Discovery of the Proton and Neutron

Section 1 of 7Timeline of the nucleus

1Nuclear model

• Nuclear model Experiments showed the atom has a small, positive nucleus at its centre.

2Rutherford and the proton

• Rutherford and the proton Further work showed that the nucleus could be split into smaller positive particles, the protons.

3Unexplained mass

• Unexplained mass Nuclei were heavier than their protons alone could account for, so scientists suspected another particle but could not prove it.

4Chadwick, 1932

• Chadwick, 1932 James Chadwick proved that a neutral particle, the neutron, also sits in the nucleus.

Section 2 of 7The proton

How it was identified

• How it was identified Ernest Rutherford found that the positive charge of any nucleus could be divided into smaller positive units.
• The smallest unit carried the same charge as a hydrogen nucleus.

Elementary charge

• Elementary charge This amount of positive charge is called the elementary charge, shown as +1 in relative terms.

Meaning of the name

• Meaning of the name The name comes from a Greek root meaning first or original, because protons are the basic unit of positive charge.

Section 3 of 7The neutron

The evidence

• The evidence For around twenty years after the nuclear model, scientists saw signs of an extra particle but could not confirm it.

Chadwick's result

• Chadwick's result In 1932 Chadwick showed the particle exists and has a mass similar to that of a proton.

No charge

• No charge A neutron is electrically neutral, so it has a relative charge of 0 and is not deflected by electric fields.

Section 4 of 7Subatomic particles compared

Particle	Relative charge	Relative mass	Location
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	-1	Very small (about 1/2000)	Energy levels around the nucleus

Section 5 of 7Working out the charge of a particle

1

Count the protons in the particle. An alpha particle is a helium nucleus, so it has 2 protons.

2

Count the electrons. A bare nucleus has none, because electrons are not in the nucleus.

3

Add the charges: total charge = number of protons - number of electrons.

4

For an alpha particle this gives 2 - 0 = +2.

Section 6 of 7Key terms

Term	Meaning
Proton	A positively charged particle in the nucleus with a relative charge of +1 and relative mass of 1.
Neutron	An uncharged particle in the nucleus with a relative mass of 1.
Elementary charge	The smallest unit of charge found on a proton; an electron carries the same amount but negative.
Nucleus	The dense central part of an atom made of protons and neutrons.
Alpha particle	A helium nucleus made of 2 protons and 2 neutrons, with a charge of +2.

Section 7 of 7Exam technique

1

Stating that the neutron has a small negative charge - it is neutral with a charge of zero.

2

Writing that electrons are found in the nucleus; only protons and neutrons are there.

3

Mixing up the scientistsRutherford is linked with the proton and Chadwick with the neutron.

4

Giving a helium nucleus a charge of +4 by counting neutrons; only protons contribute charge, so it is +2.