Exam practice

Niels Bohr (1913).

nuclear fission.

2 (calcium is 2, 8, 8, 2; it becomes Ca²⁺, 2, 8, 8).

covalent (three single covalent bonds between nitrogen and hydrogen).

CaCl₂ (Ca valency 2, Cl valency 1, crossed over).

84 amu. NaHCO₃ = 23 + 1 + 12 + 3 × 16 = 23 + 1 + 12 + 48 = 84.

It is a boon (a blessing) because a little nuclear fuel makes a huge amount of electricity without smoke or carbon dioxide, and the radiation from radioactive substances is used to treat cancer, sterilize medical instruments and preserve food. It is a bane (a curse) because it can be used to make nuclear weapons that kill on a massive scale (Hiroshima and Nagasaki, 1945), accidents at power plants can release radiation (Chernobyl 1986, Fukushima 2011), the radiation causes sickness and cancer, and the waste stays dangerous for thousands of years. So it helps or harms depending on how it is used.

Helium has 2 electrons in its K shell, its only shell (a duplet), and neon has 8 in its outermost L shell (2, 8, an octet). Their outermost shells are full and stable, so they have no tendency to lose, gain or share electrons. That is why they don’t react, and are found in nature as single atoms.

Methane (CH₄) is formed from carbon and hydrogen, both non-metals. Carbon (2, 4) has 4 valence electrons and each hydrogen 1; instead of transferring electrons, carbon shares one pair with each of four hydrogen atoms. Carbon gets an octet and each hydrogen a duplet. Bonds formed by sharing electrons are covalent, so methane is a covalent compound.

An atom is the smallest particle of an element that can take part in a chemical reaction. It is made of a tiny, positively charged nucleus, containing protons and neutrons, with electrons moving around it in shells. An atom has equal numbers of protons and electrons, so it is electrically neutral. Its diameter is about 10⁻¹⁰ m.

The particles smaller than an atom that atoms are made of are subatomic particles. There are three: protons (charge +1, mass 1 amu, in the nucleus), neutrons (no charge, mass 1 amu, in the nucleus) and electrons (charge −1, mass about 1/1840 amu, moving in shells around the nucleus). The number of protons is the atomic number of the element.

An atomic model is a picture or object that shows how an atom is built. Scientists’ models changed as they learned more:

• Dalton (1808): atoms are tiny solid balls that can’t be divided.
• Thomson (1904), after discovering the electron: a ball of positive charge with electrons stuck in it, like seeds in a fruit.
• Rutherford (1911), from the gold foil experiment: a tiny, dense, positive nucleus at the center, with electrons moving around it, and the atom mostly empty space.
• Bohr (1913): electrons move around the nucleus only in fixed shells (K, L, M, N…), each with a fixed energy; a shell holds at most 2n² electrons.

In class (Activity 14.1), we make models of the Bohr kind: a ball of clay for the nucleus, wire rings for shells and beads for electrons. A model of sodium, for example, has 11 protons and 12 neutrons in the nucleus and 2, 8 and 1 electrons on three rings. Such models help us see the electronic configuration, the valence electrons and how bonds form.

The giving out of energetic radiation by the unstable nuclei of radioactive elements (such as uranium, thorium and radium), as they change towards a more stable state, is called radioactive emission. It is of three types:

• alpha (α) rays: helium nuclei, positively charged, stopped by paper;
• beta (β) rays: fast electrons, negatively charged, stopped by a few mm of aluminium;
• gamma (γ) rays: electromagnetic rays with no charge, very penetrating, stopped only by thick lead or concrete.

Nuclear fission is the splitting of a heavy, unstable nucleus into two lighter nuclei. When a uranium-235 nucleus is hit by a neutron, it splits into barium and krypton nuclei, gives out 3 neutrons, and releases a huge amount of energy from the mass that is lost:
U-235 + n → Ba-141 + Kr-92 + 3n + energy.
The neutrons can cause a chain reaction. It is used in nuclear power plants (and atom bombs).
Nuclear fusion is the joining of light nuclei to form a heavier nucleus. In the Sun and stars, at millions of degrees and enormous pressure, four hydrogen nuclei join to form a helium nucleus, releasing an enormous amount of energy:
$4\,{}^{1}_{1}\text{H} \rightarrow {}^{4}_{2}\text{He} + 2\,{}^{\;0}_{+1}e + \text{energy}$
It is the source of the energy of the Sun and stars.

