Exercises: Electron 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.

Cell phone batteries contain lithium ($Z = 3$).

1. a) Write its electron configuration.
2. b) Explain why the third electron cannot occupy the $1s$ subshell.

Carbon ($Z = 6$) is found in pencil lead, in diamond, and in all molecules of living things.

1. a) Write its electron configuration.
2. b) State how many valence electrons (the electrons in its outermost level) it has.

The illuminated signs of many bars and cinemas use neon ($Z = 10$).

1. a) Write its electron configuration.
2. b) Check that its outermost level is full and explain why neon is such an unreactive gas.

A space probe analyses the atmosphere of a planet and finds atoms whose electron configuration is $1s^2\ 2s^2\ 2p^4$.

1. a) How many electrons does each of those atoms have?
2. b) Knowing that in a neutral atom the number of electrons equals the atomic number $Z$, which element is it?

The orange street lights on many motorways are sodium-vapour lamps ($Z = 11$).

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

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.

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?

Saline solution administered in hospitals contains $\text{Na}^+$ ions.

1. a) Write the electron configuration of the sodium atom ($Z = 11$) and that of the $\text{Na}^+$ ion.
2. b) Which noble gas does the ion's configuration match?

In table salt, chlorine ($Z = 17$) is present as the chloride ion, $\text{Cl}^-$.

1. a) Write the electron configuration of the $\text{Cl}^-$ ion.
2. b) Check that it matches the configuration of a noble gas and state which one it is.

The Moeller diagram orders the subshells along diagonals, from lower to higher energy, as shown in the figure. The first sections are: $1s$; then $2s$; then the diagonal $2p$-$3s$; then the diagonal $3p$-$4s$; and then the diagonal $3d$-$4p$-$5s$.

Arrange the following subshells in order of increasing energy: $3s$, $4s$, $1s$, $3p$, $2p$, $2s$ and $3d$.

A student writes in their exercise book that the configuration of sodium ($Z = 11$) is $1s^2\ 2s^2\ 2p^6\ 3s^3$. Find the error, explain why this configuration is impossible and write the correct configuration.

In the electronics of cell phones and computers, a highly heat-resistant ceramic is used: aluminum 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).

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 $2$.

Magnesium chloride, $\text{MgCl}_2$ ($Z$: Mg = 12, Cl = 17), is used to melt ice on roads in winter.

1. a) Write the electron configurations of the two atoms and of their ions $\text{Mg}^{2+}$ and $\text{Cl}^-$.
2. b) Based on the charges of the ions, explain why the formula of the compound is $\text{MgCl}_2$ and not $\text{MgCl}$.

In laboratory videos it can be observed that sodium ($Z = 11$) and potassium ($Z = 19$) react with water in a very similar way, giving off a flame.

1. a) Write the electron configurations of the two elements.
2. b) Point out what their valence shells have in common.
3. c) Use this similarity to explain why the two elements behave chemically in such a similar way.

For potassium ($Z = 19$), a student writes the configuration $1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^6\ 3d^1$ and reasons that "after the $3p$ subshell comes the $3d$ subshell".

1. a) Write the correct configuration using the order from the Moeller diagram.
2. b) Explain what error the student made.
3. c) According to the correct configuration, how many valence electrons does potassium have?

An $\text{X}^{2+}$ ion has the electron configuration $1s^2\ 2s^2\ 2p^6$.

1. a) Calculate how many electrons the ion has and deduce the atomic number of element X.
2. b) A $\text{Y}^{-}$ ion has exactly the same configuration: deduce its atomic number as well.
3. c) Using the periodic table, name elements X and Y.

Isotonic drinks for athletes contain, among others, $\text{K}^+$, $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ ions ($Z$: K = 19, Ca = 20, Mg = 12).

1. a) Write the electron configuration of each of the three ions.
2. b) What do the three configurations have in common?
3. c) Based on this common feature, explain why it is so easy for these three elements to form positive ions.