Atomic Structure (Oxford AQA IGCSE Combined Science Double Award): Revision Note
Atomic Structure
Atoms are the building blocks of all matter
They are incredibly small, with a radius of only 1 × 10-10 m
This means that about one hundred million atoms could fit side by side across your thumbnail
Atoms have a tiny, dense nucleus at their centre, with electrons orbiting around the nucleus
The radius of the nucleus is much smaller (over 10,000 times smaller) than the whole atom, but it contains almost all of the mass of the atom
Parts of the atom
The nucleus contains:
Protons - positively charged particles with a relative atomic mass of one unit
Neutrons – no charge, and also with a relative atomic mass of one unit
Almost all of the atom is empty space, but moving around the nucleus there are:
Electrons – negative charge with almost no mass (1/2000 the mass of a proton or neutron)
The structure of an atom
The properties of each of the particles are shown in the table below:
Table of the Relative Masses and Charges
Particle | Location | Relative Charge | Relative Mass |
---|---|---|---|
Proton | In the nucleus | +1 | 1 |
Neutron | In the nucleus | 0 | 1 |
Electron | Orbiting the nucleus | -1 | 1/2000 (Negligible) |
Examiner Tips and Tricks
There are many different models of the atom. As you progress through the topic, you will discover that the atom can also be described using the 'plum pudding' model, but for your exam, only use the model and descriptions described here!
Be careful with your terminology:
Atom = nucleus (proton and neutron) and electrons
Nucleus = protons and neutrons at the centre of the atom
Atomic & Mass Number
Atomic number
The number of protons in an atom is called its atomic number (it can also be called the proton number)
Elements in the periodic table are ordered by their atomic number
Therefore, the number of protons determines which element an atom is
The atomic number of a particular element is always the same
For example:
Hydrogen has an atomic number of 1. It always has just one proton
Sodium has an atomic number of 11. It has 11 protons
Uranium has an atomic number of 92. It has 92 protons
The atomic number is also equal to the number of electrons in an atom
This is because atoms have the same number of electrons and protons to have no overall charge
For more information, see the revision note on Atoms & Ions
Mass number
The total number of particles in the nucleus of an atom is called its mass number
The mass number is the number of protons and neutrons in the atom
The number of neutrons can be found by subtracting the atomic number from the mass number
Number of Neutrons = Mass Number - Atomic Number
For example, if a sodium atom has a mass number of 23 and an atomic number of 11:
It has 23 particles in its nucleus
11 of them are protons
So the number of neutrons would be 23 – 11 = 12
Nuclear notation
The mass number and atomic number of an atom are shown by writing them with the atomic symbol
This is called nuclear notation
Examples of nuclear notation
The top number is the mass number
This is equal to the total number of particles (protons and neutrons) in the nucleus
The lower number is the atomic number
This is equal to the total number of protons in the nucleus
The atomic and mass numbers of each type of atom in the examples above are shown in this table:
Calculating the Number of Protons, Neutrons and Electrons
Atom | Number of Protons (Atomic Number) | Number of Neutrons (Mass Number - Atomic Number) | Number of Electrons (Same as the Number of Protons) |
---|---|---|---|
Hydrogen | 1 | 1 | 1 |
Sodium | 11 | 12 | 11 |
Uranium | 92 | 143 | 92 |
Worked Example
The element symbol for gold is Au. How many protons, neutrons and electrons are in the gold atom?
Protons | Neutrons | Electrons | |
---|---|---|---|
A | 79 | 79 | 79 |
B | 197 | 79 | 118 |
C | 118 | 118 | 79 |
D | 79 | 118 | 79 |
Answer: D
Step 1: Determine the atomic and mass number
The gold atom has an atomic number of 79 (smaller number) and a mass number of 197 (larger number)
Step 2: Determine the number of protons
The atomic number is equal to the number of protons
The atom has 79 protons
Step 3: Calculate the number of neutrons
The mass number is equal to the number of protons and neutrons
The number of neutrons is equal to the mass number minus the atomic number
If there are 197 particles in the nucleus and 79 of them are protons
Then the number of neutrons is
197 - 79 = 118
The atom has 118 neutrons
Step 4: Determine the number of electrons
An atom has the same number of protons and electrons
The atom has 79 electrons
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