Exam practice

An atom has equal numbers of protons and electrons. Each proton carries one positive charge and each electron one negative charge of the same size, so they cancel exactly. The neutrons have no charge. So the atom as a whole has no charge.

Chlorine (atomic number 17) has the electronic configuration 2, 8, 7: 7 valence electrons. It needs only 1 more electron to complete its octet, so it gains 1 electron (or shares 1). The number of electrons it gains to become stable is 1, so its valency is 1.

An atom is the smallest particle of an element that can take part in a chemical reaction. It is made of a nucleus of protons and neutrons, with electrons moving around it in shells. It can’t be seen with the naked eye.

• i.valencies Mg 2, O 2 → Mg₂O₂ → divide by 2 → MgO
• ii.valencies Na 1, Cl 1 → Na₁Cl₁ → NaCl
• iii.valencies K 1, O 2 → K₂O₁ → K₂O
• iv.valencies Al 3, Cl 1 → Al₁Cl₃ → AlCl₃
• v.valencies N 3, H 1 → N₁H₃ → NH₃

A shell (or orbit) is the fixed path in which electrons move around the nucleus of an atom. Each shell has a fixed energy, so shells are also called energy levels. They are named K, L, M, N from the nucleus outwards.

The 2n² rule of Bohr and Bury gives the largest number of electrons a shell can hold, where n is the number of the shell. It is used to arrange the electrons of an atom in its shells, that is, to write its electronic configuration. K holds 2, L 8, M 18 and N 32.

The valence shell is the outermost shell of an atom. An electron in it is called a valence electron.

Valency is the number of electrons an atom of an element loses, gains or shares to become stable (to get 2 or 8 electrons in its outer shell). It depends on the number of valence electrons, which depends on the element’s electronic configuration (and so on its atomic number).

It means the atom already has a full outer shell (2 electrons for helium, 8 for neon and argon), so it doesn’t need to lose, gain or share any electron. Such elements, the inert gases, hardly ever take part in chemical reactions.

Its valency is 1. It has 1 electron in its outermost shell. Losing that 1 electron leaves 2, 8, 8, with a full outer shell of 8, which is much easier than gaining 7. The element is potassium (19 electrons).

A molecular formula is the short way of showing one molecule of an element or compound with symbols and numbers, such as H₂O or CO₂. To write it, you need to know (1) the elements (their symbols) that make up the compound, and (2) the valency of each element.

Potassium has 19 protons and 20 neutrons: atomic weight = 19 + 20 = 39 amu.
Calcium chloride is CaCl₂ (Ca = 40, Cl = 35.5):
Molecular weight = 40 + 2 × 35.5 = 40 + 71 = 111 amu.
(With Cl = 35, it is 40 + 70 = 110 amu.)

Atomic number = protons = electrons. Atomic mass = protons + neutrons, so neutrons = atomic mass − atomic number.

Chlorine’s 35 is protons + neutrons for its common kind of atom; the average over all chlorine atoms is 35.5.

No, not as a neutral atom. An atom must have as many electrons as protons to be neutral. If all its electrons are removed, what is left is just a positively charged nucleus, an ion, not an atom; it behaves completely differently and quickly grabs electrons from anything nearby. (A hydrogen atom without its one electron is a bare proton, H⁺.) Also, it is the electrons that let an atom join with other atoms in chemical reactions.

Sodium’s atomic number is 11, it has 12 neutrons, and its electronic configuration is 2, 8, 1. Count everything in her drawing.

1. She drew 6 protons. Sodium has 11 protons (its atomic number is 11); 6 protons would make it carbon.
2. She drew 8 neutrons. Sodium has 12 neutrons (11 + 12 = 23 amu).
3. She drew 10 electrons, as 2, 7, 1. Sodium has 11 electrons, arranged 2, 8, 1: the L shell must be filled with 8 before any electron goes into the M shell.

The atomic number is 19 (the number of protons), so the element is potassium (K).
Its 19 electrons are arranged 2, 8, 8, 1: draw a nucleus marked 19p⁺ and 20n⁰, with 2 electrons in the K shell, 8 in L, 8 in M and 1 in N, as in Figure 9.10.
To find the atomic weight you need the number of protons and the number of neutrons, because nearly all of an atom’s mass is in those two; the electrons are so light that they can be left out. Here, 19 + 20 = 39 amu.

