definitions - mocks 2024

energy is

is the fixed energy value that an electron in an atom may have.

Heisenberg uncertainty principle

is impossible to measure at the same time both the velocity and the position of an electron.

an orbital

is a region in space within which there is a high probability of finding an electron.

an element

is a substance that cannot be split up into simpler substances by chemical means.

is a substance all of whose atoms have the sameatomic number

The periodic law:

when elements are arranged in order of increasing atomic number the properties of the elements vary periodically.

The atomic number

of an atom is the number of protons in the nucleus of that atom.

The mass number

of an element is the sum of the number of protons and neutrons in the nucleus of an atom of that element.

Isotopes

are of the same elements that have different mass numbers due to the different number of neutrons in the nucleus.

A radio isotope

is a radioactive isotope or the radioactive form of the element.

Relative atomic mass

is the average of the mass numbers of the isotopes of the element as they occur naturally, taking their abundances into account and compared with 1/12thof the mass of the carbon – 12 isotopes.

The aufbau principle

when building up the electronic configuration of an atom in its ground state, the electrons occupy the lowest available energy level.

Pauli exclusion principle

no more than two electrons can occupy an orbital and they must have an opposite spin.

Hund’s rule

when two or more orbitals of equal energy are available electrons occupy them singly before filling them in pairs.

A compound

is a substance that is made up of two or more different elements combined together chemically.

The octet rule

states that when bonding occurs, atoms tend to reach an electron arrangement with eight electrons in the outermost shell.

An ionic bond

is the force of attraction between oppositely charged ions in a compound.

A transition element

is one that forms at least one ion with a partially filled d sub-level.

A covalent bond

is formed when two atoms share a pair of electrons.

The valency of an element

is the number of atoms of hydrogen or any other monovalent element with which each atom of that element combines.

Electronegativity

is the relative attraction that an atom in a molecule has for the shared pair of electrons in a covalent bond.

Law of conservation of mass

The total mass of the products of a chemical reaction is the same as the total mass of the reactants.

Law of conservation of matter

in any chemical reaction, matter is neither created nor destroyed, but merely changes from one form to another.

The atomic radius of an atom

is defined as half the distance between the nuclei of two atoms of the same element that are joined together by a single covalent bond.

The first ionisation energy of an element

is the energy required to remove the most loosely bound electron from a neutral gaseous atom in its ground state.

Radioactivity

is the spontaneous breaking up of unstable nuclei with the emission of one or more types of radiation.

The half life of an element

is the time taken for half of the nuclei in any given sample to decay.

The relative molecular of a substance

is the mass of one molecule of that substance compared with one twelfth of the mass of the carbon – 12 – isotope.

One mole of a substance

is the amount of that substance which contains 6 x 10 23particles of that substance.

Number of moles

mass in grams / mass of one mole.

The empirical formula of a compound

is the formula which gives the simplest whole number ratio of the elements in the compound.

The molecular formula of a compound

is the formula that gives the actual number of each atom in the molecule.

Arrhenius definition for an acid

is an acid is a substance that dissociates in water to produce H+ ions

Arrhenius definition for a base

is a base is a substance that dissociates in water to produce OH- ions.

Bronsted Lowry definition

is an acid is a proton donor.

Bronsted Lowry definition

is a base is a proton acceptor.