Sub-Atomic Particles

Section 1 of 10The three particles

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

Section 2 of 10Why atoms are neutral

1Protons give +

• Protons give + Each proton carries one unit of positive charge.

2Electrons give -

• Electrons give - Each electron carries one unit of negative charge.

3Equal numbers

• Equal numbers A neutral atom has the same number of protons as electrons.

4No overall charge

• No overall charge The charges cancel exactly, so the atom is neutral.

Section 3 of 10Atomic number and mass number

Atomic number

• Atomic number The number of protons in the nucleus; it identifies the element.
• It is the smaller number in a nuclide symbol and the order of the periodic table.

Mass number

• Mass number The total number of protons plus neutrons in the nucleus.

Neutron number

• Neutron number Neutrons = mass number − atomic number.

Section 4 of 10Worked example: sodium-23

1

Sodium has atomic number 11 and mass number 23, written ²³₁₁Na.

2

Protons = atomic number = 11.

3

Electrons in the neutral atom = 11.

4

Neutrons = 23 − 11 = 12.

Section 5 of 10Ions

Positive ion

• Positive ion Formed when an atom loses electrons, so protons outnumber electrons.

Negative ion

• Negative ion Formed when an atom gains electrons, so electrons outnumber protons.

Section 6 of 10Key facts to remember

Nucleus

• Nucleus Contains protons and neutrons, so it is positively charged overall and holds nearly all of the atom's mass.

Electron cloud

• Electron cloud Electrons move around the nucleus in shells and are attracted to it by electrostatic forces.

Isotope link

• Isotope link Atoms of one element can have different numbers of neutrons but always the same number of protons.

Section 7 of 10Worked examples: atoms and ions

Species	Protons	Neutrons	Electrons
3517Cl atom	17	35 − 17 = 18	17
3517Cl- ion	17	18	18, because one extra electron has been gained
2311Na+ ion	11	12	10, because one electron has been lost

Section 8 of 10How the nucleus was discovered

1Plum pudding model

• Plum pudding model The atom was once pictured as a ball of positive charge with negative electrons embedded in it.

2Alpha scattering

• Alpha scattering Rutherford's team fired alpha particles at thin gold foil;
  • most passed straight through but a few were deflected through large angles.

3Nuclear model

• Nuclear model The results showed a small, dense, positive nucleus surrounded by mostly empty space.

4Bohr's refinement

• Bohr's refinement Bohr proposed that electrons orbit the nucleus in fixed energy levels or shells.

Section 9 of 10Key terms

Term	Meaning
Proton	Positive particle in the nucleus with relative mass 1.
Neutron	Neutral particle in the nucleus with relative mass 1.
Electron	Negative particle with negligible mass that occupies shells.
Atomic number	Number of protons in an atom.
Mass number	Number of protons plus neutrons.
Ion	An atom or group of atoms that has gained or lost electrons and so has an overall charge.
Nucleon	A particle found in the nucleus, either a proton or a neutron.

Section 10 of 10Exam technique

1

Counting electrons in the mass number; they have negligible mass.

2

Swapping atomic number and mass number; the larger number is the mass number.

3

Changing the number of protons to make an ion. Only electrons are gained or lost.