Chemistry Definitions (Leaving Cert)

Particulate Nature of Matter

Is the idea that materials are made up of small particles.

Matter

Is the scientific name for materials.

Diffusion

Is the spreading of gas molecules from an area of high concentration to low concentration.

Atom

Is the smallest particle of an element which still retains the properties of that element.

Cathode Rays

Streams of negatively charged particles called electrons.

They travel in straight lines from the cathode to the anode,

are DEFLECTED BY ELECTRIC AND MAGNETIC FIELDS, and have sufficient energy to move a small object such as a paddle wheel.

Sub-atomic Particles

The three particles inside an atom: proton (+), neutron (0), and electron (-).

Continuous Spectrum

Is when white light is passed through a glass prism, the white light is broken up into an array of colours.

Emission Line Spectrum

Is when gas is put in a discharge tube, which replaces white light, and the emitted light is shown by coloured lines on a black background.

Quantisation

Is when electrons can only have a fixed amount of energy.

Energy Level

Is defined as the fixed energy value that an electron can have.

Ground State

Of an atom is one in which the electrons occupy the lowest available energy level.

Excited State

Of and atom is one in which the electrons occupy higher energy levels than those available in the ground state.

Atomic Absorption Spectrum

Is when white light is passed through a gaseous sample of an element, it is found that the light that emerges has certain wavelengths missing.

Atomic Absorption Spectrometer

Is used to measure the concentrations of certain heavy metals.

Heisenberg's Uncertainty Principle

States that it 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 possibility of finding an electron.

An Orbit

Is the path of an electron traveling with a certain velocity.

A Sublevel

Is a subdivision of a main energy level and consists of one or more orbitals of the same energy.

An Element

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

A Triad

Is a group of three elements with similar chemical properties in which the atomic weight is approximately equal to the average of the other two.

Newland's Octives

Are the arrangements of elements in which the first and the eight element, counting from a particular element, have similar properties (The law only worked for the first 16 elements and some elements hadn't been discovered, along with the noble gasses).

Mendeleev's Periodic Table

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

The Atomic Number

Is the number of protons in the nucleus of that atom.

The 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.

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 atoms of the same element 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 abundances into account.

~Expressed on a scale in which the atoms of the carbon-12 isotope have a mass of exactly 12 units.

The Principle of Mass Spectrometry

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

The Electronic Configuration

Shows the arrangement of electrons in an atom of an element.

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 Theory

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

A 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

Is a charged atom or groups of atoms

Ionic Bonding

Is the force of attraction between oppositely charged ions in a compound. Ionic Bonds are always formed by the complete transfer of electrons from atom to atom.

Crystal Lattice

The three dimensional arrangement of ions.

Transition Metal

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

A Molecule

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

Covalent Bonding

Is when a pair of electrons are shared between two atoms and each of the bonded atoms contributes one electron to the shared pair.

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.

A Sigma Bond

Is formed by the head-on overlap of two orbitals.

A Pi Bond

Is formed by the sideways overlap of p orbitals.

Electronegativity

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

Polar Covalent

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

Intramolecular Bonding

Is bonding that takes place within a molecule i.e it hold the atoms together. e.g. Covalent bonding and Polar Covalent bonding

Intermolecular

Are the forces of attraction that exist between molecules. e.g. Dipole-dipole forces, hydrogen bonding and Van Der Waals forces.

Van Der Waals Forces

Are weak attractive forces between molecules resulting in the formation of temporary dipoles. They are the only forces of attraction between non- polar molecules.

Dipole-dipole Forces

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

Hydrogen Bonds

Are particular types of Dipole-dipole attractions between molecules in which hydrogen atoms are bonded to nitrogen, oxygen, fluorine.

The hydrogen atom carries a partial positive charge and is attracted to the electronegative atom in another molecule.Thus, the hydrogen bond acts as a bridge between two elctronegative atoms in separate molecules.

The Law of Conservation of Matter

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

The Law of Conservation of Mass

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

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 in a single covalent bond.

The First Ionisation Energy

Of an atom is the minimum energy required to completely remove the most loosely bound electrons from a neutral gaseous atom in its ground state.

The Second Ionisation Energy

Is the energy required to remove an electron from an ion with one positive charge in the gaseous state.

