Isotopes

Section 1 of 10What isotopes are

Definition

• Definition isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

Naming

• Naming written as the symbol or name, a dash and the mass number, such as C-14 or carbon-14.
• Carbon-14
  • 6 protons, 6 electrons and 14 – 6 = 8 neutrons.
• Written form
  • it can also be shown with the mass number as a superscript before the symbol.
• Hydrogen
  • has three isotopes that differ only in neutron number.

Section 2 of 10Chemical and physical properties of isotopes

1Same chemistry

• Same chemistry isotopes have the same number of outer electrons and so the same electronic configuration, which determines chemical behaviour.

2Neutrons

• Neutrons they are neutral and add mass only.
• Appearance
  • a sample of C-14 looks identical to C-12.

3Different physical properties

• Different physical properties extra mass affects density, melting point and boiling point.

4Heavy water

• Heavy water deuterium oxide has relative formula mass 20 compared with 18 for water,
  • so it is about 20% heavier yet looks, tastes and feels like water.
• Hazard
  • it should not be drunk because it interferes with biochemical reactions in cells.

Section 3 of 10Relative atomic mass

Symbol

• Ar, found beneath the symbol in the Periodic Table, with the atomic number above it.

Standard

• Standard carbon-12 is the reference atom, with a fixed mass of 12 units.

Definition

• Ar is the average mass of the isotopes of an element compared with 1/12 of the mass of a carbon-12 atom.
• Examples
  • carbon is 12, magnesium is 24 , hydrogen is 1 (one-twelfth of carbon).
    • (twice as heavy as carbon)

Section 4 of 10Calculating Ar from isotopes

Calculating Ar from isotopes

Multiply each isotope percentage by its mass number.

Add all the products together.

Divide the total by 100.

Equation

• Ar = [ + (% isotope 2 × mass 2) + …] / 100.
  • (% isotope 1 × mass 1)

Extending

• Extending add more terms when there are more isotopes.
• Rubidium example
  • / 100 = 85.6.
    • (72 × 85 + 28 × 87)

Section 5 of 10Mass number compared with relative atomic mass

Difference

• Difference mass number is a whole number for one isotope, while Ar is a weighted average over all isotopes.

Lithium

• Lithium its Ar is 6.94, shown as 7 when rounded to a whole number.
• Why it looks the same
  • Periodic Tables often show Ar rounded, so it appears equal to a mass number.

Chlorine

• Chlorine its Ar is 35.5 because of the mixture of Cl-35 and Cl-37, so it is the exception where Ar is not used as the mass number.

Section 6 of 10Isotopes of hydrogen and carbon

Isotope	Protons	Neutrons	Electrons
Hydrogen-1	1	0	1
Hydrogen-2 (deuterium)	1	1	1
Hydrogen-3 (tritium)	1	2	1
Carbon-12	6	6	6
Carbon-14	6	8	6

Reading the table

• Reading the table the proton and electron numbers never change within an element.

What changes

• What changes only the neutron number, and therefore the mass number.

Section 7 of 10Steps for exam questions on isotopes

1

Identify the proton number from the symbol or the Periodic Table.

2

Subtract the proton number from the mass number to find neutrons.

3

State that the electrons equal the protons in a neutral atom.

4

For chemical behaviour, refer to the identical electronic configuration.

5

For physical behaviour, refer to the different mass.

1Typical statement

• Typical statement isotopes have the same chemical properties because they have the same number of electrons in the outer shell.

2Typical statement 2

• Typical statement 2 their densities and boiling points differ because their masses differ.

Section 8 of 10Worked examples

1. Chlorine has two isotopes: 75% Cl-35 and 25% Cl-37. Calculate Ar. [2]2 marks

   Show mark scheme

2. State the number of protons, neutrons and electrons in carbon-14 and in carbon-12. [3]3 marks

   Show mark scheme

3. Explain why two isotopes of an element have the same chemical properties. [2]2 marks

   Show mark scheme

4. Magnesium has isotopes Mg-24 (79%), Mg-25 (10%) and Mg-26 (11%). Calculate Ar. [2]2 marks

   Show mark scheme

Section 9 of 10Key terms

Term	Meaning
Isotope	Atom of the same element with the same protons but a different number of neutrons.
Relative atomic mass (Ar)	Average mass of an element isotopes compared with 1/12 of a carbon-12 atom.
Carbon-12	The standard atom, with a fixed mass of 12 units.
Abundance	The percentage of each isotope in a natural sample.
Heavy water	Water made with deuterium, relative formula mass 20.

Section 10 of 10Exam technique

1

Saying isotopes have different numbers of electrons — they differ only in neutrons.

2

Thinking isotopes react differently — their chemistry is the same.

3

Calculating Ar by averaging the mass numbers without weighting — use the percentages.

4

Treating mass number and Ar as identical — Ar is an average over isotopes.