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Neutrons and protons, subatomic particles of the nucleus, are held tightly together by nuclear forces.

If it weren’t for these forces, then protons would be pushed away from the nucleus.

● There is also an attractive force between

negatively charged electrons and positively

charged protons. It is this force which keeps

the electrons moving around the nucleus

What do you notice about the size of the

electrons?

While the protons and neutrons are roughly the

same size, an electron is about 1/2000th the size

of the proton.

In 1911, after conducting experiments,

Rutherford proposed his model of the atom which

comprised of:

● Centre which contained the nucleus (neutrons

and protons)

● Electrons that orbited the nucleus

Rutherford’s model is close to what we use today,

however he also proposed that the orbiting

electrons would eventually lose energy which we

now know to be false.

In 1913, Bohr who was a scientist who studied

with Rutherford, modified the model. He proposed:

● Only electrons with specific amounts of

energy could exist at each level

● To move from each level, electrons would need

to either gain or lose energy.

This was more correct than Rutherford’s model,

however did not fully explain all known properties

of atoms.

Combining Rutherford, Bohr’s and other

scientists knowledge, the current

model of the atom:

● Contains an electron cloud due to

the speed of electrons moving

around the nucleus

● Electrons do not have a set path,

they can be found in regions called

electron shells

Atomic Number: The number of protons in the nucleus OR the number of electrons in an atom

WHY? More specifically the atomic number represents the charge of the nucleus. Therefore, it is more correct

to say that it is equal to the number of protons, and the positive and negative charges of an atom

must be balanced, thus it the number of electrons must be the same.

Atomic Number: The number of protons in the nucleus

Mass Number: The number of protons and neutrons in the nucleus