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Ions & Ionic Bonds (CIE IGCSE Chemistry)
Revision Note
The formation of ions
How are ions formed?
- An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons
- 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, 6 and 7 will be the same as that of a noble gas - such as helium, neon, and argon
- 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
Formation of a cation
Formation of positively charged sodium ion
Formation of an Anion
Formation of negatively charged chloride ion
Exam Tip
- We can determine the charge an ion will have from the group the element is found in:
- Elements in Group 1 form ions with a 1+ charge
- Elements in Group 2 form ions with a 2+ charge
- Elements in Group 6 form ions with a 2- charge
- Elements in Group 7 form ions with a 1- charge
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What is ionic bonding?
What is ionic bonding?
- Ionic compounds are formed when metal atoms react with non-metal atoms
- Metal atoms lose their outer electrons which the non-metal atoms gain to form positive and negative ions
- The positive and negative ions are held together by strong electrostatic forces of attraction between opposite charges
- This force of attraction is known as an ionic bond and they hold ionic compounds together
Dot-and-cross diagrams
- Dot and cross diagrams are diagrams that show the arrangement of the outer-shell electrons in an ionic or covalent compound or element
- The electrons are shown as dots and crosses
- In a dot and cross diagram:
- Only the outer electrons are shown
- The charge of the ion is spread evenly which is shown by using brackets
- The charge on each ion is written at the top right-hand corner
Electrostatic forces between the positive Na ion and negative Cl ion
Ionic bonds between Group I & Group VII elements
Group I & Group VII Ions
- Sodium is a Group 1 metal so loses one outer electron to another atom to gain a full outer shell of electrons
- A positive sodium ion with the charge 1+ is formed, Na+
- Chlorine is a Group 7 non-metal so gains one electron to have a full outer shell of electrons
- A negative chloride ion with a charge of 1– is formed, Cl–
- The ions are then attracted to one another and held together by electrostatic forces
- The formula of the ionic compound is thus NaCl
- The large square brackets should encompass each atom and the charge should be in superscript and on the right-hand side, outside the brackets
Diagram representing the formation of the ionic bond in sodium chloride
Sodium loses one electron and chlorine gains an electron
Dot-and-cross diagram of sodium chloride
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