ABSOLUTE ZERO
The lowest possible temperature—at which the kinetic energy of the particles of matter is at a minimum. The value of absolute zero is 0 on the Kelvin temperature scale, equal to −459.67° on the Fahrenheit scale and −273.15° on the Celsius scale.
ALPHA DECAY
The process by which an unstable nucleus attains a lower-energy, more stable state. The nucleus emits an alpha particle, which consists of two protons and two neutrons bound together.
ATOMIC FORCE MICROSCOPY
A technique that produces faithful images of atoms by scanning a surface with an extremely sharp tip and sensing the force between tip and surface.
ATOMIC MASS NUMBER
Often just “mass number,” the total number of protons and neutrons in the nucleus. Compare atomic number and atomic weight.
ATOMIC NUMBER
The number of protons in the nucleus of an atom. All the atoms of a particular element have the same atomic number.
ATOMIC WEIGHT
Also called relative atomic mass, the average mass of an atom of a particular element measured in atomic mass units (see dalton). Different isotopes have different atomic masses—this is why the atomic weight is an average.
BETA DECAY
The process by which an unstable nucleus attains a lower-energy, more stable state. Inside the nucleus, a neutron becomes a proton and an electron, the latter being ejected from the nucleus as a beta particle.
BOSE-EINSTEIN CONDENSATE
A state of matter in which bosonic atoms (atoms that can share identical energy quantum states) are cooled to near absolute zero, so that their wave functions merge, and they act as one particle.
BOSON
A particle or group of bound particles with integer spin (0, 1, 2 etc.). Unlike fermions, bosons can be in the same quantum state as other particles of the same kind. Examples are photons, helium-4 atoms and the Higgs boson, also known as the God particle. Compare fermion.
COMPOUND
A substance made of two or more different elements, in which the atoms of the elements are in a definite ratio. An example is water (hydrogen and oxygen, 2:1). The atoms of a compound are bonded, with ionic or covalent bonds.
COVALENT BOND
A bond between atoms in which electrons are shared, in molecular orbitals. A molecule is a group of two or more atoms bonded together covalently.
DALTON (DA)
Also called unified atomic mass units. A measure of atomic and molecular mass. One dalton is equal to one-twelfth the mass of a carbon-12 atom.
DOUBLE SLIT EXPERIMENT
An experiment originally conceived in 1801 to investigate and demonstrate the wave nature of light, but important in modern physics, where it highlights the wave nature of subatomic particles, such as electrons.
ELECTROMAGNETISM
The force between particles that carry electric charge. Along with the strong interaction, the weak interaction and gravity, one of the four fundamental interactions.
ELEMENT
A substance made of one type of atom, defined by the number of protons in the nuclei of those atoms. Examples include hydrogen, oxygen, and carbon.
ELECTRON
A fundamental particle that carries a negative electric charge, found in all atoms.
FERMION
A particle or group of bound particles with half-integer spin (–1/2, 1/2, 3/2 etc.). Unlike bosons, fermions cannot share the same quantum state as other identical particles. Examples are protons, neutrons, electrons and helium-3 atoms. Compare boson.
FIELD EMISSION MICROSCOPY
A technique in which electrons emitted from the surface of a sharp metal point produce a hugely magnified image of the atomic structure at the surface of the tip.
FIELD ION MICROSCOPY
A technique similar to field emission microscopy, in which atoms of a rarefied gas adhere to a sharp metal tip, and are then ionized and expelled from the tip, producing a hugely magnified image of the atoms at the surface of the tip.
GAMMA RAY
A form of electromagnetic radiation with very high frequency (so with very high-energy photons), typically produced during nuclear reactions and radioactive decay.
GLUON
A fundamental particle that carries the strong interaction. Gluons hold together quarks to form protons and neutrons.
GOD PARTICLE
Nickname for the Higgs boson, a particle associated with the Higgs field, a quantum field that is responsible for giving fundamental particles their mass.
HADRON
Any composite particle, made of two or more quarks, that takes part in the strong interaction. The proton and the neutron are hadrons.
HALF-LIFE
The time it takes for half of the atoms of a radioactive isotope to decay.
IONIC BOND
A bond between ions—atoms that have gained or lost electrons. Positive and negative ions held together by ionic bonds typically form crystals.
ISOTOPES
Two or more configurations of the atoms of a particular element, with the same number of protons but differing numbers of neutrons. All elements have at least two isotopes.
LASER
A source of coherent light—light that is of a precise and well-defined wavelength and whose waves are all in phase (in step).
LEPTON
Any particle of matter that, unlike the hadrons, does not take part in the strong interaction. Electrons are leptons.
MAGNETIC RESONANCE IMAGING (MRI)
A medical imaging technique that uses strong magnetic fields and radio waves to interact with the magnetic fields of nuclei.
MAGNETISM
A phenomenon involving particles with spin. In many systems of particles, the spins of the individual particles cancel out, and these systems do not exhibit magnetism.
MASS SPECTROMETRY
An analytical technique that separates mixtures of different molecules by ionising them, accelerating them to high speed, and then bending them with magnetic fields, the amount of deflection determined by their masses.
MOLE
An amount of an element or compound consisting of a number of particles equal to Avogadro’s number (600,000 billion billion). One mole of an element has a mass equal to the atomic weight in grams.
MOLECULAR ORBITAL
An orbital formed by the overlap of the orbitals of an atom. As with atomic orbitals, a molecular orbital can hold up to two electrons. Molecular orbitals are the basis of covalent bonds.
