chemisrty

• Chemists use symbols and formulae to represent elements and compounds
• Word equations and balanced chemical equations represent the changes that happen in chemical reactions
• Methods to separate mixtures

  • Filtration
  • Crystallisation
  • Simple distillation
  • Fractional distillation
  • Chromatography
• The method chosen to separate a mixture depends on the type of mixture
• Atom


  Consists of a nucleus containing protons and neutrons, surrounded by electrons in shells
• The numbers of subatomic particles in an atom can be calculated from its atomic number and mass number
• Mendeleev made an early periodic table
• In the modern periodic table, elements are in order of atomic number in periods and groups
• Electronic structures


  Model how electrons are arranged in atoms
• Elements in the same group of the periodic table show trends in physical properties, such as boiling point
• Elements in the same group have the same number of electrons in their outer shell, so similar chemical properties
• Understanding how to approach exam questions helps to boost exam performance
• Question types will include multiple choice, structured, mathematical and practical questions
• Chemists use symbols and formulae to represent elements and compounds
• Word equations and balanced chemical equations represent the changes that happen in chemical reactions
• Atom


  The smallest part of an element that can exist
• Element


  A substance made of one type of atom only
• Chemical symbol


  • Consists of one or two letters
  • Always starts with a capital letter, with any other letter in lower case
• Chemical symbols


  • O (oxygen)
  • Na (sodium)
• There are over 100 different elements
• The names and symbols of the elements are shown in the periodic table
• Elements are arranged into groups with similar properties
• Groups are numbered from 1 to 7, then 0
• In the periodic table, metals are on the left of the stepped line, and non-metals are on the right
• There are different ways to separate mixtures, such as filtration, crystallisation, simple distillation, fractional distillation and chromatography
• The method chosen depends on the type of mixture
• Pure substance


  Consists only of one element or one compound
• Mixture


  Two or more different substances, not chemically joined together
• The substances in a mixture can be elements, compounds, or both</b>
• Being part of a mixture does not change the chemical properties of the substances that are in it
• Mixtures can be separated by physical processes
• Physical processes do not involve chemical reactions, and no new substances are made
• Ionic compounds have high melting points due to strong electrostatic forces of attraction between oppositely charged ions, while covalent compounds have low melting points as there is no charge separation.
• The strength of an ionic bond depends on the size of the ions involved, with smaller ions having stronger attractions than larger ones.
• Covalent bonds are formed by the sharing of electrons between atoms.
• Metals form positive ions when they react with non-metals, resulting in ionic bonds.
• Non-metal atoms share their outermost electron pairs to form covalent bonds, resulting in higher melting points compared to metals.
• Metals generally have lower melting points than non-metals because metals form metallic bonds with delocalised electrons, which allows them to flow freely through the lattice structure.
• Polar molecules have unequal sharing of electron pairs, resulting in partial charges on the atoms.
• Chemical changes occur when two or more substances react to produce one or more completely different substances.