There are 118 elements, too many to study and remember one by one. Classifying them puts elements with similar properties together, so by learning one element in a group we can predict the properties of the others. This makes their study easier, faster and easier to remember.

Sodium (2, 8, 1) has 1 valence electron and becomes stable by losing 1 electron. Chlorine (2, 8, 7) has 7 valence electrons and becomes stable by gaining 1 electron. They have different numbers of valence electrons, so they are in different groups (1 and 17), but each loses or gains just one electron, so both have valency 1.

The periodic table places elements with similar properties in the same group, and shows how properties change along each period. So instead of learning 118 elements separately, we can learn the pattern: from an element’s place we can tell its number of shells, its valence electrons, its valency, and whether it is a metal or non-metal.

In a chemical reaction the atoms of the reactants break their old links and join in new ways. The products are new substances, made of the same atoms arranged differently, so they have new properties. Sodium is a soft, dangerous metal and chlorine a poisonous gas, yet their product, sodium chloride, is the salt we eat.

Metals have 1–3 valence electrons and lose them, while non-metals have 5–7 and gain electrons. Their properties are opposite. The periodic table puts elements in order of atomic number, so the number of valence electrons rises across each period: elements that lose electrons (metals) end up on the left and in the middle, and elements that gain electrons (non-metals) on the right.

The periodic table is a table of all the elements arranged in rows (periods) and columns (groups) in order of increasing atomic number, so that elements with similar properties fall in the same column.

“The physical and chemical properties of elements are a periodic function of their atomic numbers.” That is, when elements are arranged by increasing atomic number, similar properties repeat at regular intervals.

Metals are the elements that lose electrons; they have 1–3 valence electrons. They are on the left and in the middle of the periodic table: group 1 (the alkali metals, apart from hydrogen), group 2 (the alkaline earth metals), groups 3–12 (the transition metals) and the lower parts of groups 13 to 16 (such as aluminium, tin and lead). Most elements are metals.

The metallic character increases down a group. Each element has one more shell than the one above, so its valence electrons are farther from the nucleus and held less tightly, and they are lost more easily. Cesium, near the bottom of group 1, is far more reactive than lithium at the top.

The metallic character decreases from left to right. Along a period, the number of valence electrons and the pull of the nucleus increase, so atoms lose electrons less easily and gain them more easily. A period starts with very reactive metals (like sodium) and ends with non-metals (like chlorine) and an inert gas.

A chemical reaction is a chemical change in which atoms combine, split apart or exchange places to form new substances with new properties. Example: zinc and dilute hydrochloric acid react to form zinc chloride and hydrogen.

A chemical equation is the short way of writing a chemical reaction with symbols and formulas, the reactants on the left and the products on the right of an arrow. Example: 2Na + Cl₂ → 2NaCl.

Count the atoms of each element on both sides. Where the numbers differ, put coefficients (whole numbers) in front of the formulas until each element has the same number of atoms on both sides. Never change the formulas themselves.
Example: H₂ + O₂ → H₂O has 2 O on the left but 1 on the right. Put 2 before H₂O: H₂ + O₂ → 2H₂O. Now there are 4 H on the right, so put 2 before H₂: 2H₂ + O₂ → 2H₂O. Check: 4 H and 2 O on each side.

• i.MgO + H₂ → Mg + H₂O (already balanced: 1 Mg, 1 O, 2 H on each side)
• ii.CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
• iii.2K + Cl₂ → 2KCl
• iv.4Na + O₂ → 2Na₂O
• v.2H₂ + O₂ → 2H₂O

In (i), the equation balances on paper, but in a laboratory hydrogen isn’t strong enough to pull the oxygen away from magnesium. Hydrogen does do this to copper oxide: CuO + H₂ → Cu + H₂O.

Sulphuric acid is H₂SO₄, and the salt formed is zinc sulphate, ZnSO₄:
Zn + H₂SO₄ → ZnSO₄ + H₂
(Already balanced: 1 Zn, 2 H, 1 S and 4 O on each side.)

This is group 2, the alkaline earth metals.

1. Each element has 2 valence electrons (Be 2, 2; Mg 2, 8, 2; Ca 2, 8, 8, 2), so each has valency 2.
2. They are metals: they lose their 2 valence electrons in reactions.
3. Their atomic size increases from Be to Ca, as one shell is added at each step.
4. Their metallic character and reactivity increase going down, from beryllium to calcium.
5. They have similar chemical properties: for example, each forms an oxide with the formula MO (BeO, MgO, CaO).