0All substances are made from atoms. Each atom is made of a nucleus - containing protons and neutrons - surrounded by electrons.
The atomic number is the number of protons in an atom. The elements are arranged in the periodic table in ascending order of atomic number.
The mass number of an atom is the total of protons plus neutrons. Atoms of the same element with different numbers of neutrons (and hence different mass numbers) are called isotopes of that element.
Atomic structure:
All material things are made from atoms. There are just over one hundred different types of atom, called elements. Atoms can join together in millions of different combinations to make all the substances on Earth and beyond.
Structure of the atom
Every atom is made of a nucleus consisting of protons and neutrons. The nucleus is surrounded by electrons. Protons and electrons are oppositely charged. Neutrons have no charge. This means the nucleus of an atom is always positively charged.
An atom has a neutral overall charge because it has the same number of electrons as protons.
Protons and neutrons have the same mass. Electrons have such a small mass that this can usually be taken as zero.
Comparing the charge and mass of electrons, protons and neutrons
| Proton | Neutron | Electron | |
|---|---|---|---|
| Charge | +1 | 0 | -1 |
| Mass | 1 | 1 | 0.0005 (almost zero) |
The atomic number (also called the proton number) is the number of protons in an atom.
The mass number (also called the nucleon number) is the total number of protons and neutrons in an atom.
The elements are arranged in the periodic table in ascending order of atomic number so it's easy to find the name or symbol for an atom if you know the atomic number.
Section of the periodic table showing elements ascending by atomic number.
Isotopes:-
The chemical symbol for carbon
The full chemical symbol for an element shows its mass number at the top and atomic number at the bottom. Here is the full symbol for an isotope of carbon, carbon-12 or 12C:
It tells us that a carbon atom has:
- Six protons (because its proton number, at the bottom, is 6)
- Six electrons (because the number of protons and electrons in an atom is the same)
The symbol also tells us that the total number of protons and neutrons in a carbon atom is 12. Note that you can work out the number of neutrons from the mass number and atomic number. In this example, it is 12 - 6 = 6 neutrons.
Isotopes of hydrogen:-
Most hydrogen atoms consist of just one proton and one electron, but some also have one or two neutrons.
Isotopes of hydrogen
| Isotope | Protons | Electrons | Neutrons |
|---|---|---|---|
 | 1 | 1 | 1 - 1 = 0 |
 | 1 | 1 | 2 - 1 = 1 |
 | 1 | 1 | 3 - 1 = 2 |
Isotopes of chlorine:-
Chlorine atoms contain 17 protons and 17 electrons. About 75 per cent of chlorine atoms have 18 neutrons, while about 25 per cent have 20 neutrons.
Isotopes of chlorine
| Isotope | Protons | Electrons | Neutrons |
|---|---|---|---|
 | 17 | 17 | 35 - 17 = 18 |
 | 17 | 17 | 37 - 17 = 20 |
Energy shells:-
Electrons are arranged in different shells around the nucleus. The innermost shell - or lowest energy shell - is filled first. Each succeeding shell can only hold a certain number of electrons before it becomes full. The innermost shell can hold a maximum of two electrons, the second shell a maximum of eight, and so on. The table gives the maximum capacity of the first three shells.
Maximum capacity of the first three shells
| Energy shell | Maximum number of electrons |
|---|---|
| First | 2 |
| Second | 8 |
| Third | 8 |
A lithium atom, for example, has three electrons. Two are in the first energy shell, and one in the second.
A carbon atom has six electrons. Two are in the first energy shell, and four in the second energy shell.
Arrangement of electrons in a lithium atom
Arrangement of electrons in a carbon atom
A calcium atom has 20 electrons. Two are in the first energy shell, and eight in the second energy shell, eight in the third energy shell and two in the fourth energy shell.
Electronic structure:-
The electronic structure of an atom is a description of how the electrons are arranged. It can be shown as numbers or as a diagram.
Electron configuration diagram for lithium
Take lithium for example. The diagram shows each energy shell as a circle around the nucleus, with each electron represented by a dot. In the exam, do not worry about colouring in the electrons. Just make them clear and ensure they are in the right place. Sometimes you will be asked to use a cross rather than a dot.
The electronic configuration for lithium is written as 2.1 (showing that lithium atoms have three electrons, two in the first shell and one in the second shell).
The table describes the electronic structures of four more elements.
Electronic structures of elements
| Element | Numeric format | Electrons | Periodic table group |
|---|---|---|---|
|
| F 2.7 | Fluorine atoms have nine electrons. Two of these fit into the first energy shell. The remaining seven fit into the second energy shell. | Group 7 |
|
| Ne 2.8 | Neon atoms have ten electrons. Two of these fit into the first energy shell. The remaining eight electrons fit into the second energy shell. Because its highest occupied energy shell is full, neon is stableand unreactive. | Group 0 - that is, the eighth group |
|
| Na 2.8.1 | Sodium atoms have 11 electrons. Two of these fit into the first energy shell, eight into the second energy shell. The last one fits into the third energy shell. | Group 1 |
|
| Ca 2.8.8.2 | Calcium atoms have 20 electrons. Two of these fit into the first energy shell, eight into the second energy shell, another eight into the third energy shell. The last two fit into the fourth energy shell. | Group 2 |