Screening Effect

In an atom with one electron, the electron experiences the full attractive force of the protons. In an atom with many electrons, the inner energy levels shield that attractive force from the outer electrons.

Increase in the Effective Nuclear Charge

Is when the number of protons in the nucleus increases without an screening effect of the electrons in the inner energy levels. (Happens across periods / Nuclear Charge is the Atomic Number)

Radioactivity

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

Nuclear Reaction

is a process that alters the composition, structure and energy of an atomic nucleus.

Transmutation

Is the changing of an element into another.

Half life

Of an element is the time it takes for half of the nucleus in any given sample to decay.

A radioisotope

Is a radioactive isotope.

Radiocarbon Dating

Is a technique used to determine the age of an object containing carbon. it is based on the ration of carbon-14 to carbon-12 in the object.

Mole

One mole of a substance is the amount of that substance that contains 6*10^23 particles of that substance.

The relative molecular mass

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

Gas

Is a substance that has no well defined boundaries but diffuses rapidly to fill any container in which it is place.

Boyle's Law

States that, at a constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure.

Charles' law

States that, at a constant pressure, the volume of a fixed mass of gas is directly proportional to the temperature on the Kelvin scale.

Gay-Lussac's Law of Combining Volumes

States that, in a reaction between gases, the volumes of the reacting gases and the volumes of any gaseous products are in the same ratio of the small whole numbers provided the volumes are measured at the same temperature and pressure.

Avogadro's Law

States that equal volumes of gases contain equal numbers of molecules under the same conditions of temperature and pressure.

Ideal gas

is one that perfectly obeys all the assumptions of the kinetic theory of gases under all conditions of temperature and pressure

Real Gases

Differ from the ideal gases because 1. forces of attraction and repulsion do exist between molecules and 2. the volume of the molecules is not negligible.

Molecular formula

Of a compound is a formula which shows the number and type of each atom present in a molecule of that compound

Empirical formula

Of a compound is the formula showing the simplest of whole number ratio of the numbers of different atoms present in the molecule.

Arrhenius definition: acid

Is a substance that dissociates in water to give H+ ions.

Arrhenius definition: strong acid

Is a substance that almost completely dissociates in water to give hydrogen ions.

Arrhenius definition: weak acid

Is a substance that only slightly dissociates in water to give hydrogen ions.

Arrhenius definition: base

Is a substance that dissociates in water to give OH- ions.

Arrhenius definition: strong base

Is a substance that almost completely dissociates in water to give hydroxide ions

Arrhenius definition: weak base

Is a substance that slightly dissociates in water to give hydroxide ions.

Bronsted-Lowry definition: Acid

a proton donor.

Bronsted-Lowry definition: Base

a proton acceptor.

Conjugate base

An acid changes into this when it donates a proton

Conjugate acid

A base changes into this when it accepts a proton.

Conjugate acid-base pair

Is any pair consisting of and acid and a base that differ by one proton.

A salt

is a substance formed when a hydrogen ion is replaced from an acid by a metal or an ammonium ion.

Neutralisation

is the reaction between an acid and a base to for a salt and water.

amphoteric

a substance that can act as an acid or a base.

Solution

Is a completely perfect mixture of a solute and a solvent. In a solution the solute is evenly distributed throughout the solvent.

Concentration

Of a solution is the amount of solute that is dissolved in a given volume of a solution.

Molarity

of a solution is the number of moles of solution per litre of solution.

One Molar

solution is one that contains one mole dissolved in one litre of solution

Standard Solution

is a solution whose concentration is accurately known.

Primary solution

is a substance that can be obtained in a stable, pure, soluble solid form that can be weighed out and dissolved in water to make a solution of accurately know concentration.

Titration

Is a laboratory procedure where a measured volume of one solution is added to a known volume of another solution until the reaction is complete.

Oxidation

Of an element takes place when it loses electrons.

Reduction

Of an element takes place when. It gains electrons.

Oxidising agent

Is a substance that brings about the oxidation in other substances. (It itself is reduced)

Reducing agent

Is a substance that brings out the reduction in other substances. (It itself is oxidised)

Oxidation number

Is the charge that an atom has or appears to have when electrons are distributed according to certain rules.