Chemistry topic 1



  • the nucleus is where most of the mass is concentrated
  • an atom has the radius of 1x10-10m and it neutral because opposite charges cancel out, a nucleus has a radius of 1 × 10-14 m it has a positive charge due to having protons
  • protons have a mass of 1 and a +1 charge
  • neutrons have a mass of 1 and a 0 charge 
  • electrons have a mass of 0 and a -1 charge
  • electrons orbit the nucleus in shells 
  • number of protons= number of electrons
  • atoms have no overall charge
  • atomic number tells you how many protons there are
  • mass number tell you the total number of protons + neutrons
  • mass number- atomic number= amount of neutrons
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  • An element is a substance that is made entirely from one type of atom
  • element are atoms with the same atomic number
  • there is around 100 elements
  • isotopes are different forms of the same element 
  • isotopes have the same number of protons but different amount of neutrons
  • isotopes have the same atomic number but different mass number
  • relative atomic mass is the average mass taking into account abundances and masses
  • relative atomic mass (Ar)= sum of (abundance x mass)/ sum of abundances of all the isotopes
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  • compounds are substances formed from 2+ elements, which are in fixed proportions and held together by chemical bonds
  • making bonds involves giving away, taking or sharing electrons
  • its usually difficult to seperate compounds, a chemical reaction is needed 
  • the properties of a compound are usually different from its original elements 
  • a formula shows what elements are in a compound
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  • chemical changes are shown using chemical equations
  • methane + oxygen -> carbon dioxide + water
  • in this reaction methane and oxygen are the reactants and carbon dioxide and water are the products
  • symbol equations need to be balanced meaning they need the same number of atoms on the reactants side as the products side
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  • a mixture contains two or more substances that are not chemically combined
  • mixtures can be seperate using physical methods such as filtration, crystalisation, chromatography, simple and fractional distillation
  • air is a mixture of mainly nitrogen, oxygen, carbon dioxide and argon
  • crude oil is a mixture of different length hydrocarbons
  •  properties of a mixture are a mixture of the properties of the elements that form it
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  • chromatography is a way of seperating dyes in an ink
  • it involves drawing a pencil line at the the bottom of the chromatography paper and adding a spot of ink the the line then placing this in  a beaker of water making sure the line isnt covered by the water or ethanal (solvent).
  • place a lid on the beacker to stop the solvent evapourating
  • the solvent seeps up the paper carrying the ink.
  • when the solvent has nearly reached the top of the paper, take it out and leave it to dry. 
  • the end result is a chromatogram
  • a pure substance will have only one spot
  • the stationary phase is the chromatography paper
  • the mobile phase is the solvent
  • the solvent used depends on whats being tested, some compounds dissolve well in water, but sometimes other solvents like ethanol are needed
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  • filtration is used to seperate insoluble solids from liquids
  • evapouration is used to seperate soluble solids from solutions, as its heated the solvent will evapourate and the solution will get more concentrated, crystals are formed as the product
  • crystalisation is used to seperate soluble solids from solutions but is used when the salt decomposes when heated, at the point of crystalisation take it off the heat and allow to cool the salt should form crystals as it becomes insoluble in the cold, highly concentrated solution, after this filter the crystals and leave them in the warmth to dry
  • filtration and crystalisation can be used to seperate rock salt, rock salt is more commonly known as grit which consists of salt and sand 
  • grind the mixture and mix with water, stir this until the salt dissolves. filter the mixure to get rid of the sand and then evaporate the water from the salt leaving dry crystals
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  • simple distillation is used to seperate out solutions, the solution is heated and the past of the solution with the lowest bp evapourates first. the vapour then cools and condenses and is collected. the rest of the solution is left behind in the flask. this can be used to get pure water from seawater
  • the problem with simple distillation is that you can only seperate things with very different bp because if theyre similar they might mix again
  • fractional distillation is used to seperate a mixture of liquids eg crude oil. put your mixture in a flask and put a fractionating collumn on top, then heat. the different liquids will have different bp so they will evapourates at different times
  • the column is cool at the bottom and hot at the top 
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  • at the start of the 19th century, john dalton described atoms as solid spheres
  • in 1897 j j thomson concluded they werent spheres, he said that they contain electrons and made up the plum pudding model in which a ball of positive charge had electrons embedded in it 
