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Absolute zero The lowest
possible temperature: 0K or
–459.67°F (–273.15°C).
Acceleration The rate of change
of velocity. Acceleration is caused
by a force that results in a change
in an object’s direction and/or speed.
Acid A chemical that, when
dissolved in water, liberates
hydrogen ions and turns litmus red.
Algorithm In mathematics and
computer-programming, a logical
procedure for making a calculation.
Alkali A base that dissolves
in water and neutralizes acids.
Alpha particle A particle made
of two neutrons and two protons,
which is emitted during a form of
radioactive decay called alpha
decay. An alpha particle is identical
to the nucleus of a helium atom.
Amino acids Organic chemicals
with molecules that contain amino
groups (NH 2 ) and carboxyl groups
(COOH). Proteins are made from
amino acids. Each different protein
contains a specific sequence
of amino acids.
Angular momentum A measure
of the rotation of an object, which
takes into account its mass, shape,
and spin speed.
Antiparticle A particle that is the
same as a normal particle except
that it has an opposite electrical
charge. Every particle has an
equivalent antiparticle.
Atom The smallest part of an
element that has the chemical
properties of that element. An atom
was thought to be the smallest part
of matter, but many subatomic
particles are now known.Atomic number The number
of protons in an atom’s nucleus.
Each element has a different
atomic number.ATP Adenosine triphosphate.
A chemical that stores and
transports energy across cells.Base A chemical that reacts with
an acid to make water and a salt.Beta decay A form of radioactive
decay in which an atomic nucleus
gives off beta particles (electrons
or positrons).Big Bang The theory that
the universe began from an
explosion of a singularity.Black body A theoretical object
that absorbs all radiation that falls
on it. A black body radiates energy
according to its temperature, so
may not in fact appear black.Black hole An object in space that
is so dense that light cannot escape
its gravitational field.Bosons Subatomic particles
that carry forces between
other particles.Brane In string theory, an
object that has between zero
and nine dimensions.Cell The smallest unit of an
organism that can survive on its
own. Organisms such as bacteria
and protists are single cells.Chaotic system A system whose
behavior over time changes
radically in response to small
changes to its initial condition.Chromosome A structure made
of DNA and protein that contains
a cell’s genetic information.Cladistics A system for classifying
life that groups species according
to their closest common ancestors.Classical mechanics Also known
as Newtonian mechanics. A set
of laws describing the motion of
bodies under the action of forces.
Classical mechanics gives accurate
results for macroscopic objects
that are not traveling close to
the speed of light.Color charge A property of quarks
by which they are affected by the
strong nuclear force.Continental drift The slow
movement of continents around
the globe over millions of years.Covalent bond A bond
between two atoms in which
they share electrons.Dark energy A poorly understood
force that acts in the opposite
direction to gravity, causing the
universe to expand. About three
quarters of the mass-energy of
the universe is dark energy.GLOSSARY