C. Chemistry: The Nature of Matter · Atoms, Elements and CompoundsMolecular Compounds and Covalent Bonds
C2.7 C2.6 C2.4 A1.2About 75 minutes
Learning goals- I can explain how nonmetal atoms share electrons to form covalent bonds and molecules.
- I can draw Lewis structures for simple molecules such as H₂, Cl₂, H₂O, NH₃, CH₄ and CO₂.
- I can explain what a molecular formula tells me, and how it differs from an ionic formula.
- I can name binary molecular compounds using prefixes, and compare the properties of ionic and molecular compounds.
Words to know
covalent bond
A bond formed when two atoms share a pair of electrons. Covalent bonds usually form between nonmetal atoms.
molecule
A group of atoms joined by covalent bonds that acts as one particle.
molecular compound
A compound made of molecules, usually formed from two or more nonmetals.
molecular formula
A formula that shows the actual number of each kind of atom in one molecule, such as H₂O₂.
lone pair
A pair of valence electrons that is not shared with another atom.
Lewis structure
A diagram of a molecule showing shared pairs (as lines or dot pairs) and lone pairs.
diatomic element
An element whose atoms are found in pairs: H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂.
Sharing instead of giving
In the last lesson, a metal gave electrons to a nonmetal. But what happens when two nonmetals meet? Both want to gain electrons. Neither wants to give any away. Their solution: share.
Most of the substances in living things, and many around your home, are built this way: water, sugar, the oxygen you breathe, the methane in natural gas, the plastic in your water bottle and the proteins in your muscles.
Covalent bonds
A covalent bond is a pair of electrons shared between two atoms. Each atom counts the shared electrons as part of its own valence shell, so both can reach a full shell.
The simplest example is hydrogen gas, H₂. Each hydrogen atom has 1 electron and needs 2 for a full first shell. When two hydrogen atoms share their electrons, each "has" 2. The pair of shared electrons is the bond: H–H.
In water, oxygen has 6 valence electrons and needs 2 more. It shares one pair with each of two hydrogen atoms. Now oxygen is surrounded by 8 electrons and each hydrogen by 2.
How many bonds can an atom make?
An atom usually forms as many covalent bonds as the number of electrons it needs to fill its valence shell. This is its bonding capacity.
- Hydrogen and the halogens (F, Cl, Br, I): need 1 → form 1 bond.
- Oxygen and sulfur (group 16): need 2 → form 2 bonds.
- Nitrogen and phosphorus (group 15): need 3 → form 3 bonds.
- Carbon and silicon (group 14): need 4 → form 4 bonds.
Sometimes two atoms share two pairs (a double bond, drawn =) or three pairs (a triple bond, ≡). Each oxygen in CO₂ makes a double bond with carbon, and the two nitrogen atoms in N₂ are held by a triple bond.
Diatomic elements
Seven elements exist as diatomic molecules (pairs of atoms sharing electrons) when they are pure: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂. A memory trick: start at element 7 (nitrogen) and trace the shape of a 7 on the periodic table through O, F, then down through Cl, Br and I, and don't forget hydrogen. These are elements, not compounds, because each molecule contains only one kind of atom.
What a molecular formula tells you
Here is the big difference between the two kinds of compounds (C2.7):
- An ionic formula, like CaCl₂, shows the simplest ratio of ions in a huge lattice. It is always reduced.
- A molecular formula shows the actual number of each atom in one molecule. It is not reduced. Hydrogen peroxide is H₂O₂ (two H and two O in each molecule), not HO. Glucose is C₆H₁₂O₆.
The structure decides the formula. If you know the structure (the atoms and bonds), you can write the formula; and a molecular formula tells you exactly which atoms make up one molecule.
Naming binary molecular compounds
Two nonmetals can combine in more than one way (CO and CO₂ are very different!), so the name must say how many of each atom there are. Use prefixes:
mono = 1, di = 2, tri = 3, tetra = 4, penta = 5, hexa = 6
- Name the first element, with a prefix if there is more than one atom (never use mono- on the first element).
