Definitions for OCR Chemistry, Atoms, bonds and groups.

All the definitions needed for the first chemistry exam OCR AS LEVEL Atoms bonds and groups. F321

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Unit Keywords Definitions
1.1.2 Isotopes Isotopes are atoms of the same element but different numbers of neutrons.
1.1.2 Atomic (proton) Atomic (proton) number is the number of protons in the nucleus of an atom.
1.1.2 Mass (nucleon) Mass (nucleon) number is the number of particles (neutron and protons) in the nucleus.
1.1.2 Ion An ion is a positively or negatively charged atom or (covalently bonded) group of atoms (a molecular ion).
1.1.3 Relative isotopic Relatively isotopic mass is the mass of an isotope compared with one twelfth of the mass of an atom of
mass carbon 12.
1.1.3 Relative atomic Relative atomic mass is the weighted mean mass of an atom compared to one twelfth of the mass of
mass carbon 12.
1.1.3 Relative molecular Relative molecular mass is the weighted mean mass of a molecule compared with one twelfth of the
mass mass of an atom of carbon 12.
1.1.3 Relative Formula Relative formula mass is the weighted mean mass of a formula unit compared with one twelfth of the
Mass mass of an atom of carbon-12.
1.1.4 Amount of Amount of substance is the quantity whose unit is the mole. Chemist's use "amount of substance" as a
substance means of counting atoms.
1.1.4 Avogadro constant The Avogadro constant is the number of atoms per mole of carbon 12 isotope (6.02 x 1023 mol-1.
1.1.4 Mole A mole is the amount of any substance containing as many particles as there are carbon atoms in exactly
12g of carbon -12 isotope.
1.1.4 Molar Mass Molar mass is the mass per mole of a substance. The units of molar mass are g mol-1.
1.1.5 Empirical Formula Empirical formula is the simplest whole number ratio of atoms of each element present in a compound.
1.1.5 Molecule A molecule is a small group of atoms held together by a covalent bond.
1.1.5 Molecular formula Molecular formula is the actual number of atoms of each element in a molecule.

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Molar Volume Molar volume is the volume per mole of a gas.
1.1.7 Concentration Concentration of a solution is the amount of solute, in mol, dissolved per 1dm3.
1.1.7 Standard Solution Standard solution is a solution of known concentration. Standard solutions are normally used in titrations to
determine the unknown information about another substance.
1.1.8 Species Species is any type of particles that takes part in a chemical reaction.
1.1.…read more

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First ionisation The first ionisation energy of an element is the energy required to remove on electron from each atom in
energy one mole of gaseous atoms to form one mole of gaseous 1+ ion.
1.2.1 Electron Shielding Electron is the repulsion between electrons in different inner shells. Shielding reduces the net attractive
force from the positive nucleus on the outer shell electrons.
1.2.1 Successive Successive ionisation energies are a measure of the energy required to remove each electron in turn. E.g.…read more

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Polar covalent A polar covalent bond has a permanent dipole.
1.2.11 Polar molecule A polar molecule has an overall dipole, when you take into account any dipoles across the bonds
1.2.12 Intermolecular An intermolecular force is an attractive force between neighbouring molecules.
1.2.13 Permanent A permanent dipole-dipole force is a weak attractive force between permanent dipoles in neighbouring
dipole-dipole polar molecules.
1.2.12 Van der Waals' Van der waals' forces are attractive forces between induced dipoles in neighbouring molecules.
1.2.…read more

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Precipitation A precipitation reaction is the formation of a solid from a solution during a chemical reaction. Precipitates
reaction are often formed when two aqueous solutions are mixed together.…read more


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