  • in 1909 rutherford and marsden did the alpha particle scattering experiments where they fired positively charged alpha particles at a thin sheet of gold. they expected it to pass through but some deflected, which shown evidence of a nucleus. he made the nuclear model in which theres a postively charged nucleus which most of the mass is found, a cloud of negative electrons surrounds this 
  • bohr proved the existance of electron shells. if it were a cloud, the electrons would be attracted to the nucleus and the atom would collapse. he proposed that each shell is a fixed distance from the nucleus
  • rutherford did further experiments which found protons
  • chadwick found neutral particles called neutrons, we use the nuclear model
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  • electrons always occupy shells 
  • lowest energy shells are filled first, these are the ones closest to the nucleus
  • a maximum of 2 in the 1st shell 8 in the 2nd and 8 in the 3rd
  • atoms are happier with a full shell like noble gases in group 0
  • in most atoms the outer shell is not full so htey try to react to fill it
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  • in the early 1800's, elements were arranged by atomic mass
  • scientists had no idea of atomic structure or protons
  • early periodic tables were not complete and some elementswere placed in the wrong place this is because they didnt take into account the properties of the elements
  • in 1869 mendeleev left gaps and predicted new elements
  • he put elements in order of atomic mass however did switch some so that the properties mathced
  • eg Te and I, iodine has a smaller atomic mass but is placed after tellurium 
  • when the elements were discovered and fitted his table it was the accepted version of the periodic table 
  • he predicted ekasilicon which we know today as germanium
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  • metals are found to the left of the zigzag line and non metals are found to the right
  • elements with similar properties form columns called groups
  • the group number tells you the amount of electrons in the outer shell, the exception being group 0 which all have a full outer shell
  • the rows are called periods, each period represents another full shell of electrons
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  • metals are elements which form positive ions when they react
  • non metals generally form negative ions when they react
  • outer electrons that are further away from the nucleus feel a weaker attraction meaning that not much energy is needed to remove the elctrons
  • non metals have outer electrons which are close to the nucleus meaning they feel a strong attractionso its harder for them to form ions
  • all metals have metallic bonding which causes them to have similar basic physical properties including; being strong, malleable, good conductors of heat and electricity and they also have high melting and boiling points
  • non metals tend to be dull looking, more brittle, arent always soldi at rtp, they usually have a lower density and dont generally conduct electricity
  • transition metal are found between group 2 and 3, they have the normal properties of a metal but... they can have more than one ion, are often coloured and compounds that contain the are colourful, they also make good catalysts 
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  • they are named the alkali metals and all have 1 electron in their outer shell which makes them very reactive and gives them similar properties
  • as you go down the group the outer electron is more easily lost as the attraction between the nucleus and the electrons decrease because it gets further away as you go go down, they form +1 ions
  • lower melting and boiling points as you go down
  • higher relative atomic mass as you go down
  • in a reaction with water they act vigorously to produce a metal hydroxide and hydrogen, the lower down the more violent the reaction is, the amount of energy given out by the reaction also increases as you go lower
  • a reaction with chlorine reacts vigorously when heated in chlorine gas to form white metal chloride salts, as you go down the reaction is more vigorous
  • a reaction with oxygen results in a metal oxide
  • group 1 metals are much more reactive than transitional metals, they are also less dense, strong and hard and have lower boiling points
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  • the group 7 elements are also called the halogens
  • halogens have cloured vapours, fluorine is a very reactive poisonous yellow gas, chlorine is a fairly reactive poisonous dense green gas, bromine is a dense poisonous red borwn volatile liquid and iodine is a dark grey crysalline solid or a purple vapour
  • they all exist as diatomic (go around in pairs)
  • as you go down they become less reactive because the outer shell is further from the nucleus, they also have a higher melting and boiling points and have higher relative atomivc masses
  • they form -1 ions called hailides and the compounds that form have ionic structures
  • a displacement reaction can occur between a more reacive one and a less reactive one
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  • they are also called the noble gases and are all colourless
  • they all have a full outer shell, this means they dont really react much
  • they exist as monatomic gases which are single atoms not bonded to each other
  • they are non flammeable 
  • as you go down the boiling point increases, this is because there is an increase in the number of electrons in each atom leading to greater intermolecular forces between them which need to be overcome
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