Atoms & Ions (Oxford AQA IGCSE Physics)
Revision Note
Neutral Atoms
Although atoms contain particles of different charge, the total charge within an atom is zero
This is because the number of electrons is equal to the number of protons
Atomic structure of a lithium atom
The following table sets out the calculation of the total charge in the lithium atom:
Total Charge Calculation Table
Relative charge | Number of Particles in a Lithium Atom | Charge x Number of Particles | Total Charge | |
|---|---|---|---|---|
Protons | +1 | 3 | (+1) x 3 = +3 | (+3) + 0 + (-3) = 0 |
Neutrons | 0 | 4 | 0 x 4 = 0 | |
Electrons | -1 | 3 | (-1) x 3 = -3 |
Worked Example
A nucleus of carbon-12 is shown below.
How many electrons are there in an atom of carbon-12?
Answer:
Step 1: Count the number of protons in the carbon nucleus
There are 6 protons in the carbon atom
Step 2: Determine the number of electrons
Remember, the number of electrons in an atom is equal to the number of protons
Therefore there must be 6 electrons in the carbon atom
Ions
If an atom gains or loses electrons, an ion is formed
An ion is an electrically charged atom
Positive ions are therefore formed when atoms lose negative electrons
There will be more positive protons than negative electrons
Negative ions are therefore formed when atoms gain negative electrons
There will be more negative electrons than positive protons
Atomic structure of an atom, a negative ion and a positive ion
The number of electrons that an atom gains or loses is the same as the charge
For example, if an atom loses 2 electrons, then the charge on the resulting ion will be +2
If an atom gains 1 electron, then the charge on the resulting ion will be -1
Examiner Tip
You may hear the term 'net charge'. This just means the 'overall' charge of the atom. If an atom has 5 protons, 5 neutrons and 6 electrons, it has a net negative charge because it's a negative ion (more electrons than protons).
Remember which way around the charges are by:
Protons are positive
Neutrons are neutral
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