C1 Atomic structure

Question 1

What is a mixture?

Answer 1

Made up of two or more substances (elements or compounds) that are not chemically combined together

Question 2

What is an element?

Answer 2

A substance of only one type of atom

Question 3

What is a compound?

Answer 3

Two or more elements combined chemically in fixed proportions

Question 4

What is an ion?

Answer 4

An atom that has an electric charge due to the loss or gain of electrons

Question 5

What are isotopes?

Answer 5

Atoms of the same element with the same number of protons but a different number of neutrons

Question 6

What is the mass number an atom?

Answer 6

The total number of protons and neutrons in the nucleus

Question 7

Differences between compounds and mixtures

Answer 7

Compounds
- fixed composition
- chemical reactions must be used to separate the elements in a compound
- chemical bonds between atoms of the different elements in a compound

Mixtures
- no fixed composition
- different elements or compounds in a mixture can be separated easily (by physical means)
- no chemical bonds between atoms of the different substances in a mixture

Question 8

Evidence for electrons in atoms

Answer 8

• Discovered by J.J. Thomson
• Proposed the plum pudding model
• Tiny negatively charged electrons embedded in a cloud of positive charge
• Atoms themselves carry no overall charge, so any charges in an atom must balance out

Question 9

Evidence for the nucleus

Answer 9

• proposed by Ernest Rutherford
• Rutherford shot a beam of positively charged particles (called alpha particles) at a thin sheet of gold foil
• Expected the particles to pass straight through the gold atoms with their diffuse cloud of positive charge, based on Thomson’s plum pudding model
• Rutherford’s atomic model became known as the Nuclear Model

Question 10

Results of the Rutherford scattering

Answer 10

• Very few alpha particles were repelled back towards the source of alpha particles
• Most of the alpha particles went straight through the foil
• Some of the alpha particles were deflected at an angle

Question 11

What did the results of the Rutherford scattering suggest?

Answer 11

Repelled: the positive charge must be concentrated at a tiny spot in the centre of the atom (the nucleus)
Straight through: most of the atom is empty space

Question 12

The relative charge and mass of sub-atomic particles

Answer 12

Proton
- relative charge = +1
- relative mass = 1

Neutron
- relative charge = 0
- relative mass = 1

Electron
- relative charge = -1
- relative mass = very small < 0

Question 13

Atomic number

Answer 13

The number of protons in each atom of an element

Question 14

Evidence for electrons in shells

Answer 14

• Niels Bohr revised the atomic model again in 1914
• He noticed that light was given out when atoms were heated only with a specific amount of energy
• He suggested that the electrons must be orbiting the nucleus at set distances, in certain fixed energy levels (shells)
• The energy must be given out when excited electrons fall from a high to low energy level

Question 15

Evidence for neutrons in the nucleus

Answer 15

• Scientists at the time speculated that there were two types of sub-atomic particles inside the nucleus
• They had evidence for protons but a second sub-atomic particle in the nucleus was also proposed to explain the missing mass that had been noticed in atoms
• These neutrons must have no charge and have the same mass as a proton
• Because neutrons have no charge, it is very difficult to detect them in experiments
• It was not until 1932 that James Chadwick did an experiment that could only be explained by the existence of neutrons

Question 16

Methods for separating mixtures

Answer 16

• Filtration
• Crystallisation
• Simple distillation

Question 17

When is filtration used?

Answer 17

Used to separate insoluble substances in a solvent from those that are soluble