Atomic structure

• Elements 


  All chemicals are elements and can be found in the periodic table

  Elements consist of one type of atom only and they are also the simplest form of substance that you can find

  Atoms of each element are represented by a chemical symbol

  There are about 100 different elements
• Molecules


  In some elements the atoms are joined in groups of 2 or more

  A particle containing atoms grouped in this way is called a molecule

  Molecules can be compounds or elements
• Compounds


  Formed from elements by chemical reactions

  Chemical reactions always involve the formation of one or more new substances and often involve a detectable energy change

  Compounds contain 2 or more elements chemically joined in fixed proportions and can be represented by formulae using the symbols of the atoms from which they are formed. Compounds can only be separated into elements by chemical reactions.
• Chemical reactions 


  Chemical reactions can be represented by word equations or equations using symbols or formulae.
• Students should be able to

  Use names and symbols of first 20 elements, Group 1 and 7 elements and any other elements in spec

  Name compounds of these elements from given formulae or symbol equations

  Write word equations, formulae and balanced chemical equations for reactions in spec

  Write balanced half and ionic equations when appropriate
• Mixtures


  Consists of 2 or more elements or compounds not chemically combined together

  The chemical properties of each substance in the mixture are unchanged
• Mixtures


  Can be separated by physical processes such as filtration, simple and fractional distillation, crystallisation and chromatography

  These physical processes don’t involve chemical reactions and no new substances are made
• Students should be able to

  Describe explain and give examples of the specified processes of separation

  Suggest suitable separation and purification techniques for mixtures when given appropriate information.
• Solids


  Particles are close together

  Particles are regularly arranged

  Particles vibrate
• Liquids


  Particles are slightly further apart

  Quite disordered and changes positions with frequent collisions
• Gases


  Particles are very far apart

  Totally randomly arranged

  Fast moving with occasional collisions
• Limitations of particle model

  All particles are solid spheres - reality is that atoms are mainly empty space and not always spherical

  No forces between particles - reality is that atoms have interactions which can distort their shape

  All particles are the same size - reality is that sizes of particles in substances can vary
• What happens as water freezes?

  At room temp water particles have a lot of KE

  These water particles can move past one another

  When water is put in freezer the temp of the water begins to drop

  The water particles have less KE

  The temp reaches 0

  The particles form an orderly pattern and can no longer move past one another meaning the water becomes a solid called ice
• Changes of state


  Ice to water is melting

  Water to steam is boiling

  Steam to water is condensing

  Water to ice is freezing
• Cooling curve for gas

  Diagram

  
• Heating curve for gas

  Diagram

  
• Sublimation 


  Process where solid turns to gas and gas turns to solid without converting to liquid inbetween
• Evaporation 


  Temp humidity and wind speed affect rate of evaporation

  Faster molecules at the surface of the liquid can escape from the water as a gas - this is evaporation

  Evaporation is also a physical process that changes liquid to gas
• Evaporation


  SA and amount of liquid affect rate of evaporation - more temp, faster moving molecules, more evaporation

  More humidity = less evaporation

  Wind speed = more wind = more evaporation

  SA = more SA = less evaporation

  More amount = less evaporation, less amount = more evaporation
• Difference between boiling and evaporating

  Boiling occurs at boiling point and occurs throughout the liquid

  Evaporating occurs at a temp below boiling point and only occurs at the surface of the liquid
• History of the atom - 1800 - Atomic Theory by Dalton

  Atoms are tiny particles that make up elements

  Atoms can't be divided

  All atoms of the elements are the same, and are different from other elements
• 1900 - Plum Pudding model by JJ Thomson


  Amorphous blob of positive charge with electrons embedded inside

  Cathode rays were streams of electrons
• 1907 - Nuclear Model by Rutherford


  The results from the alpha particle scattering experiment (alpha particles were fired at gold foil and some deflected while others repelled)led to the conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was positively charged.
• 1913 - Atomic Model by Bohr


  Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. The theoretical calculations of Bohr agreed with experimental observations.

  Outer orbits hold more than inner orbits
• 1932 - Chadwick - Nuetrons in nucleus with nuetral charge


  Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge. The name proton was given to these particles.

  The experimental work of James Chadwick provided the evidence to show the existence of neutrons within the nucleus. This was about 20 years after the nucleus became an accepted scientific idea.

  There are no electrons in the nucleus
• Structure of an atom

  Rutherford showed that an atom is made up of 3 sub atomic particles - protons, nuetrons and electrons.

  In an atom, the number of electrons is equal to the number of protons in the nucleus. Atoms have no overall electrical charge.
• Relative charges


  Table

  
• Atomic and Mass number

  Atomic number - number of protons in nucleus (can also be used to find number of electrons in atom)

  Mass number - number of protons and nuetrons in nucleus

  Mass number is always the bigger number while atomic number is always the smaller number
• Number of protons


  The number of protons in an atom of an element is its atomic number. All atoms of a particular element have the same number of protons. Atoms of different elements have different numbers of protons.
• Size of atoms


  Atoms are very small, having a radius of about 0.1 nm (1 x 10-10 m).

  The radius of a nucleus is less than 1/10 000 of that of the atom (about 1 x 10-14 m).

  Almost all of the mass of an atom is in the nucleus.
• Relative mass


  Table

  
• Atomic and mass number


  Further spec notes

  
• Isotopes


  All atoms of the same elements have the same number of protons. The proton number identifies the element.

  However the number of nuetrons can vary.

  Atoms with the same number of protons but different number of nuetrons are isotopes.
• Isotopes of hydrogen


  Hydrogen 1 - most common atom - has one electron and one proton but no nuetron

  Hydrogen 2 - Deuterium - I proton and 1 nuetron and 1 electron

  Hydrogen 3 - Tritium - 1 electron and 1 proton but 2 nuetrons
• Relative atomic mass


  The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element.

  Students should be able to calculate the relative atomic mass of an element given the percentage abundance of its isotopes
• RAM


  Formula

  
• Electron shells


  Atoms have 4 shells where electrons can live

  Shell 1 - 2 electrons

  Shell 2 - 8 electrons

  Shell 3 - 8 electrons

  Shell 4 - remaining electrons (up to 18)
• Electron shells


  Diagram

  
• Electronic structure


  The electrons in an atom occupy the lowest available energy levels (innermost available shells). The electronic structure of an atom can be represented by numbers or by a diagram.
• Conservation of Mass


  New substances are made during chemical reactions

  However the same atoms are present before and after a reaction

  The total mass of reactants is always equal to the total mass of products

  This idea is known as The Law of Conservation of Mass