Elements, compounds, mixtures & molecules

• Hydrogen, oxygen, nitrogen and group 7 elements such as iodine, chlorine, bromine and fluorine exist as diatomic molecules so they are written with 2 atoms
• A molecule is a group of two or more atoms that are held together by chemical bonds e.g O2, CO2, N2, H2O
• Molecules can contain different elements not just one e.g O2 is a molecule with one element and H2O is a molecule with two elements
• Compounds are pure substances that contain 2 or more different elements chemically bonded together
• Table salt is a compound called sodium chloride which has the formula NaCl
• The difference between compounds and molecules is that compounds are made up of two or more different elements and molecules are made up of 2 or more atoms which could be the same type (O2) or different (CO2)
• Pure substances contain only one element or compound and are not mixed with anything else e.g., water that contains H2O molecules is pure, whereas if salt is added to it then becomes impure.
• • Ionic compounds contain metal and non-metal elements joined together as particles called ions
  • The metal element’s symbol is always written first
• • The non-metal element always takes on the name ending ‘– ide’ unless oxygen is also present,
  • For example, PbS is called lead sulfide and MgCl2 is called magnesium chloride
  • When oxygen is present the name ending is usually ‘-ate’
  • For example, CuSO4 is copper sulphate, KClO3 is potassium chlorate and Na2CO3 is sodium carbonate
• • A few covalent compounds are known by their common names, and you are expected to learn them, for example:
  • H2SO4 is sulfuric acid
  • NH3 is ammonia
  • CH4 is methane
  • HCl is hydrochloric acid (or hydrogen chloride if it is a gas)
  • C2H5OH is ethanol
  • HNO3 is nitric acid
• Covalent compounds contain non metals
• There are about 100 different elements
• The atom is the smallest part of an element that can exist
• • Elements take part in chemical reactions in which new substances are made in processes that most often involve an energy change
  • In these reactions atoms combine together in fixed ratios that will give them full outer shells of electrons, often producing compounds
• • There is an unlimited number of compounds, and the chemical formula is what tells you the ratio of atoms in a compound
  • For example, H2O is a compound containing 2 hydrogen atoms and 1 oxygen atom
• • The properties of compounds are usually quite different from the elements that form them
• The number of electrons changes during resections due to the atom losing or gaining electrons but the atomic number stays the same