Stability of Atoms and Ions

Isolated atoms are rarely found in nature: except for noble gases, elements occur forming compounds or metals.

1. a) Explain what it means for an atom to be "stable" and what atoms gain by bonding with one another.
2. b) What happens to the energy of the system when atoms form a bond? Does it increase or decrease?

The octet rule guides the chemistry of most elements.

1. a) State it in your own words.
2. b) Which electrons in the atom does it refer to?
3. c) What configuration do atoms reach when they satisfy it? Give an example of an element that already possesses it naturally.

Neon ($Z = 10$) is used in illuminated signs because it is a completely inert gas: it does not react with anything.

1. a) Write its electron configuration.
2. b) How many electrons does it have in its outer shell?
3. c) Explain, from this configuration, why noble gases do not form compounds under normal conditions.

An element has the electron configuration $1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^4$.

1. a) What is its atomic number and which element is it?
2. b) How many valence electrons does it have?
3. c) How many electrons is it missing to complete its octet?

When sodium combines with chlorine, each sodium atom loses an electron and becomes the $\text{Na}^+$ ion. The diagram represents the loss of the electron.

1. a) Define what a cation is and what an anion is.
2. b) State the sign of the charge for each type of ion.
3. c) Is the $\text{Na}^+$ ion a cation or an anion? Reason your answer.

Magnesium ($Z = 12$) reacts by losing electrons and forming the $\text{Mg}^{2+}$ ion.

1. a) How many electrons does each magnesium atom lose?
2. b) How many electrons does the formed ion have in total?
3. c) Write the process as a half-equation: $\text{Mg} \rightarrow \text{Mg}^{2+} + \ldots$ (complete the number of electrons released).

The chlorine ($Z = 17$) in swimming pool water acts as a disinfectant and, when reacting, each atom gains an electron forming the $\text{Cl}^{-}$ ion.

1. a) How many electrons does the chloride ion have?
2. b) Is it a cation or an anion?
3. c) What is its net electrical charge? Justify your answer by counting protons and electrons.

A particle contains $12$ protons and $10$ electrons.

1. a) Calculate its net charge.
2. b) Is it a neutral atom, a cation or an anion?
3. c) Identify the element and write the symbol of the ion.

Chlorine ($Z = 17$) and argon ($Z = 18$) are neighbors in the periodic table.

1. a) Write the electron configuration of the chlorine atom and that of the $\text{Cl}^{-}$ ion.
2. b) Compare the configuration of the chloride ion with that of the argon atom: what do you observe?
3. c) Using this result, explain why chlorine tends to gain an electron when it reacts.

The aluminum ($Z = 13$) in soft drink cans and the sulphur ($Z = 16$) in matches are very different elements, but both form stable ions.

1. a) Write the electron configuration of each atom.
2. b) Predict the most stable ion that each will form by applying the octet rule.
3. c) Write the configuration of each ion and name the noble gas whose configuration it matches.

An $\text{X}^{2-}$ ion has $18$ electrons.

1. a) How many electrons does the neutral atom from which it originates have?
2. b) What is its atomic number? Identify the element.
3. c) Write the electron configuration of the atom and that of the ion.

To abbreviate configurations, noble gas notation is used: that of potassium ($Z = 19$) is written as $[\text{Ar}]\ 4s^1$.

1. a) Write its full electron configuration, without abbreviating.
2. b) How many valence electrons does potassium have?
3. c) Which ion will it form most easily? Write its configuration and explain why it is exceptionally stable.

Lewis diagrams represent valence electrons as dots around the element's symbol.

1. a) State how many valence electrons sodium ($Z = 11$), magnesium ($Z = 12$), oxygen ($Z = 8$), and chlorine ($Z = 17$) have.
2. b) Draw in your notebook the Lewis diagram for each of the four atoms.
3. c) How many unpaired electrons does each atom have? Relate this to the number of bonds each element usually forms.

You can do it on paper and send a photo with your answer.

Calcium ($Z = 20$), present in bones and dairy products, always forms the $\text{Ca}^{2+}$ ion; it never appears as $\text{Ca}^{+}$ or $\text{Ca}^{3+}$.

1. a) Write the electron configuration of the calcium atom.
2. b) Write the configurations that the ions $\text{Ca}^{+}$, $\text{Ca}^{2+}$, and $\text{Ca}^{3+}$ would have.
3. c) Using the octet rule, justify why only the $\text{Ca}^{2+}$ ion is stable.

Element X has atomic number $Z = 12$ and element Y has atomic number $Z = 17$.

1. a) Write their electron configurations and predict the stable ion that each will form.
2. b) Deduce the formula of the ionic compound formed by X and Y, taking into account that the overall compound must be electrically neutral.
3. c) Write the configuration of each ion in the compound and state which noble gases they are isoelectronic with.

Essay: "Stability drives atoms".

