Topic 1: Atomic structure and the periodic table

Everything around you is built from tiny particles called atoms. In this topic you will learn what atoms are made of, how our picture of the atom changed over time, and how arranging the elements into the periodic table helps us predict the way substances behave.

Atoms, elements and compounds

An atom is the smallest particle of an element that can exist. An element is a substance made of only one type of atom, and there are about 100 different elements, each shown by a one or two letter symbol such as O for oxygen or Na for sodium. When two or more different elements are chemically joined together they form a compound, for example water (H2O) or carbon dioxide (CO2). Compounds have fixed proportions of each element and can only be separated back into elements by chemical reactions, not simply by physical means. Chemical reactions can be summarised using equations, where the atoms present at the start (reactants) are rearranged to make new substances (products). Because atoms are never created or destroyed in a reaction, equations must be balanced so the number of each type of atom is the same on both sides.

Mixtures and separating them

A mixture contains two or more substances that are not chemically joined together, so each substance keeps its own properties and the mixture can be separated using physical methods. These methods do not involve chemical reactions and so do not change the substances themselves. Filtration separates an insoluble solid from a liquid, while crystallisation and simple evaporation recover a soluble solid from its solution. Distillation separates a liquid from a solution by boiling and then condensing the vapour, and fractional distillation separates two or more liquids with different boiling points, such as the gases in air or the parts of crude oil. Chromatography separates substances that dissolve in a solvent and travel at different speeds across paper, which is useful for separating coloured inks or food dyes.

How the atomic model developed

Our idea of the atom has changed many times as new evidence appeared. At first atoms were thought to be tiny solid spheres that could not be divided. After the electron was discovered, the plum pudding model suggested the atom was a ball of positive charge with negative electrons scattered through it. The alpha scattering experiment, in which positively charged particles were fired at thin gold foil, showed that most particles passed straight through but a few bounced back. This proved that the mass and positive charge were concentrated in a tiny central nucleus, leading to the nuclear model. Niels Bohr then suggested electrons orbit the nucleus at fixed distances called shells or energy levels, and later work showed the nucleus contains both protons and neutrons. This story is a good example of how scientific models are tested and improved over time.

Sub-atomic particles and isotopes

Atoms are made of three sub-atomic particles. Protons have a relative charge of +1 and a relative mass of 1, neutrons have no charge and a relative mass of 1, and electrons have a charge of -1 with a mass so small it is usually taken as zero. Protons and neutrons sit in the central nucleus, while electrons move around it in shells. Atoms have no overall charge because the number of protons equals the number of electrons. The atomic number tells you how many protons an atom has and defines which element it is, while the mass number is the total number of protons plus neutrons. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, giving them different mass numbers. The relative atomic mass of an element is the average mass of its isotopes, taking into account how common each one is.

Electronic structure

Electrons are arranged in shells around the nucleus, and each shell can only hold a limited number of electrons. The first shell holds up to 2 electrons, and the next two shells each hold up to 8 electrons. Electrons fill the lowest energy shell nearest the nucleus first before starting the next one. We can write the arrangement as numbers separated by commas, so a sodium atom with 11 electrons is written as 2,8,1. This electronic structure controls how an atom reacts, because it is the electrons in the outer shell that take part in bonding. The number of electrons in the outer shell of an atom is equal to the group number of that element in the periodic table, which is why elements in the same group behave in similar ways.

The modern periodic table

The periodic table lists all the elements in order of increasing atomic number, arranged in rows called periods and columns called groups. Elements in the same group have the same number of outer shell electrons and therefore have similar chemical properties, which makes the table a powerful tool for predicting how an element will behave. Early chemists such as Mendeleev arranged elements before protons were known, ordering them by atomic mass but leaving gaps for undiscovered elements and even swapping the order of some pairs so that similar elements lined up. His predictions about the missing elements later turned out to be correct, which gave strong support to his table. Once protons were discovered, ordering by atomic number explained why his arrangement worked so well.

Metals and non-metals

Most elements are metals, and they are found on the left and towards the bottom of the periodic table, while non-metals are found on the right. Metals tend to be shiny, good conductors of heat and electricity, malleable and have high melting points. Non-metals are often dull, poor conductors and have lower melting and boiling points, with many being gases at room temperature. The difference comes from electronic structure, because metal atoms have few outer shell electrons that they lose easily to form positive ions, while non-metal atoms tend to gain electrons to form negative ions or share electrons. When metals react with non-metals the electrons transferred lead to ionic compounds, and this is a key reason the position of an element in the table tells you so much about its chemistry.

Group 0, Group 1 and Group 7 trends

Group 0, the noble gases, are very unreactive because their atoms already have a full outer shell of electrons, so they have little tendency to react. As you go down Group 0 the boiling points increase because the atoms get larger and the forces between them grow stronger. Group 1, the alkali metals such as lithium, sodium and potassium, each have one outer electron, are very reactive and become more reactive going down the group as the outer electron is more easily lost. They react with water to form an alkaline solution and hydrogen gas. Group 7, the halogens such as chlorine, bromine and iodine, each have seven outer electrons and become less reactive going down the group because it is harder to gain an extra electron when the atom is larger. A more reactive halogen can displace a less reactive one from a solution of its salt.