# The Nuclear Model (Also seen in GCSE Physics 1 and 2)

We know from Rutherford’s experiment that the structure of an atom consists of positively charged protons and neutral neutrons in one place called the nucleus. The nucleus sits in the middle of the atom and has negatively charged electrons orbiting it. At GCSE we used charges and masses for the constituents relative to each other, the table above shows the actual charges and masses.

Almost all of the mass of the atom is in the tiny nucleus which takes up practically no space when compared to the size of the atom. If we shrunk the Solar System so that the Sun was the size of a gold nucleus the furthest electron would be twice the distance to Pluto.

If the nucleus was a full stop it would be 25 m to the first electron shell, 100 to the second and 225 to the third.

# Notation (Also seen in GCSE Physics 2)

We can represent an atom of element X in the following way:

Z is the proton number. This is the number of protons in the nucleus. In an uncharged atom the number of electrons orbiting the nucleus is equal to the number of protons.

In Chemistry it is called the atomic number

A is the nucleon number. This is the total number of nucleons in the nucleus (protons + neutrons) which can be written as A = Z + N.

In Chemistry it is called the atomic mass number

N is the neutron number. This is the number of neutrons in the nucleus.

# Isotopes (Also seen in GCSE Physics 1 and 2)

Isotopes are different forms of an element. They always have the same number of protons but have a different number of neutrons. Since they have the same number of protons (and electrons) they behave in the same way chemically.

Chlorine  If we look at Chlorine in the periodic table we see that it is represented by

. How can it have 18.5 neutrons? It can’t! There are two stable isotopes of Chlorine

, which accounts for ~75% and which accounts for ~25%. So the average of a large amount of

Chlorine atoms is

.

# Specific Charge

Specific charge is another title for the charge-mass ratio. This is a measure of the charge per unit mass and is simply worked out by worked out by dividing the charge of a particle by its mass.

You can think of it as a how much charge (in Coulombs) you get per kilogram of the ‘stuff’.

We can see that the electron has the highest charge-mass ratio and the neutron has the lowest.

# Ions (Also seen in GCSE Physics 2)

An atom may gain or lose electrons. When this happens the atoms becomes electrically charged (positively or negatively). We call this an ion.

If the atom gains an electron there are more negative charges than positive, so the atom is a negative ion.

Gaining one electron would mean it has an overall charge of -1, which actually means -1.6 x 10-19C.

Gaining two electrons would mean it has an overall charge of -2, which actually means -3.2 x 10-19C.

If the atom loses an electron there are more positive charges than negative, so the atom is a positive ion.

Losing one electron would mean it has an overall charge of +1, which actually means +1.6 x 10-19C.

Losing two electrons would mean it has an overall charge of +2, which actually means +3.2 x 10-19C.