chemistry - topic 1

chemistry - topic 1

1. atomic structure and the periodic table

2. Atoms

   • electrons
     • they move around the nucleus in electron shells
     • the number of the electrons is the same as the protons
     • they are negatively charged and tiny
     • they have virtually no mass
     • the volume of their orbits determines the size of the atom
   • in an ion, the number of protons not the same as the number of electrons
   • the nucleus
     • it has a positive charge
     • it contains protons and neutrons
     • most of its mass is concentrated in the nucleus
   • atomic number and mass number
     • the atomic number tells you how many protons there are
     • the mass number tells you the total number of protons and neutrons in the atom

3. elements

   • isotopes
     • isotopes are different forms of the same element which have the same number of protons but a different number of neutrons.
     • isotopes have the same atomic number but a different mass number
     • relative atomic mass is used instead of mass number when referring to the element as a whole as it is an average which takes into account the different isotopes of the different isotopes
   • relative atomic mass = sum of (isotope abundance x isotope mass number) sum of abundances of all the isotopes

4. compounds

   • creation of compounds
     • when elements react with each other they form compounds
     • they are formed from two or more elements
     • when elements create bonds with each other they give away, take or share electrons, the nucleus' are not affected at all
     • when a metal reacts with a non-metal with each other they produce ions - the metal atoms loose electrons and the non-metals gain them. the opposite charges mean the ions are strongly attracted to each other (ionic bonding)
     • non metals form compounds through covalent bonding where electrons are shared between the two atoms
     • compounds often have very different properties than that of the elements that make them up

5. mixtures and hromatography

   • mixures
     • there is no chemical bond between elements in mixtures
     • they can be made up of elements or compounds
     • examples are air or crude oil
   • chromatography
     • 1) draw a pencil line near the bottom of the filter paper
     • 2) add a spot of ink on the line and put the end of the paper into the solvent
     • 3) the solvent depends on what is being tested. some work well in water but some require another solvent like ethanol
     • 4) the solvent seeps up the paper carrying the ink with it
     • 5) the different dyes travel up the paper at different rates and it separates them out.

6. other sepparation techniques

   • filtration
     • you use filter paper to filter out the insoluble solid from the liqid
   • evaporation
     • 1) pour a solution into an evaporating dish
     • 2) slowly heat the solution
     • 3) the solvent will evaporate leaving behind the dry crystals
   • crystalisation
     • 1) pour the solution into an evaporation dish
     • 2) when you see crystals start to form remove from the heat then filter the crystals out of the solution
     • 3) dry the solution in a drying oven, a warm place or with a paper towel

7. distilation

   • simple distilation
     • used for separating out a liquid from a solution
     • the solution is heated and the liquid with the lowest boiling temperature evaporates first
     • the vapour then cools and condenses into another beaker
     • however you can only separate substances with very different boiling points
   • fractional distilation
     • it is used to separate a mixture of liquids
     • 1) put the solution into a flask and put a fractionating column on top
     • 2) the different liquids will have different boiling points so will evaporate at different points
     • 3) the liquid with the lowest boiling point evaporates first. when the temperature of the boiling point matches that on the thermoniter, it will reach the top of the column.
     • 4) the liquids with higher boiling temperatures might also begin to evaporate, but the column is cooler at the top so it will condense before it reached the top
     • 5) when the first liquid has been collected then you can increase the temperature until the next liquid reaches the top

8. the histiry of the atom

   • John Dalton
     • described atoms as solid spheres
   • J J Thompson
     • concluded that negatively charged particles (electrons) existed - 'plum pudding model'
   • Ernest Rutherford
     • fired alpha particles at a thin sheet of gold and found that particles were mainly made up of empty space with electrons randomly dotted around the empty space
   • Niels Bohr
     • proposed that electrons travelled around the nucleus of the atom in electron shells
   • further experiments proved the existence of protons and nutrons to come to the modern day model of the atom

9. periodic table

   • development of the periodic table
     • before the discovery of neutrons and therefore atomic number (which is how the modern periodic table is organised) elements were either organised by physical and chemical properties or their relative atomic mass.
     • early periodic tables were not complete and because they were ordered by their atomic mass, some elements were placed in the wrong group - not taking account of their properties
     • Dmitri Mendeleev - organised the periodic table by atomic mass but also taking into account their properties
   • the modern periodic table
     • organised in columns depending on the elements characteristics called 'groups'
     • the group number tells you how many electrons are in an elements outer shell and each row represents the number of electron shells it has
   • metals and non-metals
     • most elements are metals (mainly the transition metals)
     • when metals bond they form positive ions whereas non-metals form negative ions
     • non-metals tend to be duller,more brittle,not always solid at room temp, less dense and not conduct electricity
     • when metals bond they form metalic bonds which cause them to be strong, conductive, malleable , sonorous and have high boiling and melting points
   • group 1 elements
   • group 7 elements
     • halogens are all non-metals and have coloured vapours
     • as you go down group 7 the elements become less reactive and have higher boiling/melting points
   • group 0 elements