Electronic configuration

The Sun is made mostly of hydrogen ($Z = 1$) and floating party balloons are filled with helium ($Z = 2$). Write the electron configuration of each element and indicate in which subshell its electrons are located.

In Bohr's model, electrons followed exact orbits around the nucleus, like planets around the Sun. Today, the quantum model refers to orbitals. The figure compares the two models.

Explain in your own words what an orbital is and how it differs from an orbit in Bohr's model.

Each orbital can hold a maximum of $2$ electrons. An $s$ subshell has a single orbital and a $p$ subshell has $3$ orbitals, as shown in the figure.

Calculate the maximum number of electrons that fit in an $s$ subshell and in a $p$ subshell.

Chlorine ($Z = 17$) is used to disinfect swimming pool water.

1. a) Write its electron configuration.
2. b) State how many valence electrons it has and how many it needs to complete its outermost level.

The space between the two glass panes of many double-glazed windows is filled with argon ($Z = 18$), a gas that reacts with practically nothing.

1. a) Write its electron configuration.
2. b) Explain, by looking at its outermost level, why argon is so unreactive.

The potassium ($Z = 19$) in bananas has one more electron than argon. According to the Moeller diagram, when the $3p$ subshell is full, $4s$ starts to be occupied, which has lower energy than $3d$, as shown in the figure.

1. a) Write the electron configuration of potassium.
2. b) State in which subshell its valence electron is located.

In the electronics of mobile phones and computers, a highly heat-resistant ceramic is used: aluminium nitride, formed by $\text{Al}^{3+}$ ions and $\text{N}^{3-}$ ions ($Z$: Al = 13, N = 7), as shown in the figure.

1. a) Write the electron configuration of each of the two ions.
2. b) Check that both have the same number of electrons (they are said to be isoelectronic).
3. c) Explain why, having the same number of electrons, one has a charge of $3+$ and the other a charge of $3-$ (think about the protons in each nucleus).

Calcium ($Z = 20$) is the mineral in bones and coral reefs.

1. a) Write its electron configuration.
2. b) How many valence electrons does it have?

The electron configuration of an element ends in $3p^5$.

1. a) Write its complete configuration and deduce its atomic number.
2. b) How many electrons is it short of completing level $3$ (including its $3d$ orbitals)?
3. c) Predict the most likely ion that this element will form and write the configuration of that ion.

Some pharmacy antacids contain magnesium oxide, $\text{MgO}$, formed by $\text{Mg}^{2+}$ ions and $\text{O}^{2-}$ ions ($Z$: Mg = 12, O = 8).

1. a) Write the electron configuration of each ion.
2. b) Check that the two ions have the same number of electrons.
3. c) From the configurations of the neutral atoms, explain why it is advantageous for magnesium to lose $2$ electrons and for oxygen to gain $2$.