C1 - STRUCTURE + HISTORY

• Atom


  • All matter is made of atoms.
  • Atoms themselves are made of smaller subatomic particles called protons, neutrons and electrons
• Atoms are very small – approximately 0.10.1 nanometers in diameter (1×10^-10 metres)
• atomic structure

  
• The nucleus contains an atom’s protons and neutrons, while an atom’s electrons orbit the nucleus in the electron shells
• proton = +1 charge and mass of 1
• neutron = no charge and a mass of 1
• electron = -1 charge and mass of 0.005
• In any individual atom, the number of protons is equal to the number of electrons, so atoms have no overall electrical charge (positive charges are cancelled out by an equal number of negative charges).
• mass number = proton + neutron

  atomic symbol = element

  atomic number = number of electrons

  number of electrons = number of protons

  
• 1. Each electron shell must be filled before a new shell can begin to be filled up.
  2. The first electron shell can hold a maximum of 2 electrons.
  3. Every subsequent electron shell can hold a maximum of 8 electrons.

  
• Electrons orbit the nucleus in electron shells, which have fixed distances from the nucleus. Electron shells that are closer to the nucleus house lower energy electrons.
• protons determine what element an atom is
• Isotopes are atoms with the same number of protons, but different numbers of neutrons
• Molecule


  A group of two or more atoms that are held together by chemical bonds
• Compound


  A substance that contains two or more different elements, with the atoms held together by chemical bonds
• Molecules


  Can be compounds if they contain two or more different elements
• Helium existing as a single atom would not count as a molecule
• Compounds


  • The elements involved are always found in the same proportions
• Chemical formula


  Uses chemical symbols and numbers to represent the composition of a compound
• Some compounds can contain millions or billions of atoms
• Non-molecular compounds


  Compounds where the atoms are held together by ionic bonds, forming large structures rather than discrete molecules
• Mixture


  Two or more substances that are not chemically combined, just physically mixed together
• Mixtures can be separated using physical methods like filtration, crystallization or distillation
• Everything is made up from tiny little particles that can't be broken down any further and that they're separated from each other by empty space
• Atomic theory


  The idea that everything is made up from tiny little particles that can't be broken down any further and that they're separated from each other by empty space
• Atomic theory originally proposed by Democritus

  Around 500 BC
• John Dalton


  Described atoms as solid spheres and suggested that different types of spheres might make up the different elements
• J.J. Thompson came up with the plum pudding model


  1897
• J.J. Thompson's experiments showed that atoms simply couldn't be solid spheres and instead that they must have contained negatively charged particles which we now know to be electrons</b>
• Ernest Rutherford and his students made a big discovery


  1909
• Rutherford's experiment


  1. Took positively charged alpha particles and fired them at a thin sheet of gold

  2. Some alpha particles were deflected to the side and a small number were even deflected back
• Rutherford's nuclear model


  Proposed that the positive charge of the atom was concentrated in a compact nucleus, with the negative charge existing in a cloud around this central nucleus
• Rutherford's model had one important flaw - there didn't seem to be anything stopping the cloud of negative electrons from rushing in towards the positive nucleus, meaning the atom should just automatically collapse
• Niels Bohr suggested a solution to the flaw in Rutherford's model


  1913
• Bohr's model


  Suggested the electrons orbited the nucleus in a similar way to how the planets orbit the sun, and that they were held in shells
• Bohr's model of the atom, with the electrons orbiting the nucleus, is what prevents the atom from collapsing
• Further experiments by Rutherford found that the positive charge in the nucleus is actually made up of small discrete particles which we now know as protons
• A short while later, James Chadwick provided evidence for neutral particles in the nucleus which we now call neutrons
• The current model of the atom is pretty much the same as Bohr's model, with just a few small changes based on further experiments
• Separating soluble and insoluble solids from liquids

  1. Filtration

  2. Evaporation

  3. Crystallization