The Atom

All elements are made of atoms. Atoms are made up of 3 types of particale- protons, neutrons and electrons.

Electrons- Electrons have -1 charge. They whizz around the nucleus in orbitals. The orbitals take up most of the volume of the atom.

Nucleus- Most of the mass of the atom is concentrated in the nucleus. The diameter of the nucleus is rather titchy compared to the whole atom. The nucleus is where you find the protons and neutrons.

The mass and charge of these subatomic particles is really small, so relative mass and relative charge are used instead.

Proton, 1 (relative mass), +1(relative charge)

Neutron, 1 (relative mass), 0 (relative charge)

Electron, 1/2000 (relative mass). -1 (relative charge)

You can figure out the number of protons, neutrons and electrons from the nuclear symbol.

Mass number- This tells you the total number of protons and neutrons in the nucleus.

Atomic number- This is the number of protons in the nucleus- it identifies the element. All atoms of the same element have the same number of protons.

For neutral atoms, which have no overall charge, the number of electrons is the same as the number of protons.

The number of neutrons is just mass number-atomic number, i.e. top minus bottom in the nuclear symbol.

Negative ions have more electrons than protons, e.g. Br-. The negative charge means that there's 1 more electron than there are protons. Br has 35 protons, so Br- must have 36 electrons. The overall charge = 35-36=-1

And positive ions have fewer electrons than protons. It kind of makes sense if you think about it.

e.g. Mg2+. The 2+ charge means that there's 2 fewer electrons than there are protons. Mg has 12 protons, so Mg2+ must have 10 electrons. The overall charge= 12-10=+2

Make sure you learn this definition and totally understand what it means-

Isotopes of an element are atoms with the same number of protons but different numbers of electrons.

It's the number and arrangement of electrons that decides the chemical properties of an element. Isotopes have the same configuration of electrons, so they've got the same chemical properties.

Isotopes of an element do have slightly different physical properties thought, such as different densities, rates of diffusion, etc. This is because physical properties tend to depend more on the mass of the atom.