Atoms & Ions (Oxford AQA IGCSE Combined Science Double Award)

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

Nucleus and electrons in a lithium atom
A lithium atom has three protons, four neutrons and three electrons
  • 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.

A carbon 12 nucleus with 6 protons and 6 neutrons, for IGCSE & GCSE Physics revision notes

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 atomic structure of an atom, a positive ion and a negative ion
The difference between positive and negative ions
  • 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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