chemistry definitions unit 1

• Atomic number


  number of protons in (the nucleus of) an atom.
• Mass numebr


  total number of protons and neutrons in (the nucleus of) an atom
• Isotopes


  atoms which have the same number of protons (so they are atoms of the same element) but they have a different number of neutrons (so they have a different mass number).
• Cation

  positive ion.
• Anion


  negative ion.
• Compound


  substance formed when two or more elements are chemically combined
• Ionic bond


  attraction between oppositely charged ions.
• Covalent bond


  formed by a shared pair of electrons
• Diatomic


  there are two atoms covalently bonded in a molecule
• Metallic bonding


  the attraction between delocalised electrons and the positive ions in a regular lattice
• Malleable


  can be hammered into shape
• Ductile


  can be drawn out into wires
• Alloy


  mixture of two or more elements at least one of which is a metal and the resulting mixture has metallic properties.
• Nanometre


  10-9 m.
• Nanoparticle

  is a structure that is 1 - 100 nm in size.
• Allotrope


  different forms of the same element in the same physical state.
• Element


  a substance that consists of only one type of atom and it cannot be broken down into anything simpler by chemical means
• Compound

  a substance that consists of two or more elements chemically combined
• Atom


  is the simplest particle of an element that can exist on its own in a stable environment.
• Molecule


  a particle that consists of two or more atoms chemically bonded together
• period


  a horizontal row in the Periodic Table
• group


  vertical column in the Periodic Table
• relative atomic mass


  s the mass of the atom compared with that of the carbon-12 isotope, which has a mass of exactly 12, and it is the weighted mean of the mass numbers.
• Percentage yield


  =actual yield/theoretical yield x 100
• Water of crystallisation


  water that is chemically bonded into the crystal structure.
• Hydrated

  solid crystals contain water of crystallisation
• Dehydration


  means removal of water of crystallisation.
• anhydrous


  substance does not contain water of crystallisation
• degree of hydration


  number of moles of water of crystallisation chemically bonded in 1 mole of the compound
• empirical formula

  simplest whole number ratio of the atoms of each element in a compound
• molecular formula


  actual number of atoms of each element in a molecule
• base


  substance that reacts with an acid producing a salt and water.
• alkali


  soluble base.
• strong acid


  ionises completely in water.
• weak acid


  ionises partially in water.
• strong alkali


  ionises completely in water
• weak alkali


  ionises partially in water.
• Neutralisation


  reaction between an acid and an alkali producing a salt and water
• salt


  compound formed when some or all of the hydrogen ions in an acid are replaced by metal ions or ammonium ions.
• pure substance


  is a single element or compounds (and is not mixed with any other substance).