1. a) Using the electron configurations of sodium ($Z = 11$), magnesium ($Z = 12$), oxygen ($Z = 8$), and chlorine ($Z = 17$), explain what ion each element forms and why.
2. b) Deduce the formula of the compounds $\text{NaCl}$, $\text{MgO}$, and $\text{MgCl}_2$ based on the electrical neutrality of the overall compound.
3. c) Conclude by explaining what role noble gas configuration plays throughout the ion formation process.

These species seem very different: $\text{N}^{3-}$, $\text{O}^{2-}$, $\text{F}^{-}$, $\text{Na}^{+}$, $\text{Mg}^{2+}$, and $\text{Al}^{3+}$. Data: $Z(\text{N}) = 7$, $Z(\text{O}) = 8$, $Z(\text{F}) = 9$, $Z(\text{Na}) = 11$, $Z(\text{Mg}) = 12$, $Z(\text{Al}) = 13$.

1. a) Calculate the number of electrons in each species.
2. b) What do all of them have in common?
3. c) Explain what it means for them to be isoelectronic and state which noble gas they are isoelectronic with.

When converting into an ion, an atom changes in size: the radius of the sodium atom is $186\ \text{pm}$ and that of the $\text{Na}^{+}$ ion is $102\ \text{pm}$; that of the chlorine atom is $99\ \text{pm}$ and that of the $\text{Cl}^{-}$ ion is $181\ \text{pm}$. The four sizes are compared to scale in the figure.

1. a) Write the electron configurations of the two atoms and the two ions.
2. b) Explain why the cation is much smaller than the atom from which it originates.
3. c) Explain why the anion is much larger than its atom.

The species $\text{O}^{2-}$, $\text{F}^{-}$, $\text{Na}^{+}$, and $\text{Mg}^{2+}$ all have $10$ electrons, but their radii are $140\ \text{pm}$, $133\ \text{pm}$, $102\ \text{pm}$, and $72\ \text{pm}$, respectively. The figure compares the four radii to scale.

1. a) Explain why, for the same number of electrons, the size of the species depends on the number of protons in the nucleus.
2. b) Order the four species from largest to smallest radius and relate that order to their atomic number.
3. c) Without further data, predict which will have a larger radius between the $\text{N}^{3-}$ ion and the $\text{Al}^{3+}$ ion, and justify your prediction.

An $\text{M}^{3+}$ ion has $10$ electrons.

1. a) Determine the atomic number of the element and identify it.
2. b) Write the electron configuration of the neutral atom and that of the ion.
3. c) Which noble gas is the ion isoelectronic with?
4. d) Reason why this element loses $3$ electrons instead of gaining $5$ to complete the octet.

These are the energies required to remove successive electrons from a magnesium atom ($Z = 12$): $E_1 = 738\ \text{kJ/mol}$, $E_2 = 1451\ \text{kJ/mol}$, and $E_3 = 7733\ \text{kJ/mol}$. The figure represents the three energies.

1. a) Locate between which ionisations the large jump occurs and justify it using the electron configuration of magnesium.
2. b) Explain why magnesium forms the $\text{Mg}^{2+}$ ion and never $\text{Mg}^{3+}$ in its compounds.
3. c) Predict, reasoning from their configurations, between which ionisations the large jump will appear in sodium ($Z = 11$) and in aluminum ($Z = 13$).

Removing an electron from a neon atom costs $2081\ \text{kJ/mol}$, but removing it from the $\text{Na}^{+}$ ion, which has exactly the same electron configuration, costs $4562\ \text{kJ/mol}$. In contrast, it costs a neutral sodium atom only $496\ \text{kJ/mol}$ to lose its $3s$ electron. The figure compares the three ionisation energies.

1. a) Explain why it is harder to ionise $\text{Na}^{+}$ than $\text{Ne}$ if both have the same electrons.
2. b) Using the data, reason why sodium easily forms the $\text{Na}^{+}$ ion but never appears as $\text{Na}^{2+}$ in chemical compounds.
3. c) Connect your answers to the idea of noble gas configuration stability.

The octet rule has known exceptions: in $\text{BeCl}_2$ beryllium is left with only $4$ electrons around it, and in $\text{BF}_3$ boron has $6$.

1. a) Draw in your notebook the Lewis diagrams for both molecules.
2. b) Explain what it means for beryllium and boron to have an "incomplete octet".
3. c) In $\text{PCl}_5$ phosphorus is surrounded by $10$ electrons ("expanded octet"): comment on why the octet rule remains a useful guide even if it is not a strict law.

You can do it on paper and send a photo with your answer.

Hydrogen is a special case: it can lose its electron to become $\text{H}^{+}$, gain one to form the hydride ion $\text{H}^{-}$, or share it, as in the $\text{H}_2$ molecule.

1. a) Write the electron configuration of the three species.
2. b) Which noble gas is the $\text{H}^{-}$ ion isoelectronic with? Explain why for hydrogen the octet rule becomes a "duet rule".
3. c) Sodium hydride, $\text{NaH}$, is used in laboratories as a reagent: using the configurations of sodium ($Z = 11$) and hydrogen, justify that it is formed by the ions $\text{Na}^{+}$ and $\text{H}^{-}$.