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Substance that contains at least two different elements, chemically combined
Made up of many substances that can be easily separated
A substance that cannot be broken down chemically (and is made up of all the same type of atom)
Chemical reaction where one element ’swaps out’ a less reactive element from a compound
A change in properties in a general direction
E.g. there is a trend of decreasing reactivity down the group
The atomic number is the number of protons.
The number of electrons = number of protons.
The number of neutrons = mass number – atomic number
Protons and neutrons have a relative mass of 1, whilst electrons have a relative mass of 1/8360.
Protons have a charge of +1, electrons -1, and neutrons are neutral.
These are different forms of the same element.
Same atomic number (same protons)
Different mass number (change in neutrons)
Relative atomic mass =
∑(isotope abundance% x isotope mass number) ÷ 100
Atoms fill to a maximum of 2 electrons in the first shell, 8 in the second and 8 in the third.
Each shell is filled before starting the next shell.
Chromatography – separates out different liquids. An Rf value can be calculated to compare the different parts.
Filtration – Separates a solid (S) from a liquid (L).
Crystallisation – Separates out a solid that has dissolved in a liquid. The liquid evaporates leaving the solid behind.
Distillation – Separates out liquids that have different boiling points, or to keep a liquid from a S+L mixture.
The same number of atoms of each element are needed on each side of an equation:
The Periodic Table
The Periodic Table is arranged in such a way that elements with the same properties can be found together in 'groups'.
Elements in the same group have the same number of electrons in their outshell (these are known as valance electrons).
Elements in the same period have the same number of electron shells.
Group 1 (Alkali Metals)
React with water to form an alkaline solution:
Lithium + water —> lithium hydroxide + hydrogen
React vigorously when heated with chlorine gas:
Sodium + chlorine —> sodium chloride
Reactivity increases down the group – the outer negative electron is further from the positive nucleus so more easily lost.
Lower melting and boiling points down the group.
Group 7 (Halogens)
Exist as pairs of atoms.
Less reactive down the group – the outer shell is further from the nucleus so harder to gain an electron.
Higher melting and boiling points down the group.
A more reactive halogen will displace a less reactive halogen.
Group 0 (Noble Gas)
Full outer shells so are inert.
Boiling point increases down the group as atoms have more electrons so stronger intermolecular forces form between molecules.
Strong, dense, shiny, good conductors of heat and electricity. Have more than one ion, forming coloured compounds.
History of the Atom
Periodic Table History
Four years before Mendeleev published his version of the table, Newlands noticed similarities between elements with atomic weights that had a difference of seven.
He called this The Law of Octaves, which were so named because he thought they were similar to music octaves.
Newlands did not think to leave any gaps for undiscovered elements, and had placed more than one element into some boxes in order tfor the pattern to hold true. Because of this, the Chemical Society refused to publish his paper.
Mendeleev arranged his table in order of atomic weight, and noticed a pattern emerging too. Where the elements didn't fit the pattern, he moved them - something Newlands didn't do. Mendeleev is most famous for leaving gaps for undiscovered elements, rather than trying to get it all fit together. He went one step further and actually predicted the properties of the undiscovered elements... and got them right!