GCSE Chemistry revision

Atoms, elements and compounds

A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes

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An atom consists of a central nucleus containing protons and neutrons, with electrons orbiting the nucleus.
Hydrogen is the simplest element, with just one proton in its nucleus and one electron orbiting it.
Helium is the next smallest element, with two protons and two neutrons in the nucleus and two electrons orbiting it.

1. Structure of an Atom AQA

An atom consists of a central nucleus containing protons and neutrons, with electrons orbiting the nucleus.
Hydrogen is the simplest element, with just one proton in its nucleus and one electron orbiting it.
Helium is the next smallest element, with two protons and two neutrons in the nucleus and two electrons orbiting it.

There are around 100 different elements, all organised into the periodic table where each box represents a different element.
Each element is represented by a nuclear symbol, which includes a one- or two-letter symbol, the atomic number, and the mass number.
Some element symbols are not obvious from their names — for example, sodium is Na and iron is Fe.

2. The Periodic Table AQA

There are around 100 different elements, all organised into the periodic table where each box represents a different element.
Each element is represented by a nuclear symbol, which includes a one- or two-letter symbol, the atomic number, and the mass number.
Some element symbols are not obvious from their names — for example, sodium is Na and iron is Fe.

The atomic number is the number of protons in the nucleus of an atom and is unique to each element.
Any atom with a given number of protons must be the element that corresponds to that atomic number — for example, any atom with six protons is carbon.
If an atom has three protons, the periodic table tells us it must be lithium, which has an atomic number of three.

3. Atomic Number AQA

The atomic number is the number of protons in the nucleus of an atom and is unique to each element.
Any atom with a given number of protons must be the element that corresponds to that atomic number — for example, any atom with six protons is carbon.
If an atom has three protons, the periodic table tells us it must be lithium, which has an atomic number of three.

Isotopes are different forms of the same element that have the same number of protons but a different number of neutrons.
The number of neutrons in an atom can vary without changing which element it is, as long as the number of protons stays the same.
Because isotopes only diﬀer in the number of neutrons, they behave in almost identical ways chemically.

4. What Are Isotopes? AQA

Isotopes are different forms of the same element that have the same number of protons but a different number of neutrons.
The number of neutrons in an atom can vary without changing which element it is, as long as the number of protons stays the same.
Because isotopes only diﬀer in the number of neutrons, they behave in almost identical ways chemically.

Carbon-12, the most common form of carbon, has six protons, six neutrons, and six electrons, giving it a mass number of 12.
Carbon-13 is a rarer isotope that still has six protons and six electrons, but has seven neutrons, giving it a mass number of 13.
The mass number of an isotope is the total number of protons and neutrons in its nucleus.

5. Isotopes of Carbon AQA

Carbon-12, the most common form of carbon, has six protons, six neutrons, and six electrons, giving it a mass number of 12.
Carbon-13 is a rarer isotope that still has six protons and six electrons, but has seven neutrons, giving it a mass number of 13.
The mass number of an isotope is the total number of protons and neutrons in its nucleus.

Isotopic abundance refers to how common or rare a particular isotope is within a sample of an element, usually expressed as a percentage.
Copper has two stable isotopes: copper-63 with an abundance of 69.2% and copper-65 with an abundance of 30.8%.
The abundances of all isotopes of an element must add up to 100%, as they account for all atoms of that element.

6. Isotopic Abundance AQA

Isotopic abundance refers to how common or rare a particular isotope is within a sample of an element, usually expressed as a percentage.
Copper has two stable isotopes: copper-63 with an abundance of 69.2% and copper-65 with an abundance of 30.8%.
The abundances of all isotopes of an element must add up to 100%, as they account for all atoms of that element.

The relative atomic mass (Aᵣ) is the average mass of all the isotopes of an element, taking into account their relative abundances.
The formula for relative atomic mass is: Aᵣ = (Σ(isotope abundance × isotope mass) / Σ(abundances of all isotopes))
For copper, the calculation is ((69.2 × 63) + (30.8 × 65) / 69.2 + 30.8) = (6361.6 / 100) = 63.6 (to one decimal place).
The relative atomic mass of copper is 63.6, meaning the average mass of a copper atom is 63.6 atomic mass units.

7. Calculating Relative Atomic Mass AQA

The relative atomic mass (Aᵣ) is the average mass of all the isotopes of an element, taking into account their relative abundances.
The formula for relative atomic mass is: Aᵣ = (Σ(isotope abundance × isotope mass) / Σ(abundances of all isotopes))
For copper, the calculation is ((69.2 × 63) + (30.8 × 65) / 69.2 + 30.8) = (6361.6 / 100) = 63.6 (to one decimal place).
The relative atomic mass of copper is 63.6, meaning the average mass of a copper atom is 63.6 atomic mass units.