Describe the dot and cross diagrams for the ionic bonds formed between lithium and fluorine, and sodium and sulphur.

Here are the descriptions of the dot and cross diagrams for the ionic bonds formed:

a) Lithium and fluorine

1. Valence Electrons: Lithium (Li) has 1 valence electron. Fluorine (F) has 7 valence electrons.
2. Electron Transfer: Lithium transfers its single valence electron to fluorine.
3. Dot and Cross Diagram Description:
   • The lithium ion is represented as $\text{Li}^{+}$, having lost its valence electron. Its electron configuration is now stable (like helium).
   • The fluoride ion is represented as $[\text{F}]^{-}$. It has 8 electrons in its outermost shell: 7 original valence electrons from fluorine (represented by crosses, $\times$) and 1 electron gained from lithium (represented by a dot, $\cdot$). These 8 electrons are shown surrounding the F symbol within square brackets, with a $-1$ charge outside.
4. Ionic Compound: The ionic compound formed is LiF.

b) Sodium and sulphur

1. Valence Electrons: Sodium (Na) has 1 valence electron. Sulphur (S) has 6 valence electrons.
2. Electron Transfer: Two sodium atoms each transfer 1 valence electron to one sulphur atom.
3. Dot and Cross Diagram Description:
   • Each sodium ion is represented as $\text{Na}^{+}$, having lost its valence electron. Its electron configuration is now stable (like neon).
   • The sulphide ion is represented as $[\text{S}]^{2-}$. It has 8 electrons in its outermost shell: 6 original valence electrons from sulphur (represented by crosses, $\times$) and 2 electrons gained from the two sodium atoms (represented by dots, $\cdot$). These 8 electrons are shown surrounding the S symbol within square brackets, with a $-2$ charge outside.
4. Ionic Compound: The ionic compound formed is $\boxed{\text{Na}_2\text{S}}$.

c) Magnesium and sulphur

1. Valence Electrons: Magnesium (Mg) has 2 valence electrons. Sulphur (S) has 6 valence electrons.
2. Electron Transfer: Magnesium transfers its 2 valence electrons to sulphur.
3. Dot and Cross Diagram Description:
   • The magnesium ion is represented as $\text{Mg}^{2+}$, having lost its two valence electrons. Its electron configuration is now stable (like neon).
   • The sulphide ion is represented as $[\text{S}]^{2-}$. It has 8 electrons in its outermost shell: 6 original valence electrons from sulphur (represented by crosses, $\times$) and 2 electrons gained from magnesium (represented by dots, $\cdot$). These 8 electrons are shown surrounding the S symbol within square brackets, with a $-2$ charge outside.
4. Ionic Compound: The ionic compound formed is $\boxed{\text{MgS}}$.

d) Aluminum and nitrogen

1. Valence Electrons: Aluminum (Al) has 3 valence electrons. Nitrogen (N) has 5 valence electrons.
2. Electron Transfer: Aluminum transfers its 3 valence electrons to nitrogen.
3. Dot and Cross Diagram Description:
   • The aluminum ion is represented as $\text{Al}^{3+}$, having lost its three valence electrons. Its electron configuration is now stable (like neon).
   • The nitride ion is represented as $[\text{N}]^{3-}$. It has 8 electrons in its outermost shell: 5 original valence electrons from nitrogen (represented by crosses, $\times$) and 3 electrons gained from aluminum (represented by dots, $\cdot$). These 8 electrons are shown surrounding the N symbol within square brackets, with a $-3$ charge outside.
4. Ionic Compound: The ionic compound formed is $\boxed{\text{AlN}}$.