- Name the second element with a prefix and the ending -ide.
- Drop the final a or o of the prefix before "oxide": monoxide (not monooxide), tetroxide.
Examples: CO carbon monoxide; CO₂ carbon dioxide; N₂O dinitrogen monoxide; SO₃ sulfur trioxide; CCl₄ carbon tetrachloride; PCl₅ phosphorus pentachloride; SF₆ sulfur hexafluoride. A few have common names that everyone uses: water (H₂O), ammonia (NH₃), methane (CH₄).
| Property | Ionic compounds (e.g. NaCl) | Molecular compounds (e.g. sugar, water) |
|---|
| Elements involved | Metal + nonmetal | Nonmetals only |
| Electrons | Transferred | Shared |
| State at room temperature | Solid (crystals) | Solid, liquid or gas |
| Melting point | High (NaCl: 801 °C) | Low to moderate (water: 0 °C; table sugar breaks down at about 186 °C) |
| Solubility in water | Many dissolve | Some dissolve (sugar); many don't (oil, wax) |
| Conducts electricity when dissolved? | Yes: free-moving ions | Usually no: most molecules have no charge |
Worked exampleExample 1: Predict a formula from bonding capacity
Predict the formula of the simplest compound of carbon and chlorine, draw its structure in words, and name it.
- Carbon forms 4 bonds; chlorine forms 1 bond.
- To use all 4 of carbon's bonds, attach 4 chlorine atoms, each by a single bond.
- Formula: CCl₄. Structure: carbon in the centre, with a Cl on each of four sides (each Cl also has 3 lone pairs).
- Name: one carbon (no mono- for the first element) + four chlorines → carbon tetrachloride.
CCl₄, carbon tetrachloride.
Worked exampleExample 2: Ionic or molecular?
Classify each compound as ionic or molecular, and name it: (a) KCl, (b) NO₂, (c) MgF₂, (d) P₂O₅.
- (a) K is a metal, Cl a nonmetal → ionic: potassium chloride (no prefixes).
- (b) N and O are both nonmetals → molecular: nitrogen dioxide.
- (c) Mg is a metal → ionic: magnesium fluoride.
- (d) Both nonmetals → molecular: diphosphorus pentoxide (the "a" of penta is dropped before oxide).
Ionic: KCl, MgF₂. Molecular: NO₂, P₂O₅. Only molecular compounds use prefixes.
Hands-on activityBuild molecules, then compare sugar and salt
Part A: models. You need: gumdrops or mini marshmallows in 3 or 4 colours (or modelling clay), toothpicks. Safety: toothpicks are sharp; keep them away from young children.
Let one colour be H, one O, one C, one N. Use the bonding capacities: each H has 1 toothpick, O has 2, N has 3, C has 4 (use two toothpicks side by side for a double bond). Build H₂, H₂O, NH₃, CH₄ and CO₂. Check: does every atom have the right number of bonds? Write the formula and name of each.
Part B: sugar versus salt (grown-up does the heating). You need: a grown-up, two small pieces of aluminium foil folded into cups, 1/4 teaspoon each of sugar and salt, a frying pan, the stove, eye protection.
- Put the foil cups side by side in the dry pan, with sugar in one and salt in the other.
- The grown-up heats the pan on medium heat for about 3 to 5 minutes, with the kitchen fan on. Do not touch the pan or foil: they get very hot.
- Watch and record what happens to each. Turn off the heat and let everything cool completely before cleaning up.
- Which has the lower melting point? Which is ionic and which is molecular? Explain using the table above.
What to look for: the sugar melts and then turns brown (it begins to break down, a chemical change, as it caramelizes), while the salt does not change at all: a stove can't get the pan near 801 °C. This matches the rule that molecular compounds usually melt at much lower temperatures than ionic compounds.
Just for funWhy are two chlorine atoms such great roommates?
They share everything, and they always do it in pairs.
Your turn
A pair of electrons shared between two atoms (usually nonmetals).
2How many covalent bonds does nitrogen usually form? Why?
