Chem - end of years

• Acids release hydrogen ions into water solutions while bases accept them
• The pH of a neutral solution is 7
• A mixture consists of two or more elements or compounds not chemically combined together. The chemical properties of each substance in the mixture are unchanged.
• An atom is the smallest part of an element that can exist.
• An alloy is a solid material consisting of two or more metals, metal and non-metal, or metalloid fused together.
• A compound is formed when atoms from different elements combine to form a new substance with unique physical and chemical properties that cannot be separated by simple physical means.
• Mixtures can be separated by physical processes such as filtration, crystallisation, simple distillation, fractional distillation and chromatography
• Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided.
• Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided.
• The results from the alpha particle scattering experiment led to the conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged. This nuclear model replaced the plum pudding model.
• The experimental work of James Chadwick provided the evidence to show the existence of neutrons within the nucleus.
• The sum of the protons and neutrons in an atom is its mass number
• The relative atomic mass of an element is an average value that takes account of the number of the isotopes of the element.
• To calculate the relative atomic mass:

  1. Multiply the % abundance of each isotope by its mass.
  2. Add these numbers together.
  3. Divide by the total abundance (which when using % abundance, will be 100%)
• The elements in the periodic table are arranged in order of atomic (proton) number
• Elements in the same group in the periodic table have the same number of electrons in their outer shell (outer electrons) and this gives them similar chemical properties.
• Mendeleev overcame some of the problems by leaving gaps for elements that he thought had not been discovered and in some places changed the order based on atomic weights.
• Elements that react to form positive ions are metals.
• Elements that do not form positive ions are non-metals.
• The elements in Group 0 of the periodic table are called the noble gases
• noble gases are unreactive and do not easily form molecules because their atoms have stable arrangements of electrons.
• Lithium reacts with water because it is a highly reactive metal that loses an electron to form a positive ion.
• The elements in Group 7 of the periodic table are known as the halogens and have similar reactions because they all have seven electrons in their outer shell.
• In Group 7, the reactivity of the elements decreases going down the group.
• In Group 7, the further down the group an element is the higher its relative molecular mass, melting point and boiling point.
• Many transition elements have ions with different charges, form coloured compounds and are useful as catalysts.
• There are three types of strong chemical bonds: ionic, covalent and metallic.
• ionic bonding the particles are oppositely charged ions.
• covalent bonding the particles are atoms which share pairs of electrons
• metallic bonding the particles are atoms which share delocalised electrons.
• Ionic bonding occurs in compounds formed from metals combined with non-metals.
• Covalent bonding occurs in most non-metallic elements and in compounds of non-metals.
• Metallic bonding occurs in metallic elements and alloys.
• In chemical equations, the three states of matter are shown as (s), (l) and (g), with (aq) for aqueous solutions.
• the 2 forces are attraction and repulsion that exist between 2 molecules and this is called intermolecular force
• In pure metals, atoms are arranged in layers, which allows metals to be bent and shaped.
• Pure metals are too soft for many uses and so are mixed with other metals to make alloys which are harder.
• Metals are good conductors of electricity because the delocalised electrons in the metal carry electrical charge through the metal.
• Metals are good conductors of thermal energy because energy is transferred by the delocalised electrons.
• we use symbols and formulae to represent elements and compounds