Revision notes: Matter and the Atomic Structure

These notes work through Matter and the Atomic Structure one content standard at a time, so you can revise each part and check you have the definitions the marking scheme expects. Keep them beside past-paper questions and use them to confirm your answers. This chapter is the foundation for the whole of SPM Chemistry: every later idea, from bonding to the periodic table, rests on the picture of the atom built here.

2.1 Matter and the kinetic theory of matter

Matter is anything that has mass and occupies space, and it is made up of tiny, discrete particles, atoms, molecules or ions. The kinetic theory of matter states that these particles are always moving and possess kinetic energy, and that the higher the temperature, the faster and more energetically they move.

The three states. In a solid, particles are closely packed in an orderly arrangement, held by strong forces, and only vibrate about fixed positions, so a solid has a fixed shape and a fixed volume. In a liquid, particles are still close but not orderly, held by weaker forces, and can slide past one another, so a liquid has a fixed volume but takes the shape of its container. In a gas, particles are far apart in a random arrangement, with negligible forces between them, and move freely at high speed, so a gas has neither a fixed shape nor a fixed volume.

Diffusion, the movement of particles from a region of higher concentration to a region of lower concentration, is the everyday evidence that matter is made of moving particles. Diffusion is fastest in gases and slowest in solids, and it speeds up as temperature rises.

2.2 Changes in the state of matter

The three states interconvert when a substance is heated or cooled; these are physical changes. Melting (solid to liquid) happens at the melting point; freezing (liquid to solid) at the freezing point; boiling (liquid to gas) at the boiling point; condensation returns gas to liquid; and sublimation takes a solid directly to gas (for example iodine and dry ice).

The heating curve. When a solid is heated steadily, its temperature rises until it reaches the melting point, where it stays constant even though heat is still supplied. This plateau occurs because the heat energy is used to overcome the forces of attraction holding the particles in the lattice, not to raise their kinetic energy. Once melting is complete, the temperature rises again.

A cooling curve shows the reverse: a plateau appears at the freezing point because heat is released as the forces of attraction re-form, so the temperature holds steady while the liquid solidifies. Being able to read these flat sections and explain them in terms of particles and energy is one of the most reliably examined skills in the chapter.

2.3 Development of the atomic model

The atomic model was not discovered all at once; it was refined by several scientists as new evidence appeared, and the syllabus asks you to sequence their contributions.

• John Dalton, the atom is a tiny, indivisible solid sphere.
• J. J. Thomson, discovered the electron and proposed the “plum-pudding” model: a sphere of positive charge with electrons embedded in it.
• Ernest Rutherford, proposed the nuclear model: a small, dense, positively charged nucleus at the centre with electrons moving around it and most of the atom being empty space.
• Niels Bohr, electrons move around the nucleus in fixed shells (energy levels).
• James Chadwick, discovered the neutron in the nucleus.

2.4 Atomic structure and subatomic particles

An atom consists of a central nucleus containing protons and neutrons, surrounded by electrons that occupy shells. The three subatomic particles differ in relative mass and relative charge:

• Proton: relative mass 1, relative charge +1, found in the nucleus.
• Neutron: relative mass 1, relative charge 0, found in the nucleus.
• Electron: relative mass 1/1840 (effectively negligible), relative charge −1, found in the shells.

Because a neutral atom has equal numbers of protons and electrons, their charges cancel and the atom carries no overall charge. Almost all the mass of the atom is concentrated in the nucleus.

2.5 Proton number, nucleon number and isotopes

The proton number (also called the atomic number) is the number of protons in the nucleus; it defines the element and, in a neutral atom, equals the number of electrons. The nucleon number (mass number) is the total number of protons and neutrons. It follows that the number of neutrons equals the nucleon number minus the proton number.

Isotopes. Isotopes are atoms of the same element with the same proton number but different nucleon numbers, in other words, the same number of protons but different numbers of neutrons. Because they have the same electron arrangement, isotopes of an element have the same chemical properties; because they differ in mass, they differ in physical properties such as density. Familiar examples are the isotopes of carbon and of chlorine.

Isotopes have real uses, for example in medicine (radiotherapy and diagnosis), in archaeology (carbon dating) and in industry, which the exam sometimes asks you to describe in general terms.

2.6 Electron arrangement of atoms

Electrons are arranged in shells (energy levels) around the nucleus, and they fill the innermost shell first. For the first twenty elements, the first shell holds a maximum of 2 electrons, the second shell 8, and the third shell 8. The arrangement is written with dots between the shells, for example, an atom with 11 electrons has the arrangement 2.8.1.

The electrons in the outermost occupied shell are the valence electrons; they determine the element’s chemical behaviour. The number of shells occupied tells you the period, and the number of valence electrons tells you the group in the periodic table, the direct link to the next chapter.

How to use these notes

Revise one content standard at a time and test yourself by explaining it aloud. For 2.1, sketch the particle arrangement in each state and list its properties. For 2.2, draw a heating curve and explain both plateaux. For 2.3, put the scientists in order with one contribution each. For 2.4, tabulate the relative mass and charge of the three particles. For 2.5, practise finding neutrons from the two numbers and defining isotopes precisely. For 2.6, write electron arrangements for the first twenty elements until it is automatic. A one-to-one teacher can check that your heating-curve explanation talks about forces and energy rather than just “heat”, which is the single most common place marks slip away in this chapter. Then attempt the practice questions and mark your answers against the definitions above.