C1
Elements and compounds
There are 118 different types of elements that humans have discovered and each one has its own chemical symbol. Atoms are the smallest possible unit of an element
Atoms of different elements can be combined together to create compounds. Compounds have formulae that are made by combining the chemical symbols of the elements that combine to make them.
Chemical reactions
Key features
Compounds are broken up or formed
at least 1 new substance is created
measurable energy change
no atoms are created or destroyed
Chemical equations
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The products are found on the right of equations.
The chemicals that are being made are called the products.
The reactants are found on the left of equations.
The chemicals that are reacting with each other are called the reactants.
symbol equatioms
.
4Na + O2 → 2Na2O
The balanced symbol equation for the creation of sodium oxide is:
Symbol equations must be 'balanced' (have the same number of atoms of each element on each side of the equation). This is because atoms aren't created or destroyed during chemical reactions.
Mixtures
Mixtures are made of 2+ substances (elements or compounds) that haven't been combined chemically. Mixtures can be separated. Their chemical properties don't change because they have been mixed with another substance. Mixtures are common in everyday life.
Examples of mixtures:
air
vegetables and pasta and sauce
separation techniques
simple distillation
separates 2 liquids with different boiling points
Fractional distillation
The mixture is slowly heated until the liquid with the lowest boiling point boils and then condenses. Then we increase temperature slowly to collect (boil then condense) the other fractions.
Separates lots of liquids with different boiling points
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Simple distillation can also separate a solute from a solvent, when the solvent has a lower boiling point than the solute
The vapour released passes through a condenser, where the gas cools back into a liquid.
The mixture is heated until the liquid with the lower boiling point starts to boil.
We can normally separate mixtures and compounds into the elements or compounds found in them. We use physical processes to separate them - these don't create new chemical substances. Separation processes include:
crystalisation
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Crystallisation separates solutions into their different parts: dissolved solids (solutes) and liquids (solvents).
Crystallisation is performed by heating the mixture so that the solvent evaporates.
Eventually, crystals of the solute (dissolved solids) will form.
We can collect the solvent (liquid) by condensing it as it evaporates.
filtration
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Filtration separates mixtures that contain insoluble (can't dissolve) solids and liquids (which are soluble).
Filtration is performed by pouring the mixture through filter paper:
The insoluble solid is trapped by the filter paper.
The liquid runs through the filter paper and is collected below.
chromatography
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Chromatography separates solutions with a number of different solutes (solids) in the solvent (liquid).
Place a drop of the solution to be separated near the bottom of a piece of chromatography paper. Dip the very bottom of the paper into a suitable solvent. The solvent (liquid) moves up the paper and carries the solutes (solids) in the solution with it.
Different solutes (solids) move at different speeds, so they separate on the paper.
Model of the atom
Thomsons Plum Pudding model
Discovered electrons
a ball of positive charge (dough), with negatively charged electrons (currants in pudding)
Rutherford's nuclear model
Alpha particles could bounce off of atoms
atom's mass is concentrated in the atom's centre
this became the nucleus
Modern model
Niels Bohr discovered that electrons orbit (fly around) the nucleus at fixed distances.
Charges of subatomic particles
Protons
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Protons have a relative charge of +1.
Protons are found in the nucleus.
An element’s atomic number is the number of protons it has.
All atoms of the same element will have an identical number of protons.
Neutrons
Neutrons have a relative charge of 0 - they are neutral.
Like protons, they are found in the nucleus.
Electrons
.
Electrons have a relative charge of -1.
Electrons are found in fixed orbits around the nucleus.
In any atom, the total number of negative electrons equals the number of positive protons, meaning atoms have no overall electric charge.
Neutrons (0)
Groups in the periodic table
Transition metals physical properties
conduct electricity
they are shiny when freshly cut
higher melting and boiling points
higher density
greater hardness
Group 1 /alkali metals
React vigorously with cold water to produce a metal hydroxide and hydrogen
Soft
Noble gases
Low boiling points
attractive forces between atoms are stronger
Boiling point increases as you go down the group
Extremely unreactive - already have a complete shell so dont need to loose or gain electrons
Low boiling points and density
As you go down the group boiling points get lower
Reactvity of the metals increases going down the group
Reactions
With water
With oxygen
Potassium, K - Burns violently with sparks and a lilac flame; quickly melts to form a ball; disappears rapidly, often with a small explosion
Lithium, Li - Fizzes steadily; slowly becomes smaller until it disappears
Sodium, Na - Fizzes rapidly; melts to form a ball; quickly becomes smaller until it disappears
React to make metal oxides
At room temperature, oxygen reacts with the surface of the metal. This forms a white oxide, which covers the surface. The metal below the surface does not react.
Burn vigorously when heated and placed in a jar of oxygen
With chlorine
React to form chlorides
Chlorides are white solids which dissolve in water to form colourless solutions
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the atoms become larger
the outer electron becomes further from the nucleus
the force of attraction between the nucleus and the outer electron decreases
the outer electron is lost more easily
Halogens
The halogens exist as simple molecules. Each molecule is made up of a pair of halogen atoms joined by a single covalent bond
further down the group an element is, the higher its melting point and boiling point.
.
the molecules become larger
the intermolecular forces become stronger
more energy is needed to overcome these forces
Form negatively charged ions
Reactions
React with metals to produce salts
Sodium and chlorine react vigorously when heated, giving an orange flame and clouds of white sodium chloride.
Salts are ions held together with ionic bonds
reactivity of the elements decreases down the group.
Displacement reactions
A more reactive halogen can displace a less reactive halogen from solutions of its salts
isotopes
Every atom of the same element will contain an identical number of protons (the same atomic number). But atoms of the same element can have different numbers of neutrons: these atoms are known as isotopes.
hydrogen has 3 isotopes
protium has 1 proton and 0 neutrons - it is used in fuel cells and plastic production and is the most common isotopes
deuterium has 1 proton and 1 neutron - it is used in nuclear fusion.
tritium has 1 proton and 2 neutrons - it is very rare and used in thermonuclear fusion weapons
calculating relative atomic mass