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proton
A positively charged particle in the nucleus of an atom.
electron
A negatively charged particle that orbits the nucleus of an atom.
neutron
A neutrally charged particle in the nucleus of an atom.
atom
The smallest unit of a chemical element, composed of protons, neutrons, and electrons.
mass number
The total number of protons and neutrons in an atomic nucleus.
average atomic mass
The weighted average mass of the isotopes of an element.
atomic number
The number of protons in the nucleus of an atom, determining the element's identity.
atomic mass unit (amu)
A unit of mass used to express atomic and molecular weights.
isotope
Different forms of an element that have the same number of protons but different numbers of neutrons.
ion
An atom or molecule that has gained or lost one or more electrons, resulting in a net charge.
octet rule
The principle that atoms tend to bond in such a way that they each have eight electrons in their valence shells.
element
A pure substance consisting of one type of atom.
compound
A substance formed when two or more elements are chemically bonded together.
chemical reaction
A process that leads to the transformation of one set of chemical substances to another.
radioactive decay
The process by which an unstable atomic nucleus loses energy by radiation.
significant figures
The digits in a number that carry meaningful information about its precision.
dimensional analysis
A mathematical technique used to convert units from one system to another.
conversion factor
A ratio used to convert from one unit to another.
valence electron
An electron in the outermost shell of an atom that can participate in chemical bonding.
Bohr model
A model of the atom that depicts electrons in fixed orbits around the nucleus.
density
The mass of a substance per unit volume.
periodic table
A tabular arrangement of the elements, organized by increasing atomic number.
group
A vertical column in the periodic table.
period
A horizontal row in the periodic table.
unit of measurement
A definite amount of a quantity used to quantify a physical property.
base unit
A fundamental unit of measurement in the metric system.
metric scale
A system of measurement based on powers of ten.
prefix
A scaling factor added to a unit of measurement to indicate multiples or fractions.
scientific notation
A way of expressing numbers as a multiple of a coefficient and a power of ten.
pure substance
Matter that has a constant composition and distinct chemical properties.
mixture
A combination of two or more substances that retain their individual properties.
homogeneous mixture
A mixture that has a uniform composition throughout.
heterogeneous mixture
A mixture in which the composition is not uniform throughout.
extensive properties
Properties that depend on the amount of matter present.
intensive properties
Properties that do not depend on the amount of matter present.
plum pudding model
An early model of the atom proposed by J.J. Thomson, depicting electrons embedded in a positive charge.
nuclear decay
The process by which an unstable atomic nucleus loses energy and mass.
photon
A quantum of electromagnetic radiation.
wavelength
The distance between successive crests of a wave.
frequency
The number of waves that pass a given point in a certain amount of time.
Planck's constant
A fundamental constant used to describe the sizes of quanta in quantum mechanics.
speed of light
The speed at which light travels in a vacuum, approximately 3.00 x 10^8 m/s.
alkali metals
Group 1 elements in the periodic table that are highly reactive.
noble gases
Group 18 elements known for their lack of reactivity due to having a full valence shell.
