Write the electron configuration for Oxygen and balance the chemical equation for the formation of water from hydrogen and oxygen.

Oxygen (O) has an atomic number (Z) of 8, meaning it has 8 electrons.

The electron configuration for a neutral Oxygen atom is $1s^2 2s^2 2p^4$.
The s, p electron configuration for the Oxygen atom is $\boxed{1s^2 2s^2 2p^4}$.

18.b)

Reactants: Hydrogen gas ($\text{H}_2(g)$) and Oxygen gas ($\text{O}_2(g)$).
Product: Water vapor ($\text{H}_2\text{O}(g)$).
Unbalanced equation: $\text{H}_2(g) + \text{O}_2(g) \rightarrow \text{H}_2\text{O}(g)$.

To balance the oxygen atoms, place a coefficient of 2 in front of $\text{H}_2\text{O}$:
$\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(g)$.
Now, there are 4 hydrogen atoms on the right. Place a coefficient of 2 in front of $\text{H}_2$ on the left:
$2\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(g)$.
The balanced chemical equation is:

19.a)

Magnesium (Mg) has an atomic number (Z) of 12, meaning it has 12 electrons.

The electron configuration for a neutral Magnesium atom is $1s^2 2s^2 2p^6 3s^2$.

Each box represents an orbital, and arrows represent electrons (up for spin up, down for spin down).

The box orbital diagram for magnesium is:

19.b)

Magnesium ($\text{Mg}$) is in Group 2, so it forms a $\text{Mg}^{2+}$ ion.
Hydrochloric Acid ($\text{HCl}$) provides the chloride ion, $\text{Cl}^{-}$.

To form a neutral compound, one $\text{Mg}^{2+}$ ion (charge +2) combines with two $\text{Cl}^{-}$ ions (total charge $2 \times -1 = -2$).

The formula for magnesium chloride is $\boxed{\text{MgCl}_2}$.