique to that element

Question 2

Isotopes

Answer 2

Atoms of an element with:
- the same number of protons
- different numbers of neutrons

You can recognise isotopes of the same element because they will have the same atomic number (bottom number), but different mass numbers (top number).

Isotopes of an element have the same chemical properties because they have the same number of electrons.

Question 3

Calculating particle numbers

Answer 3

Use the atomic number and mass number to calculate the number of subatomic particles in an atom.

For example, for sodium, Na:
Atomic number = 11
Number of protons = 11
Number of electrons = 11

Mass number =23
Neutrons = mass number - atomic number = 23 - 11 = 12

Question 4

Relative atomic mass

Answer 4

Take care not to confuse relative atomic mass, Ar, with mass number:
Ar is the mean mass of the atoms of an element, relative to 1/12th the mass of a 12C atom.

Ar values take into account the relative abudance or percentage of each isotopes in a sample of an element. The existence of isotopes means that the Ar values of elements may not be whole numbers.

This is because some elements have different isotopes so their relative atomic masses are a weighted mean value.

Question 5

Calculating relative atomic mass

Answer 5

Ar = (mass of isotope 1 * abundance of isotope 1) + (mass of isotope 2 * abundance of isotope 2) + … / 100