Ions (Edexcel GCSE Chemistry): Revision Note
Exam code: 1CH0
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Ions
What are ions?
An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons
Negative ions are called anions and form when atoms gain electrons, meaning they have more electrons than protons
Positive ions are called cations and form when atoms lose electrons, meaning they have more protons than electrons
All metals lose electrons to other atoms to become positively charged ions
All non-metals gain electrons from other atoms to become negatively charged ions
Deducing subatomic particles in ions
An atom is neutral and has no overall charge
Ions on the other hand have either gained or lost electrons causing them to become charged
The number of subatomic particles in atoms and ions can be determined given their atomic (proton) number, mass (nucleon) number and charge
Ions have a different number of electrons to the number of protons, depending on their charge
A positively charged ion has lost electrons and therefore has fewer electrons than protons
A negatively charged ion has gained electrons and therefore has more electrons than protons
Worked Example
Determine the number of protons, neutrons & electrons of the following ions:
Mg2+ ion
F– ion
Answer 1:
The atomic number of a magnesium atom is 12 and the mass number is 24
Therefore, the number of protons in a Mg2+ ion is 12
Remember: The number of protons does not change when an ion is formed
An atom of magnesium is electronically neutral, which means that the number of protons equals the number of electrons
However, the 2+ charge in an Mg2+ ion means it has lost two electrons
So, it has 12 - 2 = 10 electrons
Number of neutrons = mass number – atomic (proton) number
Number of neutrons = 24 – 12
Number of neutrons = 12
Answer 2:
The atomic number of a fluorine atom is 9 and the mass number is 19
Therefore, the number of protons in an F– ion is 9
Remember: The number of protons does not change when an ion is formed
An atom of fluorine is electronically neutral, which means that the number of protons equals the number of electrons
However, the 1- charge in an F– ion means it has gained one electron
So, it has 9 + 1 = 10 electrons
Number of neutrons = mass number – atomic (proton) number
Number of neutrons = 19 – 9
Number of neutrons = 10
Examiner Tips and Tricks
The number of electrons that an atom gains or loses is the same as the charge
For example:
If a magnesium atom loses 2 electrons, then the charge will be 2+
If a bromine atom gains 1 electron then the charge will be 1-
Formation of Ions
This loss or gain of electrons takes place to obtain a full outer shell of electrons
The electronic structure of ions of elements in groups 1, 2, 3, 5, 6 and 7 will be the same as that of a noble gas - such as helium, neon, and argon
Deducing Dot & Cross Diagrams for Ionic Compounds
Sodium is a group 1 metal so will lose one outer electron to another atom to gain a full outer shell of electrons
A positive sodium ion with the charge 1+ is formed
Diagram showing the formation of the sodium ion
Chlorine is a group 7 non-metal so will need to gain an electron to have a full outer shell of electrons
One electron will be transferred from the outer shell of the sodium atom to the outer shell of the chlorine atom
A chlorine atom will gain an electron to form a negatively charged chloride ion with a charge of -1
Diagram showing the formation of the chloride ion
Examiner Tips and Tricks
For exam purposes you need only show the outer electrons in dot & cross diagrams.When writing about ions, we use the notation 1-, 2+ etc. to describe the charge of the ion, with the number first followed by the sign (+/-). It is incorrect to write them the other way around as this refers to the oxidation state, not the charge.
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