The structure of an atom

1. Every atom has a tiny central nucleus with electrons moving around it.
2. The nucleus contains two kinds of particle, protons and neutrons.
3. The electrons are arranged in electron shells, which surround the nucleus at increasing distances from it.
4. The nucleus is extremely small compared with the whole atom, so most of the volume of an atom is empty space through which the electrons move.
   1. If an atom were enlarged to the size of a football stadium, its nucleus would be about the size of a pea on the centre spot.
5. Almost all the mass of an atom is concentrated in its nucleus, because protons and neutrons are far heavier than electrons.
6. The positively charged nucleus attracts the negatively charged electrons, and this attraction holds the electrons in the atom.

Protons, neutrons and electrons

1. The real masses and charges of these particles are extremely small, so chemists compare them using relative mass and relative charge.
2. Relative values compare each particle with a proton, which is given a relative mass of $1$ and a relative charge of $+1$.
3. A proton is found in the nucleus and has a relative mass of $1$ and a relative charge of $+1$.
4. A neutron is found in the nucleus and has a relative mass of $1$ and a relative charge of $0$, so it is neutral.
5. An electron is found in the shells and has a relative mass of $\frac{1}{1840}$ and a relative charge of $-1$.
6. The mass of an electron is so small that it is often described as negligible, meaning it is almost nothing compared with the mass of a proton or a neutron.
7. The charge on an electron is the same size as the charge on a proton but opposite in sign, so one electron exactly cancels the charge of one proton.
8. An atom has no overall charge, because it contains equal numbers of protons and electrons.
   1. A carbon atom has 6 protons with a total charge of $+6$ and 6 electrons with a total charge of $-6$, giving an overall charge of $0$.

Proton number and mass number

1. The proton number is the number of protons in the nucleus of an atom, and it is also called the atomic number.
2. The proton number identifies the element, so every atom with 6 protons is a carbon atom and every atom with 11 protons is a sodium atom.
3. The Periodic Table lists the elements in order of increasing proton number, starting with hydrogen, which has a proton number of $1$.
4. Protons and neutrons are together called nucleons, because they are the particles found in the nucleus.
5. The mass number is the total number of protons and neutrons in the nucleus of an atom, and it is also called the nucleon number.
6. Electrons are not counted in the mass number, because their mass is negligible.
7. In a neutral atom, the number of electrons is equal to the proton number.
8. The number of neutrons in an atom is found by subtracting the proton number from the mass number, $number\; of\; neutrons=mass\; number-proton\; number$.