LEAVING CERTIFICATE CHEMISTRY DEFINITIONS

Chemistry definitions

Dalton’s Atomic Theory

All matter is made up of very small particles called atoms. All atoms are indivisible. They cannot be broken down into simpler particles. Atoms cannot be created or destroyed.

Cathode Rays

are streams of negatively charged particles called electrons. They travel in straight lines from the cathode to the anode, they are deflected by electric and magnetic fields and have sufficient energy to move a small object such as a paddle wheel.

Energy levels

- is defined as the fixed value that an electron in an atom may have.

Ground state

of an atom is one in which the electrons occupy the lowest available energy levels

Excited state

of an atom is one in which the electrons occupy higher energy levels than those available in the ground State

E2-E1=hf

E2-E1=hf

Heisenberg’s uncertainty principle

states that it is impossible to measure at the same time both the velocity and position of an electron

Orbital

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

Sublevel

- is a subdivision of a main energy level and consists of one or more orbitals of the same energy

Element

is a substance that cannot be split into simpler substances by chemical Means

Triad

is a group of three elements with similar properties in which atomic weight (relative atomic mass) of the middle element is approximately equal to the average of the other two elements.

Newlands octaves

are arrangements of elements in which the first and eighth element, counting from a particular element, have similar properties

Mendeleev’s periodic law

when elements are arranged in order of increasing atomic weight (relative atomic mass), the properties of the elements recur periodically i.e. the properties displayed by an element are repeated at regular intervals in other elements

Atomic number

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

Modern periodic table

- is an arrangement of elements in order of increasing atomic number

Modern periodic law

when elements are arranged in order of increasing atomic number, the properties of the elements recur periodically, i.e. the properties displayed by an element are repeated at regular intervals in other elements

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 atoms of the same element (i.e. they have the same atomic number) which have different mass numbers due to the different number of neutrons in the nucleus.

Relative atomic mass

The average of the mass numbers of the isotopes of the element. - As they occur naturally. - Taking their abundance into account and -expressed on a scale in which the atoms of the carbon-12 isotope have a mass -of exactly 12 units.

Principle of mass spectrometry

is that charged particles moving in a magnetic field are deflected to different extents according to their masses and are thus separated according to these masses

Electron configuration

shows the arrangement of electrons in an atom of that element.

Aufbau principle

when building up the electron configuration of an atom in its ground state, the electrons occupy the lowest available energy levels

Hund’s rule of maximum Multiplicity

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

The Pauli exclusion principle

States that no more than two electrons may occupy an orbital and they must have opposite spin

Ion

atom that has gained or lost electrons

Compound

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

Octet rule

when bonding occurs, atoms tend to reach an electron arrangement with eight electrons in the outermost energy level.

Ion (chapter5)

- is a charged atom or group of atoms.

Ionic bond

is the force of attraction between oppositely charged ions in a compound. Ionic bonds are always formed by the completed transfer of electrons from one form to another.

Transition metal

is one that forms at least one ion with a partially filled d sublevel.

Molecule

is a group of atoms joined together. It is the smallest particle of an element or compound that can exist independently.

Valency

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

Sigma bond

is formed by the head on overlap of two orbitals

Pi bond

is formed by the sideways overlap of p orbitals.

Electronegativity

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

Polar covalent bond

is a bond in which there is an unequal sharing of the pair (or pairs) of electrons. This causes one end of the bond to be slightly positive and the other to end slightly negative.

Mole

of a substance is the amount of that substance that contains 6x1023 particles of that substance.

Relative molecular mass -

of a compound is the average mass of one molecule of that compound compared with one twelfth of the mass of one atom of the carbon 12 isotope.

Valence electron

Outermost electron/ bonding only involves this electrons

Exceptions to octet rule

elements near helium- have 2 electrons / transition metals- <>

Lone Pairs –

pairs of electrons that are not involved in bonding

Bond pairs

pairs of electrons that are involved in bonding with other atoms.

The valency of an element is

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

Exception to octet rule

Hydrogen, aren’t enough electrons

(VSEPRT)

The Valence Shell Electron Pair Repulsion Theory

VSPERT

The shape of a molecule depends on the number of pairs of electrons in the valence shell of the central atom

(i.e. that lie around the central atom of the molecule)

Vseprt shapes

Linear - Pyramidal - Triangular Planar - Planar/V-Shaped - Tetrahedral

Exception to octet rule 2

Linear( BeCl2) and triangular (BCl3)

Polarity:

Polarity means that there is a positive and negative charge on the molecule and these two charges are separated by some distance.

Uses of electronegativity valuesP

1) predict polarity of covalent bonds

2) predict which compounds are ionic/ covalent

Dipole

is a pair of separated opposite charges.

Van der Waals

forces are weak intermolecular forces that arise from temporary dipoles in molecules. Only force of attraction between non polar molecules

Dipole forces

forces of attraction between the negative pole of one polar molecule and the positive pole of another polar molecule.

Hydrogen bonds

are strong dipole-dipole attractions that occur between molecules containing hydrogen bonded to highly electronegative atoms such as nitrogen(N), oxygen(O), or fluorine(F)

The hydrogen atom carries a partial positive charge and is attracted to the electronegative atom in another molecule.

Thus, it acts as a bridge between two electronegative atoms.