Atomic Structure & Periodic Table

• ATOMS

  • atoms have a radius of about 0.1 nanometres
  • The nucleus is in the middle of the atom & contains protons and neutrons. It has a radius of around 1 × 10-14 m. It has a +ve charge because of the protons. Almost the whole mass of the atom is concentrated in the nucleus.
  • The electrons move around the nucleus in electron shells. They have a -ve charged and tiny, but cover a lot of space
  • atoms are neutral - have no overrall charge as they have same no of protons as electrons
  • the atomic number tells you how many protons there are
  • the mass number tells you the total number of protons & neutrons
• ELEMENTS

  • the number of protons in the nucleus decides what type of atom it is. All the atoms of a particular element have the same number of protons and different elements have atoms with different numbers of protons
  • 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 different mass numbers
  • because many elements can exist as a number of different isotopes, relative atomic mass is used instead of mass number when referring to the element as a whole
• formula to work out the relative atomic mass of an element:

  
• COMPOUNDS

  • compounds are substances formed from 2 or more elements, the atoms of each are in fixed proportions throughout the compound and they're held togther by chemical bonds
  • making bonds involves giving atoms away, taking or sharing electrons
  • it is usually difficult to separate the original elementsof a compound out again - a chemical reaction is needed to do this
• COMPOUNDS

  • a compound which is formed from a metal and a non-metal consists of ions. The metal atoms lose electrons to form +ve ions and the non-metal atoms gain electrons to form -ve ions. The opposite charges of the ions mean that they're strongly attracted to each other - this is called ionic bonding.
  • a compound formed from non-metals consists of molecules. Each atom shares an electron with another atom - called covalent bonding
  • the properties of a compound are usually totally different from the properties of the original elements
• MIXTURES

  • unlike in a compound, there's no chemical bond between the different parts of a mixture
  • the parts of a mixture can be either elements or compounds, and they can be separated out by physical methods, such as filtration, crystallisation, simple distillation, fractional distillation, and chromatography
  • the properties of a mixture are just a mixture of the properties of the separate parts - the chemical properties of a substance aren't affected by it being a part of a mixture
• PAPER CHROMATOGRAPHY

  • draw a line near the bottom of a sheet of filter paper in pencil
  • add a spot of the ink to the line and place the sheet in a beaker of solvent. Make sure the ink isn't touching the solvent
  • place a lid on top to stop the solvent evaporating
  • the solvent seeps up the paper, carrying the ink with it
  • each different dye in the ink will move up the paper at a different rates so the dyes will separate out.
  • if any of the dyes are insoluble, they'll stay on the baseline
  • when the solvent has nearly reached the top of the paper, take it out of the beaker and leave it to dry
• FILTRATION

  • filtration separates insoluble solids from liquids
  • it can be used in purification as well. For example, solid impurities in the reaction mixture can be separated out using filtration
• EVAPORATION - separating soluble solids from solutions

  • pour the solution into an evaporating dish
  • slowly heat the solution. The solvent will evaporate and the solution will get more concentrated. Eventually, crystals will start to form
  • keep heating the evaporating dish until all you have left are dry crystals
• CRYSTALLISATION - separating soluble solids from solutions

  • pour the solution into an evaporating dish and gently heat the solution. Some of the solvent will evaporate and the solution will get more concentrated
  • once some of the solvent has evaporated, or when you see crystals start to form , remove the dish from the heat and leave the solution to cool
  • the salt should start to form crystals as it becomes insoluble in the cold, highly concentrated solution
  • filter the crystals out of the solution, and leave them in a warm place to dry. You could also use a drying oven or dessicator
• filtration & crystallisation to separate rock salt:

  • grind the mixture to make sure the salt crystals are small, so will dissolve easily
  • put the mixture in water and stir. the salt will dissolve, but the sand won't.
  • filter the mixture. the grains of sand won't fit through the tiny holes in the filter paper, so they collect on the paper instead. The salt passes through the filter paper as it's part of the solution
  • evaporate the water from the salt so that it forms dry crystals
• SIMPLE DISTILLATION - separating out a liquid from a solution

  • the solution is heated. the part of the solution that has the lowest boiling point evaporates first
  • the vapour is then cooled, condenses, and is collected
  • the rest of the solution is left behind in the flask

  the problem with simple distillation is that you can only use it to separate things with very different boiling points - if the temperature goes higher than the boiling point of the substance with the higher boiling point, they will mix again
• FRACTIONAL DISTILLATION - used to separate a mixture of liquids

  • put your mixture in a flask and stick a fractionating column on top. Then heat it
  • the different liquids will all have different boiling points - so they will evaporate at different temps
  • the liquid with the lowest boiling point evaporates first. when the temp on the thermometer matches the boiling point of this liquid, it will reach the top of the column
  • when the first liquid has been collected, raise the temp until the next one reaches the top
• JJ THOMPSON

  • at the start of the 19C John Dalton described atoms as solid spheres and said that diff spheres made up different elements
  • in 1897 JJ Thompson concluded from his experiments that atoms weren't solid spheres. His measurements of charge and mass showed that an atom must contain even smaller, negatively charged particles - electrons. The 'solid sphere' idea of atomic structure had to be changed. The new theory was known as the 'plum pudding model'.
  • the plum pudding model showed the atom as a ball of positive charge with electrons stuck in it
• ERNEST RUTHERFORD-alpha particle scattering experiment

  • in 1909 Rutherford and his student Ernest Marsden conducted the alpha particle scattering experiments. They fired +ve charged alpha particles at an extremely thin sheet of gold.
  • from the plum pudding model, they were expecting the particles to pass straight through the sheet or be slightly deflected at most. This was because the positive charge of each atom was thought to be very spread out. Although most of the particles did go straight through the gold sheet, some were deflected more than expected, and some were deflected backwards.
• ERNEST RUTHERFORD-

