A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes

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.

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.

Higher tier students should also be able to write balanced half equations and ionic equations where appropriate.

A half equation shows what happens to one species — the electrons lost or gained — and so the electrons appear in it. An ionic equation leaves out the ions that take no part, so only the particles that actually change are written.

A mixture consists of two or more elements or compounds not chemically combined together. The chemical properties of each substance in the mixture are unchanged — which is exactly why a mixture can be separated physically and a compound cannot.

Mixtures can be separated by physical processes such as filtration, crystallisation, simple distillation, fractional distillation and chromatography. These processes do not involve chemical reactions and no new substances are made.

You should be able to describe, explain and give examples of the specified processes of separation, and to suggest suitable separation and purification techniques for mixtures when given appropriate information.

That second one is where the marks are won or lost, because the mixture in the question is usually one you have never met. Work from what is being separated, not from what you did in class: an insoluble solid in a liquid means filtration; a solid dissolved in a liquid means crystallisation or distillation, depending on which part you want to keep; two liquids mean fractional distillation. Purification often needs two steps in order — filter off the insoluble impurity first, then crystallise what is left.

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.

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.

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.

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.

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.

The electrons in an atom occupy the lowest available energy levels — the innermost available shells. The electronic structure of an atom can be represented by numbers or by a diagram.

Sodium is 2,8,1: two electrons in the lowest energy level, eight in the second, and one in the third. You may answer in terms of either energy levels or shells; both are accepted.

The shells fill 2, then 8, then 8, which covers every one of the first twenty elements — and you should be able to represent all twenty in both forms.