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Ions (Edexcel GCSE Chemistry)
Revision Note
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 Tip
- 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 Tip
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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