The duplet state is the stable state of an atom with 2 electrons in its K shell when that is its only shell. Example: helium (2). The octet state is the stable state of an atom with 8 electrons in its outermost shell. Examples: neon (2, 8) and argon (2, 8, 8). Atoms in these states don’t react; other atoms lose, gain or share electrons to reach them.

The force that holds the atoms (or ions) of a compound together is called a chemical bond. It forms when atoms lose, gain or share electrons to get a duplet or an octet. The two main kinds are the electrovalent (ionic) bond, formed by transfer of electrons, as in NaCl, and the covalent bond, formed by sharing of electrons, as in H₂O.

Electrovalent bond: sodium chloride. Sodium (2, 8, 1) loses its 1 valence electron and becomes Na⁺ (2, 8); chlorine (2, 8, 7) gains that electron and becomes Cl⁻ (2, 8, 8). Both ions have an octet. The positive and negative ions attract each other, and this electrostatic attraction is the electrovalent bond: Na⁺ + Cl⁻ → NaCl. (Draw Figure 14.7.)
Covalent bond: hydrogen chloride. Hydrogen (1) and chlorine (2, 8, 7) both need one more electron, so neither can give one away. Each puts one electron into a shared pair. Hydrogen then has a duplet and chlorine an octet, and the shared pair holds them together: H–Cl. (Draw Figure 14.11.)

Ammonia (NH₃): nitrogen (2, 5) needs 3 electrons; each hydrogen (1) needs 1. Nitrogen shares one pair of electrons with each of three hydrogen atoms: three covalent bonds. N gets an octet (with its one lone pair), each H a duplet. (Draw Figure 14.14.)

Sodium chloride (NaCl): sodium (2, 8, 1) gives its one valence electron to chlorine (2, 8, 7): Na − e⁻ → Na⁺ and Cl + e⁻ → Cl⁻. The ions, both with an octet, attract: an ionic bond. (Draw Figure 14.7.)

Magnesium chloride (MgCl₂): magnesium (2, 8, 2) has 2 valence electrons, but a chlorine atom (2, 8, 7) takes only one. So magnesium gives one electron to each of two chlorine atoms: Mg − 2e⁻ → Mg²⁺ and 2Cl + 2e⁻ → 2Cl⁻. One Mg²⁺ (2, 8) and two Cl⁻ (2, 8, 8) attract each other: Mg²⁺ + 2Cl⁻ → MgCl₂. (Draw it like Figure 14.9, with magnesium’s three shells in place of calcium’s four.)

The combining capacity of an atom with other atoms is its valency. It is the number of electrons an atom loses, gains or shares to reach a duplet or octet. For example, sodium (2, 8, 1) loses 1 electron: valency 1; oxygen (2, 6) gains 2: valency 2; carbon (2, 4) shares 4: valency 4.

An ion is an atom or group of atoms that carries an electric charge because it has lost or gained electrons.
A positive ion (cation) forms when an atom loses electrons from its valence shell, leaving more protons than electrons. Metals do this. Example: Na − e⁻ → Na⁺ (11 protons, 10 electrons); Mg − 2e⁻ → Mg²⁺.
A negative ion (anion) forms when an atom gains electrons, so that it has more electrons than protons. Non-metals do this. Example: Cl + e⁻ → Cl⁻ (17 protons, 18 electrons); O + 2e⁻ → O²⁻.

We use the criss-cross method:

1. Write the name of the compound, e.g. aluminium oxide.
2. Write the symbols below it: Al, O.
3. Write the valencies above the symbols: Al 3, O 2.
4. Cross the valencies over and write them at the bottom right of the other symbol: Al₂O₃.
5. Simplify: divide by any common factor, don’t write 1, and put a radical in brackets if it has a number 2 or more.

More examples: calcium chloride, Ca (2) and Cl (1) → CaCl₂; calcium sulphate, Ca (2) and SO₄ (2) → Ca₂(SO₄)₂ → CaSO₄; aluminium nitrate, Al (3) and NO₃ (1) → Al(NO₃)₃.

The sum of the atomic masses of all the atoms in a molecule is its molecular mass. (Na = 23, Cl = 35.5, Ca = 40, C = 12, O = 16, N = 14, H = 1, Mg = 24, S = 32)

• i.NaCl = 23 + 35.5 = 58.5 amu
• ii.CaCO₃ = 40 + 12 + 3 × 16 = 40 + 12 + 48 = 100 amu
• iii.NH₃ = 14 + 3 × 1 = 17 amu
• iv.CO₂ = 12 + 2 × 16 = 44 amu
• v.MgSO₄ = 24 + 32 + 4 × 16 = 24 + 32 + 64 = 120 amu