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.

Magnesium oxide, $\text{MgO}$, is the active ingredient in some stomach antacids. Data: $Z(\text{Mg}) = 12$, $Z(\text{O}) = 8$.

1. a) Write the electron configurations of the magnesium and oxygen atoms and those of the ions they form.
2. b) Explain, from the charges of these ions, why the formula of the compound is $\text{MgO}$ and not $\text{Mg}_2\text{O}$ or $\text{MgO}_2$.
3. c) Which noble gas is each ion isoelectronic with?

Write the formula of the ionic compound formed by the following pairs of ions, knowing that the overall compound must be electrically neutral:

1. a) $\text{Ca}^{2+}$ and $\text{Cl}^{-}$, de-icing salt for roads.
2. b) $\text{Al}^{3+}$ and $\text{O}^{2-}$, the oxide layer that protects aluminum.
3. c) $\text{Na}^{+}$ and $\text{S}^{2-}$.

In each case, indicate how many ions of each type are needed and check that the total charge is zero.

When a magnesium ribbon burns in air, not only is $\text{MgO}$ formed, but magnesium nitride, $\text{Mg}_3\text{N}_2$, also appears. Data: $Z(\text{Mg}) = 12$, $Z(\text{N}) = 7$.

1. a) Write the electron configurations of the two atoms and of the $\text{Mg}^{2+}$ and $\text{N}^{3-}$ ions.
2. b) Justify the formula $\text{Mg}_3\text{N}_2$ by checking that the total charge of the compound is zero.
3. c) Which noble gas is each ion isoelectronic with? Comment on what is special about this situation.

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.

Ammonia, $\text{NH}_3$, gives its characteristic smell to many cleaners. Data: $Z(\text{N}) = 7$, $Z(\text{H}) = 1$.

1. a) Calculate the total number of valence electrons in the molecule.
2. b) Draw its Lewis structure in your notebook.
3. c) How many shared electron pairs and how many lone pairs does nitrogen have?

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

The nitrogen in chip 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 structure for the three molecules.
2. b) How many electrons are shared in each?
3. c) Explain 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.

In water, two types of forces must be distinguished: the $\text{O}-\text{H}$ covalent bonds within each molecule, which are very strong, and the forces between molecules (hydrogen bonds), which are much weaker. Observe the diagram:

1. a) Explain the difference between intramolecular and intermolecular forces.
2. b) To boil water, which forces must be overcome? And to decompose it into $\text{H}_2$ and $\text{O}_2$?
3. c) Use the above to explain why water boils at only $100\ \celsius$, but chemically decomposing it requires extremely high temperatures or electrolysis.

The $\text{CO}_2$ molecule has $16$ valence electrons and two double bonds.

1. a) Check, by counting electrons, that with this structure carbon and both oxygens satisfy the octet rule.
2. b) How many bonding electron pairs and how many non-bonding pairs are in the molecule?
3. c) Apply the same count to the $\text{N}_2$ molecule ($10$ valence electrons, triple bond) and draw both Lewis diagrams in your notebook.

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 delocalized?
3. c) If the shared electrons were to disappear, how would the iron cations interact with each other? Explain 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 aluminum, 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.

Classify the following substances according to the predominant type of bond in them, justifying each answer based on the combining elements:

1. a) $\text{MgO}$, the magnesium oxide in antacids.
2. b) $\text{CO}_2$, the gas in soda bubbles.
3. c) $\text{Cu}$, the metal in cables.
4. d) $\text{NH}_3$, the ammonia in cleaning products.
5. e) $\text{KBr}$, a salt used in photography.
6. f) $\text{N}_2$, the main gas in air.

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

1. a) Complete the rule of thumb: metal + nonmetal → ... bond; nonmetal + nonmetal → ... bond; metal + metal → ... bond.
2. b) Justify each case by thinking about which atom 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 the two 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"? Explain your reasoning.

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?

Classify the bond of each substance and indicate whether it forms molecules or a crystal lattice, justifying your answer:

1. a) The bronze of bells, an alloy of copper and tin.
2. b) Beach sand, $\text{SiO}_2$.
3. c) The sodium fluoride in toothpastes, $\text{NaF}$.
4. d) The nitrogen in air, $\text{N}_2$.
5. e) The calcium chloride in de-icing salt, $\text{CaCl}_2$.

A laboratory analyzes two unknown substances. Substance A melts at $770\ \celsius$, does not conduct electric current in the solid state, but does when molten or dissolved in water, and is very soluble. Substance B boils at $-161\ \celsius$, does not conduct in any state, and is insoluble in water.

1. a) Deduce, justifying your answer, which type of bond predominates in each substance.
2. b) Propose a known substance that could correspond to each one.
3. c) Explain the electrical conductivity data based on the structure of each substance.

Diamond and common salt are both crystal lattices with very high melting points and yet they behave very differently.

1. a) Explain what particles occupy the lattice nodes in each case and what forces hold them together.
2. b) Molten salt conducts electricity, but solid diamond does not conduct it: explain the difference by focusing on the electrons in each structure.
3. c) Reason why both substances melt at such high temperatures despite their bonds being of a different nature.

The negative electrode in cell 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 delocalized 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 delocalized electrons of graphite and the sea of electrons in a metal.

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

The nitrogen in chip 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 structure for the three molecules.
2. b) How many electrons are shared in each?
3. c) Explain 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.

Essay: "Three ways of bonding".

1. a) Compare ionic, covalent, and metallic bonding by answering three questions for each: what happens to the valence electrons?, what structure is formed, molecules or a lattice?, and which example best illustrates it, $\text{NaCl}$, $\text{CO}_2$, or $\text{Fe}$?
2. b) Finish by explaining how to predict the type of bond from the position of the elements in the periodic table.