Study Guide

Atoms, Molecules, and Ions Terms

Atoms, Molecules, and Ions Terms


A negatively charged ion.


A subunit of a single element that consists of a nucleus and electrons surrounding the nucleus.

Atomic Number (Z)

The atomic number is the number of protons in an atomic nucleus. In an elemental symbol the atomic number is a leading subscript. For example, in 126C the 6 is the atomic number.

Atomic Theory

The atomic theory is an explanation of chemical properties and processes that assumes tiny particles called atoms are the ultimate building blocks of matter.

Atomic Symbol

The atomic symbol is the letter or set of letters internationally agreed upon for a chemical element. For example, C is the atomic symbol for carbon.


A positively charged ion.

Chemical Formula

A chemical formula is a representation of a substance using symbols for its constituent elements. It is chemistry shorthand.


A substance that is formed from more than one kind of atom chemically bound together. For example, carbon dioxide (CO2) and sodium chloride (NaCl) are compounds because each one has two kinds of atoms that are bound by covalent or ionic bonds. Oxygen (O2) is not a compound, however, because it has one single kind of atom (O). Oxygen is referred to a diatomic molecule.


A complex consisting of two atoms.


A negatively charged atomic particle.

Empirical Formula

An empirical formula is the simplest of all formulas. Empirical formulas show which elements are present in a compound, with their mole ratios indicated as subscripts in their smallest whole number ratios.


An atom or molecule that has obtained a charge by either gaining or losing one or more electrons. This means that the proton number does not match the number of electrons giving the atom either a net negative or positive charge.

Ionic Compound

An ionic compound is also called a salt. It is a compound composed of cations and anions held together by electrostatic forces.

Law of Conservation of Mass

A scientific principle that states that matter cannot be created nor destroyed.

Law of Definite Proportions

The law of definite proportions states that when two pure substances react to form a compound, they do so in a definite proportion by mass. For example, when water is formed from the reaction between hydrogen and oxygen, the definite proportion is 1 g of H for every 8 g of O.

Law of Multiple Proportions

The law of multiple proportions states that when one element can combine with another to form more than one compound, the mole ratios of the elements in the compounds are simple whole-number ratios of each other. For example, in CO and in CO2, the oxygen-to-carbon mole ratios are 1:1 and 1:2 respectively.

Mass Number (A)

The mass number is the total number of protons and neutrons in an atom or ion.

Molecular Formula

Shows the number and kinds of atoms in a molecule.


A group of atoms that are held together by covalent bonds.

Monatomic Ion

A monatomic ion is an ion composed of a single atom.


A subatomic particle with no charge that resides in the nucleus of an atom. The mass of a neutron is essentially the same as the mass of a proton. Nothing but a chargeless copycat.


The hallmark of a eukaryotic cell, where the majority of the DNA is housed and replication and transcription of this DNA takes place.


An orbital describes what an electron with a given energy is located inside an atom or molecule. In other words, it describes the space that a given electron occupies.

Periodic Table

A means of organizing all known existing elements. Elements are sorted into families based on common characteristics. The periodic table also provides a quick summary of the atomic makeup of any element.

Polyatomic Ion

A polyatomic ion is a charged particle that contains more than two covalently bound atoms.

Polyatomic Molecule

A polyatomic molecule is an uncharged particle that contains more than two atoms.


A positively charged atomic particle. The number of protons in an atom determines the identity of the atom; for example, carbon atoms always have 6 protons.

Structural Formula

A structural formula is a diagram or pictorial representation that shows how the atoms in a molecule are bonded together.

Subatomic Particles

A subatomic particle is a particle smaller than an atom such as a proton, neutron, or electron.

Uncertainty Principle

The uncertainty principle is the principle that states the exact momentum and exact location of a particle cannot be known at the same time. Hence, there is some "uncertainty" about either the momentum or location of a particle at any given time.