NEUTRON
A particle, composed of three quarks, found in all atomic nuclei (except hydrogen-1). Although the quarks carry electric charge, the neutron carries no charge overall.
NUCLEAR FISSION
A nuclear reaction, used in nuclear power stations and weapons, in which large atomic nuclei break, or fission, into two parts, releasing energy. It can happen spontaneously, but can also be made to happen, in the presence of free-moving neutrons.
NUCLEAR FUSION
A nuclear reaction, occurring at the centre of stars and used in thermonuclear weapons, in which small atoms join, or fuse, releasing energy.
NUCLEAR MAGNETISM
The magnetism of nuclei in which the spins of the protons and neutrons do not cancel out. Certain nuclei, including those of hydrogen-1 atoms, have non-zero spin, and therefore act as tiny magnets.
ORBITAL
A region of space, defined by electrons’ wave functions, where electrons can be found. Each orbital can hold up to two electrons, with opposing spins.
PARTICLE ACCELERATOR
An apparatus in which subatomic particles or ions are made to travel at high speeds and then collide with each other, creating showers of other particles that allow experimenters to test theories about the subatomic interactions.
PERIODIC TABLE
A chart containing all the elements, arranged in order of increasing atomic number, in rows (periods), arranged so that elements with similar properties—the result of similar electron configurations—are in the same columns (groups).
PHOTOELECTRIC EFFECT
The phenomenon in which illuminating a metal surface with light or other electromagnetic radiation causes electrons to be emitted from the surface. Only radiation with high enough frequency (with photons of high enough energy) can eject electrons.
PHOTON
A fundamental particle. Light and other electromagnetic radiation is a stream of photons, while virtual photons are responsible for carrying the electromagnetic force between charged particles.
PROTON
A particle, composed of three quarks, found in all atomic nuclei. The quarks carry electric charge, and the proton carries an overall positive charge.
QUANTUM THEORY
(Or quantum mechanics or quantum physics.) The scientific theory that describes the behavior of fields, particles and their interactions at the atomic and subatomic scale.
QUANTUM CHROMODYNAMICS
A quantum field theory that describes and predicts the behaviour of particles affected by the strong interaction, notably quarks and gluons.
QUANTUM ELECTRODYNAMICS
The quantum field theory that describes and predicts the interactions of photons and particles with electric charge.
QUANTUM FIELD THEORY
The framework by which particles are understood as manifestations of quantized fields that permeate all of space. Each kind of particle in the Standard Model has its own field.
QUARK
A fundamental particle that takes part in the strong interaction. Protons and neutrons are made of quarks (and gluons).
RADIOACTIVITY
(Or radioactive decay.) The process by which unstable atomic nuclei attain lower-energy, often more stable, states. See alpha decay, beta decay, gamma ray.
RADIOCARBON DATING
A technique for estimating the age of once-living matter, by measuring how much of the isotope carbon-14 is present in it. Carbon-14 is taken in at a steady rate by living things, but decays, with a half-life of about 5,700 years.
RELATIVITY
Any theory in physics that acknowledges that matter and energy must behave according to the same fundamental laws wherever they are, whatever speed they are moving at, and however strong is the gravitational field.
SCANNING ELECTRON MICROSCOPY
A technique that produces a magnified image of an object by bouncing electrons, rather than light, off its surface.
SCANNING PROBE MICROSCOPY
Any technique that can be used to produce accurate images of atoms by scanning up and down and sensing the atomic-scale bumps in a surface.
SCANNING TRANSMISSION ELECTRON MICROSCOPY
A technique that produces faithful atomic-scale images by scanning an electron beam up and down across a surface, and collecting the electrons that pass through.
SCANNING TUNNELING MICROSCOPY
A technique that produces faithful atomic-scale images by scanning a probe with a very sharp tip across a surface and measuring the tiny current of electrons that “tunnel” across the gap between tip and surface.
SEMICONDUCTOR
A material that conducts electricity much better than an insulator, but not as well as a metal under normal circumstances. When energized by light or heat, or when doped appropriately, the conductivity can increase.
SPIN
A property of subatomic particles that behaves as if those particles are rotating. Spin gives charged particles magnetism, although paired, opposing spins cancel out. Atoms with unpaired electrons have residual spin, and account for the magnetic properties of certain substances.
STANDARD MODEL
The current best theory that explains the fundamental interactions between subatomic particles.
STRONG INTERACTION
(Or strong force.) The interaction between quarks, carried by virtual gluons. Hadrons (particles composed of quarks) are subject to the strong interaction.
TRIPLE ALPHA PROCESS
A nuclear reaction inside stars in which the would-be carbon-12 nuclei form from three alpha particles.
UNCERTAINTY PRINCIPLE
A major feature of quantum theory, which recognizes fundamental limits on how accurately pairs of quantities can be known—in particular, momentum and position and energy and time.
VIRTUAL PARTICLE
A particle whose fleeting existence is allowed by the uncertainty principle: the particles can exist, from borrowed energy, if the energy is “paid back” within a certain time.
WAVE FUNCTION
A mathematical description of the quantum state of a particle or collection of particles. The value of the wave function at any time and place is related to the probability of a particle being in a particular state at those times and places.
WAVE-PARTICLE DUALITY
The phenomenon in which objects once thought of as particles have wavelike behaviors, and vice versa.
WEAK INTERACTION
(Or weak force.) The interaction between certain subatomic particles, involved in radioactive decay and nuclear reactions.