Ionic, covalent and metallic bonding

Common salt, $\text{NaCl}$, is formed of ions arranged in a crystal.

1. a) Explain what happens to the valence electrons of sodium and chlorine when the compound is formed.
2. b) What particles form the salt and what force holds them together?
3. c) Draw in your notebook a small section of the crystal lattice, alternating $\text{Na}^{+}$ and $\text{Cl}^{-}$ ions.

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

No one has ever isolated "a molecule of salt": $\text{NaCl}$ forms a cubic crystal lattice in which each ion is surrounded by six oppositely charged ions.

1. a) Explain why, for ionic compounds, we speak of a "formula unit" and not a molecule.
2. b) What, then, does the formula $\text{NaCl}$ represent?
3. c) Draw in your notebook a layer of the lattice, alternating $\text{Na}^{+}$ and $\text{Cl}^{-}$ ions.

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

A water molecule, $\text{H}_2\text{O}$, contains one oxygen atom bonded to two hydrogen atoms.

1. a) What type of bond joins the oxygen to the hydrogens?
2. b) How many bonds are there in the molecule and how many electrons does oxygen share in total?
3. c) How many electrons does each hydrogen atom contribute to its bond?

Carbon dioxide, $\text{CO}_2$, is exhaled when we breathe and absorbed by plants. Data: $Z(\text{C}) = 6$, $Z(\text{O}) = 8$.

1. a) Calculate the total number of valence electrons in the molecule.
2. b) Knowing that carbon forms a double bond with each oxygen, draw in your notebook the Lewis diagram of the molecule.
3. c) How many electron pairs does carbon share? Check that it obeys the octet rule.

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

The nitrogen in crisp bags protects the food because it is a very unreactive gas. The molecules $\text{H}_2$, $\text{O}_2$, and $\text{N}_2$ have, respectively, a single bond, a double bond, and a triple bond.

1. a) Draw in your notebook the Lewis diagram for the three molecules.
2. b) How many electrons are shared in each?
3. c) Reason why the triple bond of $\text{N}_2$ makes it such an inert gas that it is used as a protective atmosphere.

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

An iron nail is a piece of metal made up solely of $\text{Fe}$ atoms.

1. a) Explain what the "sea of electrons" in metallic bonding consists of.
2. b) What does it mean for electrons to be delocalised?
3. c) If the shared electrons were to disappear, how would the iron cations interact with each other? Reason why the sea of electrons is said to be the "glue" of the metal.

Mercury is the only liquid metal at room temperature and, like copper or aluminium, it conducts electric current.

1. a) Describe the model of metallic bonding: what particles are present and how they are arranged.
2. b) Explain, using this model, why a metal conducts electricity in both the solid and liquid states.
3. c) Bronze (copper and tin) and stainless steel (iron, chromium, and nickel) also conduct: explain why mixing several metals does not "break" the metallic bond.

Electronegativity measures the strength with which an atom attracts the electrons of a bond towards itself; metals have low electronegativity and non-metals have high electronegativity.

1. a) Complete the rule of thumb: metal + non-metal → ... bond; non-metal + non-metal → ... bond; metal + metal → ... bond.
2. b) Justify each case thinking about which pulls harder on the electrons.

Given these electronegativity values: $\text{Na} = 0,9$, $\text{Cl} = 3,0$, $\text{H} = 2,1$, $\text{O} = 3,5$, $\text{C} = 2,5$, and $\text{Mg} = 1,2$; and the rule of thumb that an electronegativity difference $\Delta\text{EN} > 1,7$ corresponds to a predominantly ionic bond. The electronegativity values are represented on a scale in the figure.

1. a) Calculate $\Delta\text{EN}$ for the bonds in $\text{NaCl}$, $\text{H}_2\text{O}$ (bond $\text{O}-\text{H}$), $\text{CO}_2$ (bond $\text{C}-\text{O}$), and $\text{MgO}$.
2. b) Classify each bond as predominantly ionic or covalent.
3. c) Explain why the $\text{O}-\text{H}$ bond, despite being covalent, is said to be "polar".

Some substances form crystal lattices and others form molecules.

1. a) Explain the difference between both structures.
2. b) Give an example of a substance that forms a crystal lattice and another of a molecular substance.
3. c) In which of the two cases does it make sense to talk about an isolated "molecule"? Reason your answer.

Diamond is pure carbon. In it, each carbon atom is bonded to four other carbon atoms, forming a continuous three-dimensional structure.

1. a) What type of bond joins the carbon atoms to each other?
2. b) Is diamond a molecular substance or a crystal lattice?
3. c) How many bonds does each carbon atom form?

The negative electrode in mobile phone lithium batteries is made of graphite. In graphite, each carbon bonds to only three other carbon atoms, forming hexagonal layers, and its fourth valence electron remains delocalised throughout the entire layer.

1. a) Draw in your notebook the structure of a layer of graphite.
2. b) Explain why graphite conducts electricity even though the bond between its atoms is covalent.
3. c) Point out the analogy between the delocalised electrons of graphite and the sea of electrons in a metal.

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