Leaving Certificate Chemistry Definitions & Atomic Theory

Essential chemistry vocabulary flashcards defining core terms, laws, principles, and concepts from Leaving Certificate Chemistry notes.

Atom

The smallest particle of an element that still has the properties of that element.

Cathode rays

Streams of negatively charged particles called electrons that travel in straight lines from the cathode to anode.

Thomson’s Model of the Atom (Plum Pudding Model)

A model of the atom proposed as a positive sphere with negative particles embedded in it.

Rutherford’s Model of the Atom (The Nuclear Model)

A model proposing that atoms contain a small dense positive core called the nucleus containing protons, the atom is mainly empty space, and electrons are scattered in the space around the nucleus called the electron cloud.

Energy Level

The fixed energy value that an electron in an atom may have.

Ground State

The state of an atom in which the electrons occupy the lowest available energy levels.

Excited State

The state of an atom in which the electrons occupy higher energy levels than those available in the ground state.

Heisenberg's Uncertainty Principle

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

Atomic Orbital

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

Sublevel

A subdivision of a main energy level that consists of one or more orbitals of the same energy.

Element

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

Mendeleev's Periodic Law

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

Modern Periodic Law

When elements are arranged in order of increasing atomic number, the properties displayed by an element are repeated at regular intervals in other elements.

Atomic Number

The number of protons in the nucleus of that atom (defined by Moseley).

Mass Number

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

Isotopes

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 mass of an atom of that element compared with one twelfth of the mass of the carbon 12 isotope.

Relative Molecular Mass

The average mass of a molecule of that substance compared with one twelfth of the mass of the carbon 12 isotope.

Mass Spectrometry

An analytical technique used to: 1. Identify the presence and abundance of isotopes; 2. Measure relative atomic masses and relative molecular masses; 3. Identify unknown compounds.

Aufbau Principle

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

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.

Pauli Exclusion Principle

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

Compound

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

Ion

A charged atom or group of atoms (a cation is positively charged, while an anion is negatively charged).

Molecule

A group of atoms joined together; the smallest particle of an element or compound that can exist on its own.

Valency

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

Mole

The amount of a substance that contains $6 \times 10^{23}$ particles of that substance.

Molecular Formula

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

Arrhenius Acid

A substance that dissociates in water to produce $H^+$ ions.

Arrhenius Base

A substance that dissociates in water to produce $OH^-$ (hydroxide) ions.

Brønsted-Lowry Acid

A proton donor.

Brønsted-Lowry Base

A proton acceptor.

Conjugate Acid-Base Pair

Any pair consisting of an acid and a base that differ by one proton.

Neutralisation

The reaction between an acid and a base to form a salt and water.

Salt

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

Standard Solution

A solution whose concentration is accurately known.

Primary Standard

A substance which can be obtained in a stable, pure and soluble solid form so that it can be weighed out and dissolved in water to give a solution of accurately-known concentration.

Standardise

The process of finding the concentration of a solution by titrating it against a solution of known concentration.

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