3. Splitting atoms page 10
Interactive diagram of electron shells Interactive diagram of electron shells
Picture 3.2 Some atoms lose electrons and become positive ions. Others gain electrons and become negative ions.
A lithium atom has 3 protons and 3 electrons. It can lose one of its electrons, making it an ion. It now has more positive protons than electrons so it has an overall positive charge. Therefore it is a positive ion.

A fluorine atom will tend to gain, rather than lose, an electron. By gaining a negative electron, it has an overall negative charge. It has become a negative ion.

The positive lithium ion and the negative fluoride ion will now be attracted to each other. They can join together, or bond, to form lithium fluoride. You can find out more about the electrostatic attraction of opposite charges on page 11.

Note: Chemists sometimes draw the electrons around atoms in shells (as in picture 3.3). This helps to explain why some atoms lose electrons and others pick them up. This doesn't mean that the electrons are really going around in circles with those radii - it is just a way of picturing it.
Question 9
Look at the numbers of particles in the atoms below. In each case, decide what the charge will be. Select your answer from the menu.
elements protons electrons
sodium 11 11
sodium 11 10
oxygen 8 9
oxygen 8 7
iron 26 24
iron 26 23
hydrogen 1 1
hydrogen 1 0
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