atomic structure

• 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
• a compound is made up of at least 2 different elements
• in chemical reactions, compounds are broken up or formed
• in chemical reactions, at least 1 new substance is created
• in chemical reactions, there is measurable energy change
• in chemical reactions, no atoms are created or destroyed
• a chemical equation shows what happens in a chemical reaction using symbols or words
• the chemicals that are reacting with each other are called the reactants, found on the left of equations
• the chemicals that are being made are called the products, found on the right of equations
• symbol equations must be balanced because atoms aren't created or destroyed during chemical reactions
• mixtures are made of 2+ substances that haven't been combined chemically
• mixtures can be separated and their chemical properties don't change because they have been mixed with another substance
• the air around us is a mixture of different gases, including oxygen, nitrogen and carbon dioxide
• we can normally separate mixtures and compounds into the elements or compounds found in then, using physical processes which don't create new chemical substances
• filtration separates mixtures that contain insoluble solids and liquids
• filtration is performed by pouring the mixture through a filter paper, causing the insoluble solid to be trapped by the filter paper and so the liquid runs through the filter paper and is collected below
• crystallisation separates solutions into their different parts: dissolved solids and liquids
• crystallisation is performed by heating the mixture so that the solvent evaporates, and eventually crystals of the solute will form and we can collect the solvent by condensing it as it evaporates
• chromatography separates solutions with a number of different solutes in the solvent
• in chromatography, you should 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 moves up the paper and carries the solutes in the solution with it
• in chromatography, different solutes move at different speeds, so they separate on the paper
• distillation is a technique used to separate mixtures of liquids
• distillation uses the different boiling points of liquids to separate them
• simple distillation separates 2 liquids with different boiling points, where the mixture is heated until the liquid with the lower boiling point starts to boil, and the vapour released passes through a condenser, where the gas cools back into a liquid
• simple distillation can also separate a solute from a solvent, when the solvent has a lower boiling point than the solute
• fractional distillation separates lots of liquids with different boiling points
• in fractional distillation, the mixture is slowly heated until the liquid with the lowest boiling point boils and then condenses, then we increase the temperature slowly to collect the other fractions
• in 1837, an english physicist called j.j. thomson discovered electrons
• thomson modelled the atom as a 'plum pudding' - a ball of positive charge (dough), with negatively charged electrons (currants) mixed in with the 'dough'
• in 1909, ernest rutherford discovered that alpha particles could bounce back off atoms
• rutherford concluded that an atom's mass is concentrated in the atom's centre which was called the nucleus and contained positively charged particles called protons
• niels bohr discovered that electrons orbit the nucleus at fixed distances
• in 1932, james chadwick discovered that some particles in the nucleus have no charge at all and called them neutrons
• the 3 different sub-atomic particles have different relative charges
• in an atom, all the charges of sub-atomic particles cancel each other out, meaning that atoms have zero overall charge
• protons have a relative charge of +1 and are found in the nucleus
• an element's atomic number is the number of protons it has
• electrons have a relative charge of -1 and around found in fixed orbits around the nucleus