Unit 1 - The Core Principle of Chemistry

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Atom: smallest part of an element that can take part in a chemical change ­ no charge, neutral
Element: cannot be broken down by chemical means into other substances
Ion: atom or group of atoms that becomes electrically charged by the loss or gain of electron(s)
Molecule: smallest part of a covalent compound
Compound: formed when two or more elements are chemically bonded together
Empirical Formula: shows the simplest whole number ratio of the atoms of different elements in a
compound
Molecular Formula: shows the number of atoms of the different elements in one molecule of a
compound
Relative Atomic Mass: average (weighted) mass of the element's isotopes relative to 1/12th of the
mass of a carbon-12 atom
Amount of Substance: chemical amount
Molar Mass: mass of 1 mole of substance (g)
Enthalpy Change: overall energy exchanged with the surroundings when a change happens at
constant pressure and the final temperature is the same as the starting temperature
Standard Enthalpy Changes of Reaction: energy transferred when the molar quantities of
reactants as stated in the equation react under standard conditions
Standard Enthalpy Changes of Formation: enthalpy change when 1 mole of the compound is
formed from its elements under standard conditions
Standard Enthalpy Changes of Combustion: enthalpy change when 1 mole of the substance is
completely burnt in oxygen under standard conditions
Standard Enthalpy Changes of Neutralisation: enthalpy change when 1 mole of water is formed
by the reaction of an acid with an alkali under standard conditions
Standard Enthalpy Changes of Atomisation: enthalpy change when 1 mole of gaseous atom is
formed from the element under standard conditions
Lattice Energy: energy released when 1 mole of an ionic crystal is formed from its ion in the gaseous
state under standard conditions
Hess's Law: the total enthalpy change for a reaction is independent of the route taken, provided the
initial and final conditions are the same
Bond Enthalpy: energy required to break one mole of the bonds in a substance in the gaseous state
Mean Bond Enthalpy: mean bond dissociation enthalpy for a particular bond in a range of
compounds ­ can be used to calculate enthalpy change for reactions

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Relative Isotopic Mass: mass of an atom of an isotope of an element on a scale where an atom of
carbon-12 is 12
Relative Molecular Mass/Relative Formula Mass: sum of the relative atomic masses of all the
atoms in its molecular formula
First Ionisation Energy: energy needed to remove 1 electron from each atom in 1 mole of gaseous
atoms to form 1 mole of gaseous 1+ ion
Second Ionisation Energy: energy needed to remove 1 electron from each ion in 1 mole of gaseous…read more

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