1.1Atoms, elements and compounds
All substances are made of atoms. An atom is the smallest part of an element that can exist.
Atoms of each element are represented by a chemical symbol — O for an atom of oxygen, Na for an atom of sodium. There are about 100 different elements, and they are shown in the periodic table.
Compounds are formed from elements by chemical reactions. Chemical reactions always involve the formation of one or more new substances, and often involve a detectable energy change. Compounds contain two or more elements chemically combined in fixed proportions, and can be represented by formulae using the symbols of the atoms they were formed from. Compounds can only be separated into elements by chemical reactions.
- Atom
- The smallest part of an element that can exist.
- Element
- A substance made of only one type of atom. There are about 100.
- Compound
- Two or more elements chemically combined in fixed proportions.
Students will be supplied with a periodic table for the exam, so nothing here is about memorising symbols. What you are expected to do with it:
• use the names and symbols of the first 20 elements in the periodic table, the elements in Groups 1 and 7, and the other elements in this specification • name compounds of these elements from given formulae or symbol equations • write word equations for the reactions in this specification • write formulae and balanced chemical equations for those reactions.
Worked example
Balance the equation for the reaction of magnesium with hydrochloric acid: Mg + HCl → MgCl₂ + H₂
- Count each element on both sides. Left: 1 Mg, 1 H, 1 Cl. Right: 1 Mg, 2 Cl, 2 H.
- Chlorine is short on the left, so put a 2 in front of HCl: Mg + 2HCl → MgCl₂ + H₂.
- Recount. Left: 1 Mg, 2 H, 2 Cl. Right: 1 Mg, 2 Cl, 2 H.
- Every element now matches, and no formula has been altered — only the big numbers in front.
Answer: Mg + 2HCl → MgCl₂ + H₂
Common mistake
Never balance an equation by changing a formula. Turning H₂ into H₃ makes the numbers work and describes a substance that does not exist. Only the large numbers in front of formulae may be changed, because those say how many of each molecule react.
1.1The development of the model of the atom
New experimental evidence may lead to a scientific model being changed or replaced. This section is really about that idea, told through one example — so the marks are for what each piece of evidence forced, not for the dates.
Two things are asked for directly.
Why the scattering experiment changed the model: most alpha particles passed straight through, so the atom is mostly empty space. A few were deflected and a very few came almost straight back, so the mass and the positive charge must be concentrated in a tiny central nucleus. A plum pudding of spread-out charge could not have done that.
The difference between the two models: in the plum pudding the positive charge is spread throughout the atom with electrons embedded in it; in the nuclear model the positive charge and nearly all the mass sit in a tiny nucleus, with electrons outside it.
Details of the experimental work supporting the Bohr model are not required, and neither are the details of Chadwick's experiments.
1.3–1.4Relative electrical charges of subatomic particles
In an atom the number of electrons equals the number of protons in the nucleus, so atoms have no overall electrical charge — the +1s and the −1s cancel exactly.
The number of protons in an atom of an element is its atomic number. All atoms of a particular element have the same number of protons, and atoms of different elements have different numbers of protons. That is what makes an element the element it is: change the proton number and you have changed the element.
Common mistake
An ion is not an atom with a different number of protons — it is an atom that has lost or gained electrons. Losing an electron leaves one more proton than electron, so the charge is +1. Gaining one gives −1.
- Atomic number
- the number of protons in an atom of an element; all atoms of a particular element have the same atomic number
- Mass number
- the total number of protons and neutrons in an atom
- Isotope
- atoms of the same element with the same number of protons but different numbers of neutrons
1.5–1.8Size and mass of atoms
Atoms are very small, having a radius of about 0.1 nm, which is 1 × 10⁻¹⁰ m. The radius of a nucleus is less than 1/10 000 of that of the atom — about 1 × 10⁻¹⁴ m. Almost all of the mass of an atom is in the nucleus.
Put together, those two facts say something strange and worth holding on to: the atom is almost entirely empty space, and almost all of its mass is in a speck at the centre ten thousand times smaller than the atom itself.
The sum of the protons and neutrons in an atom is its mass number.
Atoms of the same element can have different numbers of neutrons; these are called isotopes of that element. They have the same atomic number and different mass numbers, so they behave identically in chemical reactions — the chemistry is decided by the electrons, and isotopes have the same number of those.
Worked example
An atom is written as ³⁵₁₇Cl. Give the number of protons, neutrons and electrons. Then do the same for the ion Cl⁻.
- The lower number is the atomic number: 17 protons.
- The upper number is the mass number: protons + neutrons = 35, so neutrons = 35 − 17 = 18.
- A neutral atom has as many electrons as protons: 17 electrons.
- The ion Cl⁻ has gained one electron, so it has 18 electrons. Protons and neutrons are unchanged.
Answer: Cl atom: 17 protons, 18 neutrons, 17 electrons. Cl⁻ ion: 17 protons, 18 neutrons, 18 electrons.
You are also asked to relate the size and scale of atoms to objects in the physical world. A useful comparison: if an atom were scaled up to the size of a football stadium, the nucleus would be about the size of a pea at the centre — and the rest would be empty.
1.9–1.12Relative atomic mass
The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element.
That is why relative atomic masses on the periodic table are rarely whole numbers. Chlorine is 35.5 not because any chlorine atom has half a neutron, but because a sample is a mixture of chlorine-35 and chlorine-37, and 35.5 is the weighted average.
Worked example
Chlorine exists as 75% chlorine-35 and 25% chlorine-37. Calculate its relative atomic mass.
- Relative atomic mass = (percentage × mass number) added for each isotope, all divided by 100.
- (75 × 35) = 2625.
- (25 × 37) = 925.
- 2625 + 925 = 3550, then 3550 ÷ 100.
Answer: 35.5 — which is the value printed on the periodic table.
Common mistake
Do not simply average the mass numbers. (35 + 37) ÷ 2 = 36, which is wrong, because there is three times as much chlorine-35 as chlorine-37. The abundance is the whole point of the calculation.
- Relative atomic mass
- an average value that takes account of the abundance of the isotopes of an element, which is why relative atomic masses are rarely whole numbers
Before you practise
- The model of the atom changed because new evidence appeared — name the experiment and what it showed.
- An atom is a nucleus of protons and neutrons surrounded by electrons in shells.
- Relative charges: proton +1, electron −1, neutron 0. Relative masses: proton and neutron 1, electron very nearly 0.
- An atom has equal numbers of protons and electrons, so it has no overall charge.
- Atomic number is the number of protons and identifies the element; mass number is protons plus neutrons.
- Isotopes are atoms of the same element with different numbers of neutrons.
- Relative atomic mass is a weighted mean of the isotopes, which is why it is often not a whole number.
- Almost all the mass is in the nucleus, and almost all the volume is empty space.