3. Nitrogen has 5 valence electrons and needs 3 more to have 8.
3Describe the Lewis structure of NH₃.
N in the centre with 3 single bonds to 3 H atoms and one lone pair on N.
4Name these: CO, SO₂, N₂O₄.
Carbon monoxide, sulfur dioxide, dinitrogen tetroxide.
5Write formulas: carbon disulfide, phosphorus trichloride.
6Why is hydrogen peroxide written H₂O₂ and not HO?
It is a molecular compound: the formula shows the actual atoms in one molecule (2 H and 2 O), so it is not reduced.
7List the seven diatomic elements.
H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.
More practice: warm-up
1What does the prefix tri- mean?
2Do covalent bonds usually form between metals and nonmetals, or between nonmetals?
3What is the common name for H₂O?
4How many bonds does carbon usually form?
5Is CO₂ ionic or molecular?
Molecular (two nonmetals).
More practice: on your own
1Predict the formula of the compound formed by hydrogen and sulfur, and describe its structure.
S forms 2 bonds, H forms 1 → H₂S. S in the middle with two single bonds to H and two lone pairs (like water).
Dichlorine heptoxide (hepta = 7; the a is dropped before oxide). (Hepta- was not in our list, but it follows the same pattern.)
3Why do molecular compounds such as sugar not conduct electricity when dissolved in water?
The sugar molecules are neutral: they have no charge, so there are no charged particles to carry a current.
4A substance melts at 80 °C and its water solution does not conduct electricity. Is it more likely ionic or molecular? Explain.
Molecular. It has a low melting point, and its solution does not conduct, so it does not contain ions.
5Explain why CO₂ has double bonds instead of single bonds.
Carbon needs 4 bonds and each oxygen needs 2. With only two oxygens, single bonds would give carbon only 2 bonds. Two double bonds give carbon 4 bonds and each oxygen 2.
Challenge
1Glucose is C₆H₁₂O₆. A student reduces it to CH₂O. Explain why that is wrong, and say what CH₂O would actually describe.
Glucose is molecular, so its formula must show the actual atoms in one molecule: 6 C, 12 H and 6 O. CH₂O shows only the ratio of atoms. It is the formula of a different molecule (formaldehyde), so reducing it changes the substance.
2Predict the formula of the simplest compound of silicon and fluorine, and its name. Then explain whether it should be ionic or molecular.
Si (group 14) forms 4 bonds; F forms 1 → SiF₄, silicon tetrafluoride. Both are nonmetals or metalloids that share electrons, so it is molecular.
3Explain why the name "carbon oxide" would be useless, but "sodium oxide" is fine without prefixes.
Carbon and oxygen form more than one molecular compound (CO and CO₂, with very different properties; CO is toxic), so the prefixes are needed to tell them apart. In sodium oxide, sodium is always Na⁺ and oxide is always O²⁻, so the charges fix the formula (Na₂O) and the name doesn't need to say how many of each.
Quiz
1. A covalent bond is formed when atoms...
2. What is the correct name for N₂O₃?
3. Which formula is written correctly for a molecular compound?
4. How many covalent bonds does oxygen usually form?
5. Which is a diatomic element?
6. Which property is typical of molecular compounds compared with ionic compounds?
7. Which compound is molecular?
Big idea: C2.7 asks students to describe the relationship between the structure of simple compounds and their chemical formulas. The key contrast: an ionic formula is a ratio of ions in a lattice; a molecular formula is the actual count of atoms in one molecule. C2.6 is addressed by comparing properties of ionic and molecular compounds.
Common difficulties:
- Using prefixes for ionic compounds ("dichlorine magnesium") or forgetting them for molecular ones. Check: metal present → no prefixes.
- Reducing molecular formulas. Never reduce H₂O₂ or C₆H₁₂O₆.
- Giving atoms the wrong number of bonds. Use the H-1, O-2, N-3, C-4 rule.
How to help: the stove part of the activity must be done by an adult, with the fan on; the sugar can smoke if overheated, so stop once it browns. Gumdrop models are a fun way to practise: ask your teen to build a molecule and then tell you its formula and name.