chem defs

• Atom


  Smallest part of an element that shows the element's properties
• Element


  Substance that cannot be split into simpler substances by chemical means
• Triad/Law of Triads


  • A triad is a group of three elements that:
  • Have similar chemical properties
  • The atomic weight of the middle element is approximately equal to the average of the other two
• Octave/Law of Octaves


  • If the known elements are arranged in order of their atomic weights, the properties of the elements repeated every eight elements
• Atomic number


  Number of protons in the nucleus
• Mass number


  Total number of protons and neutrons present in the nucleus
• Isotope


  Atoms of the same element that have the same atomic number but different mass numbers
• Relative atomic mass


  Average mass of all the isotopes in an element taking abundances into account relative to 1/12th of the carbon 12 isotope
• Ion


  Atom that has an electrical charge
• Energy Level


  • Fixed/discrete amount of energy of an electron in an atom
• Ground State


  All the electrons are in the lowest available energy levels- most stable state of an atom
• Excited State


  Electrons have absorbed enough energy to be promoted to higher energy levels - unstable state, electrons will drop back down to lower energy levels
• Line (atomic) Emission Spectrum


  A series of coloured lines that correspond to specific frequencies of light emitted when electrons in an element are excited
• Sublevel/Energy sublevel


  Subdivision of a main energy level and consists of one or more atomic orbitals of equal energy
• Heisenberg's uncertainty principle


  • Impossible to measure at the same time both the exact position and velocity of an electron
• Atomic Orbital


  Space around the nucleus of an atom where there is a high probability of finding an electron
• Flame test


  1. Heating an element in a bunsen burner will cause that element to produce a characteristic colour i.e. give out light of a certain frequency

  2. The element can therefore be identified
• Aufbau Principle


  • Electrons must fill the lowest available sub-levels first
• Hund's rule (of maximum multiplicity)


  • Two or more orbitals of equal energy are available, the electrons must fill the orbitals singly before filling them in pairs
• Pauli's exclusion Principle


  • Two electrons in an orbital must spin in opposite direction
• Compound


  Two or more different elements chemically combined
• Molecule


  Two or more atoms chemically combined
• Octet Rule


  • When bonding, most atoms tend to "want to" reach an electron arrangement of eight electrons in their outer energy level
• Valency


  Number of atoms of hydrogen with which an atom of the element can bond
• Ionic Bond


  Electrostatic attraction between a positive and negative ion
• Covalent Bond


  Atoms share a pair of electrons
• Polar Covalent Bond


  Atoms share a pair of electrons unequally
• Non-polar Covalent Bond


  Atoms share a pair of electrons equally
• Sigma Bond


  Head-on overlap of two atomic orbitals
• Pi Bond


  Sideways overlap of p or d atomic orbitals
• Catalyst


  Alters the rate of a chemical reaction without being used up
• Atomic (Covalent) Radius


  Half the distance between the nuclei of two atoms of the same element joined by a single covalent bond
• Electronegativity


  Measure of relative attraction an atom has for a shared pair of electrons
• Ionisation Energy


  Minimum energy required to remove the most loosely bound electron from an atom or ion
• First Ionisation Energy


  Minimum energy required to remove the most loosely bound electron from a neutral gaseous atom in the ground state
• Second Ionisation Energy


  Minimum energy required to remove the most loosely bound electron from a positive ion
• Radioactivity


  Spontaneous breaking up of unstable nuclei with the emission of one or more types of nuclear radiation
• Radioisotopes


  Radioactive isotopes- isotopes with unstable nuclei and in an attempt to become more stable spontaneously decay and emit alpha, beta and gamma radiation.
• Background radiation


  Radiation naturally present in the environment
• Half-Life


  Time taken for half of the nuclei in any given sample to decay