Chemistry Defs
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| Compound | is a substance that is made up of 2 or more elements chemically combined
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| Molecule | a group of atoms joined together and is the smallest part of an element that can exist independently
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| Electronegativity | The relative force of attraction an atom has over a shared pair of electrons in a covalent bond
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| Ionic bond | The force of attraction between oppositely charged ions in a compound
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| Ion | a charged atom or group of atoms
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| Electronegativity difference | <.4 non polar POLAR >1.7 ionic
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| Valency | the number of atoms of hydrogen or any other monovalent element with which each atom of the element combines
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| Aliphatic Compound | is an organic compound that consists of open chains of carbon atoms and closed chain compounds that resemble them in chemical properties
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| Octet Rule | states that when bonding occurs, atoms will try to reach an electron arrangement with 8 in its outer shell
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| Octane Number | measure of the tendency of a fuel to resist knocking
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| Catalytic Cracking | taking a long chain hydro-carbon molecule and breaking it into short chain compounds for which there is a greater demand
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| Homogulous Series | a series of compounds with uniform chemical type, showing gradation in physical properties, differing from the last by a CH2 unit, having a similar method of preparation having a general formula.
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| An aromatic compound | compounds that contain a benzene ring
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| Mass number | the sum of the number of protons and neutrons in he nucleus of an atom.
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| Isotopes | atoms of the same element that have different mass numbers due to different numbers of nuetrons in the nucleus.
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| Relative atomic mass | average of the mass numbers of all the isotopes of an element, as they occur naturally, taking their abundancies into account, and expressed on a scale in which the carbon-12 isotope has a mass of exactly 12 units.
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| The Afbau Principle | When building up the electronic configuration of an atom in its ground state, the electrons must occupy the lowest possible available energy level.
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| Hunds Rule of Maximum Muplicity | states that when 2 or more orbitals of equal energy are available, the electrons will occupy them singly before filling them in pairs.
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| The Pauli Exclusion Principle | states that no more than 2 electrons may occupy an orbital and they must have opposite spin.
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| Newlands Octaves | groups of elements arranges in order of increasing atomic weight where the first and eighth element of each group have similar properties
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| Mendeleevs Periodic Law | when elements are arranges in order of increasing atomic weight, the properties vary periodically
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| The atomic number | the number of protons in the nucleus of the atom
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| The modern periodic table | arrangement of elements in order of increasing atomic number
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| An energy level | fixed energy value that an electron in an atom may have
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| E=hf | Energy = planks constant x frequency of light
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| orbital | region of space in which there is a high probability of finding an electron
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| An element | a substance that cannot be split into simpler substances by chemical means
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| A triad | a group of 3 elements in which the middle element's atomic weight is approximately equal to the average of the other two.
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| Bond energy | the energy required to break one mole of covalent bonds and to separate the nuetral atoms completley from each other
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| Heat of neutralisation | the heat change when one mole of H+ ions from an acid reacts with 1 mole of OH- ions from a base
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| A transition metal | forms at least one ion with a partially filled d-sublevel
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| Van der Waals forces | weak attractive forces between molecules resulting from the formation of temporary dipoles.
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| Dipole-dipole | forces of attraction between negative pole of one molecule and the positive pole of the other
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| Hydrogen Bonding | type of dipole dipole force that occurs when hydrogen bonds with nitrogen, oxygen and fluorine. The partial positive pole of the hydrogen is attracted to the electronegative atom in another molecule. It acts as a bridge between electro-negative atoms.
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| Heat of reaction | heat change when the numbers of moles of reactants, as indicated in the balanced equation for the reaction react together completely
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| Heat of combustion | heat change when one mole of the substance in completley burned in excess oxygen
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| The kilogram calorific value | the heat energy produced when 1 kg of the substance is completley burned in oxygen.
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| Radioactivity | the spontaneous breaking up of nuclei with the emmission of one or more types of radiation
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| When an atom loses an alpha particle | it changes into an atom of an element two places before it in the periodic table. The mass number decreases by 4.
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| When an atom loses a beta particle | it changes into an atom of an element one place after it on the periodic table. The mass number stays the same
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| The half life of an element | the time taken for half the nuclei in any given sample to decay
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| The atomic radius | covalent radius is defined as half the distance between two nuclei of the same element bonded together by a single covalent bond
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| The values of atom radius increase | down any group...new shell and screening effect of inner electrons
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| The values of of atomic radius decrease | left to right along a period due to increasing nuclear charge and no screening effect.
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| The first ionisation energy | energy required to remove the most loosely bound electron from a nuetral gaseous substance.
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| The value of electronegativity | decrease down the groups: increasing atomic radius and screening effect of inner electrons
increase along the periods: increasing nuclear charge and decreasing atomic radius
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| Chemical Equilibrium | a state of dynamic balance whereby the rate of forward reaction is equal to the rate of the backward reaction
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| If a stress is applied to a system at equilibrium... | the system readjusts to relieve the stress applied
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| Le Chateliers principle | predicts that in an all gaseous reaction an increase in pressure will favour the reaction which takes place with a reduction in volume. i.e.. toward the side with the smaller amount of molecules.
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| An element | is a substance that cannot be split up into simpler substances by chemical means.
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| A triad | is a group of 3 elements with similar chemical properties which the atomic weight of the middle element is approx equal to the average of the other two.
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| Newland's Octaves | group of elements arranged in order of increasing atomic weight where the first and eighth element of each group have similar properties
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| Atomic number | number of protons in the nucleus
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| Mass number | sum of the protons and nuetrons in the nucleus
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| Isotopes | atoms o the same element that have different mass numbers due to differing numbers of nuetrons in the nuecleus.
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| Relative Atomic Mass | average of all the mass numbers of the isotopes of that element, as they occur naturally, taking their abundancies into account and expressed on a scale in which the atoms of the carbon 12 isotope has a mass of exactly 12 units.
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