AQA GCSE Single Science Chemistry: Atomic structure and the periodic table

Define the following terms; atom, compound, mixture

• Atom: The smallest part of an element that can exist

• Compound: Substances containing two or more elements, chemically combined

• Mixture: Substances consisting of two or more elements or compounds not chemically combined together.

Describe the process of crystallisation

1. A solution is placed in an evaporating basin and heated with a Bunsen burner.

2. All the water has evaporated, leaving the solid crystals behind.

Describe the process of simple distillation

1. The solution is heated and solvent vapour leaves

2. This cools in the condenser to be collected in a beaker

3. The remaining solution becomes much more concentrated- a salt is left behind

Describe the process of fractional distillation

1. The mixture is heated

2. Vapours rise through a column which is hot at the bottom, and cooler at the top

3. Vapours condense when they reach a part of the column that is below the temperature of their boiling point

4. Each liquid is led away from the column to be collected

Describe the process of chromatography

1. Ink or plant dye is dotted along the pencil line.

2. As the paper is lowered into the solvent, some of the dye spreads up the paper

3. The paper has absorbed the solvent, and the dye has spread further up the paper- producing a chromatogram

Describe the plum pudding model

• Proposed by JJ Thomson

• This follows the discovery of the electron, but at this point, neutrons and protons were not yet found

• The atom is a ball of positive charge with negative electrons embedded in it

Describe the alpha scattering experiment

• This was conducted by Ernest Rutherford

• It involved (positive) alpha particles being fired at a gold sheet- most passed straight through and some were deflected at large angles

• Therefore this provided evidence that the nucleus existed, was very small and was positively charged- resulting in a nuclear model that replaced the plum pudding model

How did Niels Bohr adapt the nuclear model?

• By suggesting that electrons orbit the nucleus at specific distances

What did James Chadwick find?

• The existence of neutrons within the nucleus

• This was around 20 years after the nucleus became an accepted scientific idea

In which order were the subatomic particles discovered?

1. Electron (1897)- by Thomson

2. Proton (1919)- by Rutherford

3. Neutron (1932)- by Chadwick

What are the relative charges of the subatomic particles?

• Electron: -1

• Proton: +1

• Neutron: 0

What are the relative masses of the subatomic particles?

• Electron: Very small

• Proton: 1

• Neutron: 1

What is the relative atomic mass of an element? How can it be calculated?

• An average value that takes account of the abundance of the isotopes of the element.

• (Isotope abundance + isotope mass number) + (Isotope abundance + isotope mass number)/ total abundance

How are elements arranged in the periodic table?

• In order of increasing atomic number

• Elements with similar properties are in the same group

• Its group number references the number of electrons in its outer shell

How did scientists arrange elements pre-Mendeleev?

• In order of atomic mass

• This meant some elements were placed in inappropriate groups- properties were dissimilar

What did Mendeleev do?

• He left gaps for elements that he thought had not been discovered

• In some places, he changed the order based on atomic weights so elements in a group shared properties

Why was Mendeleev’s periodic table finally accepted?

• Elements with properties predicted by Mendeleev were discovered

• Knowledge of isotopes explained why an order based on atomic weights was not always correct

What makes an element a metal? Where can they be found?

• Elements that react to form positive ions are metals

• Majority of elements in the periodic table are metals

• These are found to the left and toward the bottom of the periodic table

What are the physical properties of a metal?

• Shiny

• High melting points

• Good electrical conductors- delocalised electrons can carry current through a structure

• Good thermal conductors

• High density

• Malleable- layers of ions can slide over another

What are the physical properties of a non-metal?

• Dull

• Low melting points

• Poor electrical conductors

• Poor thermal conductors

• Low density

• Brittle

What are the properties of a Group 0 element?

• They are unreactive and do not easily form molecules- have a stable arrangement of electrons

• Boiling points increase with increasing relative atomic mass (as you go down the group)

What are the properties of a Group 1 element?

• They are soft (they can be cut with a knife)

• They have relatively low melting points

• They have low densities

• Reactivity increases with increasing relative atomic mass (as you go down the group)

What happens when lithium, sodium and potassium react with water?

• Lithium: lithium floats. It fizzes steadily and becomes smaller, until it eventually disappears.

• Sodium: sodium melts to form a ball that moves around on the surface. It fizzes rapidly before it disappears.

• Potassium: the metal melts and floats. It moves around very quickly on the surface of the water. The metal self-ignites, which also ignites the hydrogen gas. This results in sparks and a lilac flame.

What happens when lithium, sodium and potassium react with oxygen?

• The group 1 elements react with oxygen from the air to make metal oxides.

• Reactions grow more vigorous with increasing relative atomic mass (as you go down the group)

What happens when lithium, sodium and potassium react with chlorine?

• The group 1 elements react vigorously with chlorine.

• The products of the reactions are chlorides.

• At room temperature the chlorides are white solids.

• They dissolve in water to form colourless solutions

What are the properties of a Group 7 element?

• The halogens exist as simple molecules. Each molecule is made up of a pair of halogen atoms joined by a single covalent bond.

• Reactivity of the elements decrease as you go down the group

• Melting and boiling points increase as you go down the group

What are the properties of transition metals?

• They are good conductors of heat and electricity

• They can be hammered or bent into shape easily

• They have high melting points (but mercury is a liquid at room temperature)

• They are usually hard and tough

• They have high densities

What are properties that are unique to transition metals?

• Have ions with different charges

• Form coloured compounds

• Are useful as catalysts

What happens when transition elements react with oxygen, water and halogens?

• Oxygen: most transition elements react slowly, or not at all, with oxygen at room temperature. Some transition metals react on heating

• Water: most transition elements react slowly with cold water, or not at all.

• Halogens: some transition elements also react with halogens