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Atom
The basic unit of matter, consisting of a nucleus surrounded by electrons in shells.
Molecule
A group of two or more atoms bonded together, representing the smallest unit of a chemical compound.
Isotopes
Species with the same number of protons but different numbers of neutrons.
Protons
Positively charged particles found in the nucleus that determine the atomic number.
Neutrons
Neutrally charged particles located in the nucleus that contribute to the mass number.
Electrons
Negatively charged particles that orbit the nucleus in energy levels.
Atomic number
The number of protons in the nucleus of an atom, defining the element.
Mass number
The total number of protons and neutrons in an atom’s nucleus.
Relative atomic mass (A_r)
The weighted average mass of an atom of an element relative to 1/12th the mass of a carbon-12 atom.
Periodic Table
A tabular arrangement of elements ordered by increasing atomic number.
Groups
Vertical columns in the Periodic Table where elements have similar chemical properties.
Periods
Horizontal rows in the Periodic Table where the number of protons increases across the row.
Electron configuration
The arrangement of electrons in shells around the nucleus of an atom.
Metals
Elements that generally conduct electricity well and are found on the left side of the Periodic Table.
Non-metals
Elements that do not conduct electricity well and are found on the right side of the Periodic Table.
Amphoteric oxides
Oxides that can react with both acids and bases, such as aluminum oxide.
Valence electrons
Electrons in the outermost shell that determine an element's chemical properties.
Alkali metals
Highly reactive metals in Group 1 that lose one electron to form +1 ions.
Halogens
Elements in Group 7 that gain one electron to form -1 ions.
Noble gases
Unreactive gases in Group 0 or 18 with complete outer electron shells, making them stable.
