OCR • J249 • 56 key terms
Alpha particle
A positively charged particle consisting of two protons and two neutrons (a helium nucleus).
From: Development of the atomic model
Archimedes' Principle
A principle stating that when a body is partially or totally immersed in a fluid, it experiences an upthrust force which is equal to the weight of fluid displaced.
From: Density
Atomic number
The number of protons in the nucleus of an atom, which also dictates the number of electrons in a neutral atom.
From: Atomic structure
Atomic radius
The distance from the centre of the nucleus to the outermost stable electron energy level, typically about 1 × 10⁻¹⁰ m.
From: Atomic structure
Conservation of mass
A principle stating that mass cannot be created or destroyed; the mass of a substance before a physical change is exactly equal to the mass after.
From: Density calculations
Density
The mass per unit volume of a substance.
From: Density
Desublimation
A physical change of state where a gas turns directly into a solid.
From: Density calculations
Displacement
The process where an object pushes aside a volume of fluid equal to its own submerged volume.
From: Density
Electron
A subatomic particle with a negative charge, discovered by J.J. Thomson.
From: Development of the atomic model
Electronic balance
A digital measuring device used to determine the mass of an object precisely.
From: Density
Electrons
Subatomic particles with a negative charge (–1) and a negligible relative mass that orbit the nucleus.
From: Atomic structure
Electrostatic attraction
The force that pulls two objects with opposite electrical charges together, such as the positive nucleus and negative electrons.
From: Atomic structure
Electrostatic neutrality
The state where the total positive charge equals the total negative charge, resulting in a net charge of zero.
From: Development of the atomic model
Electrostatic repulsion
The force that pushes apart two objects with the same electrical charge (e.g., positive alpha particles repelled by a positive nucleus).
From: Development of the atomic model
Energy levels
The specific distances from the nucleus where electrons orbit, also known as electron shells.
From: Atomic structure
Energy levels (shells)
The specific, discrete distances from the nucleus where electrons are permitted to orbit.
From: Development of the atomic model
Eureka can
A container with a spout used to measure the volume of an irregular solid by collecting the water it displaces.
From: Density
Excitation
The process of an electron moving to a higher energy level after absorbing energy.
From: Development of the atomic model
Gas
A state of matter where particles are widely spaced, randomly arranged, and move quickly in all directions.
From: Density and states of matter
Gravitational field strength
The force of gravity per unit mass, typically 10 N/kg on Earth in OCR Gateway exams.
From: Density
Ground state
The lowest possible energy level an electron can occupy (closest to the nucleus).
From: Development of the atomic model
Indivisible
Unable to be divided or broken down into smaller parts (a concept disproved by later atomic models).
From: Development of the atomic model
Irregular arrangement
A disordered, random pattern of particles found in both liquids and gases.
From: Density and states of matter
Isotope
Atoms of the same element containing the same number of protons but different numbers of neutrons.
From: Development of the atomic model
Kinetic theory
A scientific model explaining the physical properties of matter in terms of the arrangement and continuous motion of its constituent particles.
From: Density and states of matter
Liquid
A state of matter where particles are closely packed but in an irregular arrangement, allowing them to flow past each other.
From: Density and states of matter
Mass
The amount of matter in an object, measured in kilograms.
From: Density calculations
Mass number
The total number of protons and neutrons in the nucleus of an atom.
From: Atomic structure
Measuring cylinder
A piece of laboratory equipment used to accurately measure the volume of liquids or displaced fluids.
From: Density
Meniscus
The curved upper surface of a liquid in a tube, which must be read from the bottom for accurate volume measurement.
From: Density
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