Atomic structure and isotopes Revision Notes for AQA GCSE Chemistry

Atomic structure and isotopes

What are atoms made of?

All atoms contain three types of particles. These particles have different properties that you need to remember:

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Particles in atoms:

Protons - have a positive charge (+1) and a mass of 1
Neutrons - have no charge (neutral) and a mass of 1
Electrons - have a negative charge (-1) and have very little mass

The protons and neutrons are found in the nucleus at the centre of the atom. The electrons move around the nucleus in shells or energy levels.

How atoms are represented

Scientists use a special way to show atoms called atomic notation. This tells us important information about any atom:

Mass number (top number) - total number of protons + neutrons
Atomic number (bottom number) - number of protons only
Chemical symbol - the letter(s) that represent the element

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Example: Reading Atomic Notation

For sodium: $^{23}Na$

This means:

Mass number = 23 (total particles in nucleus)
Atomic number = 11 (number of protons)
23 particles total = 11 protons + 12 neutrons

Size of atoms

Atoms are incredibly tiny. The whole atom has a radius of about $1 \times 10^{-10}$ metres (0.1 nanometres).

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The nucleus is even smaller - it's less than 1/10000th the size of the whole atom, but it contains almost all the atom's mass.

What are isotopes?

Isotopes are atoms of the same element that have different numbers of neutrons. This means:

They have the same number of protons (same atomic number)
They have different numbers of neutrons
They have different mass numbers
They react in exactly the same way chemically

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Example: Chlorine Isotopes

Chlorine has two main isotopes:

Chlorine-35: 17 protons, 18 neutrons (mass number = 35)
Chlorine-37: 17 protons, 20 neutrons (mass number = 37)

Both are still chlorine because they have 17 protons, but they have different masses.

Relative atomic mass

Most elements exist as a mixture of different isotopes. The relative atomic mass is the average mass of all these isotopes, taking into account how common each one is.

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Worked Example: Calculating Relative Atomic Mass

How to calculate:

Multiply each isotope's mass by its percentage abundance
Add all these values together
Divide by 100

For chlorine (75% chlorine-35 and 25% chlorine-37):

$\text{Relative atomic mass} = \frac{(75 \times 35) + (25 \times 37)}{100}$

$= \frac{2625 + 925}{100} = \frac{3550}{100} = 35.5$

This is why chlorine's relative atomic mass is 35.5, not a whole number.

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Key Points to Remember:

Atoms contain protons (+1 charge), neutrons (no charge), and electrons (-1 charge)
Protons and neutrons are in the nucleus and have mass of 1 each
Isotopes are atoms with the same protons but different neutrons
Relative atomic mass is the average mass of all isotopes of an element
The nucleus is tiny but contains almost all the atom's mass

Atomic structure and isotopes (AQA GCSE Chemistry): Revision Notes