  • he came up with an idea to explain this new evidence - the nuclear model of the atom
  • in this, there's a tiny, positively charged nucleus at the centre, where most of the mass is concentrated. A 'cloud' of negative electrons surrounds this nucleus - so most of the atom is empty space
  • when alpha particles came near the concentrated, positive charge of the nucleus, they were deflected. If they were fired directly at the nucleus, they were deflected backwards. otherwise, they passed through the empty space
• NIELS BOHR'S NUCLEAR MODEL:

  • scientists realised that electrons in a 'cloud' around the nucleus of an atom, as Rutherford described, would be attracted to the nucleus, causing the atom to collapse. Bohr's model of the atom suggested that all the electrons were contained in shells
  • Bohr proposed that electrons orbit the nucleus in fixed shells and aren't anywhere in between. Each shell is a fixed distance from the nucleus.
  • Bohr's theory of atomic structure was supported by many experiments and it helped to explain lots of other scientists' observations at the time
• FURTHER EXPERIMENTS -

  • further experiments by Rutherford and others showed that the nucleus can be divided into smaller particles, which each have the same charge as a hydrogen nucleus. These particles were named protons.
  • about 20 yrs after scientists had accepted that atoms have nuclei, James Chadwick carried out an experiment which proved evidence for neutral particles in the nucleus called neutrons. The discovery of neutrons resulted in a model of the atom which was pretty close to the modern day accepted version, known as the nuclear model
• EARLY PERIODIC TABLE

  • elements categorised by their atomic weight
  • early periodic tables were not complete and some elements were placed in the wrong group. This is because elements were placed in the order of atomic weight, and their properties not taken into account
• MENDELEEV

  • put the elements mainly in order of atomic weight but did switch that order if the properties meant it should be changed.
  • gaps were left in the table to make sure that elements with similar properties stayed in the same groups. Some of these gaps indicated the existence of undiscovered elements and allowed Mendeleev to predict what their properties might be. When they were found and they fitted the pattern it helped confirm Mendeleev's ideas
• THE MODERN PERIODIC TABLE

  • elements are in order of increasing atomic (proton) number
  • metals are found to the left and non-metals to the right
  • elements with similar properties form columns. These vertical columns are called groups. The group number tells you how many electrons there are in the outer shell.
  • the rows are called periods. each new period represents another full shell of electrons
• METALS AND NON-METALS

  • metals are elements which can form positive ions when they react
  • non metals don't generally form positive ions when they react

  All metals have metallic bonding which causes them to have similar basic physical properties:

  • they're strong, but malleable (can be bent or hammered into different shapes)
  • they're good conductors of heat and electricity
  • they have high boiling and melting points

  Non-metals tend to be dull looking, more brittle, aren't always solids at room temp, don't generally conduct electricity, and often have a lower density
• TRANSITION METALS

  • are good conductors of heat & electricity, and are very dense, strong, and shiny
  • can have more than one ion
  • are often coloured, so form coloured compounds
  • transition metal compounds often make good catalysts
• Group 1 elements-

  • are known as the alkali metals
  • all have 1 electron in their outer shell so are very reactive
  • all soft and have a low density

  As you go down group 1, the trends are:

  • increasing reactivity (the outer electron is more easily lost as the attraction between the nucleus and electron decreases, because the electron gets further away from the nucleus)
  • lower melting and boiling points
  • higher relative atomic mass
• ALKALI METALS form ionic compounds-

  these compound are generally white solids that dissolve in water to form colourless solutions
• Alkali metals - reaction with water

  • react vigorously to produce hydrogen gas and metal hydroxides
  • the more reactive (lower down in the group) an alkali metal is, the more violent the reaction
  • the amount of energy given out by the reaction increases down the group

  reaction with chlorine:

  • react vigorously when heated in chlorine gas to form white metal chloride salts
  • as u go down the group, reactivity increases so the reaction with chlorine gas gets more vigorous

  reaction with oxygen

  • they react with oxygen to form a metal oxide
• Group 1 metals VS Transition metals

  • group 1 metals are much more reactive
  • group 1 metals are much less dense, strong, and hard than the transition metals, and have much lower melting points.
• GROUP 7 elements:

  are all non-metals with coloured vapours

  • fluorine is a very reactive, poisonous yellow gas
  • chlorine is a fairly reactive, poisonous dense green gas
  • bromine is a dense, poisonous, red-brown volatile liquid
  • iodine is a dark grey crystalline solid or a purple vapour.

  they all exist as molecules - pairs of atoms

  as you go down the group, the halogens:

  • become less reactive - harder to gain an extra electron
  • have higher melting & boiling points
  • have higher relative atomic masses
• Group 7 elements - halogens

  • can share electrons via covalent bonding with other non-metals so as to achieve a full outer shell
  • the compounds that form when halogens react with non-metals all have simple, molecular structures
  • they form 1- ions called halides when they bond with metals. The compounds that form have ionic structures.
  • a displacement reaction can occur between a more reactive halogen and the salt of a less reactive one
• Group 0 elements - the noble gases

  • all have a full outer shell. As their outer shell is energetically stable they don't need to give up or gain electrons to become more stable. This means they are inert - they don't react with much at all
  • they exist as monatomic gases - single atoms not bonded to each other
  • all elements in Group 0 are colourless gases at room temperature
  • as the noble gases are inert they're non-flammable
• Properties of the Noble Gases

  • the boiling points increase as you move down the group along with increasing relative atomic mass
  • the increase in boiling point is due to the increase in the number of electrons in each atom leading to greater intermolecular forces which need to be overcome