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Atoms, Molecules, and Ions Terms


An anion is a negatively charged ion. Non-Metals typically form anions.


An atom is the smallest particles of an element that retains the chemical properties of the element. Atoms are electrically neutral, with a positively charged nucleus that binds one or more electrons in motion around it.

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 cation is a positively charged ion. Metals typically form cations.

Chemical Formula

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


A compound is a material formed from elements chemically combined in definite proportions by mass. For example, water is formed from chemically bound hydrogen and oxygen. Specifically, a single compound of water is made up of two hydrogen atoms and one oxygen atom.


A diatomic molecule is a molecule that contains only two atoms. All of the noninert gases occur as diatomic molecules. Hydrogen, oxygen, nitrogen, fluorine, and chlorine exist as H2, O2, N2, F2, and Cl2, respectively.


An electron is a fundamental constituent of matter having a negative charge and negligible mass.

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 ion is an atom or molecule that has acquired a charged by either gaining or losing electrons. An atom or molecule with missing electrons has a net positive charge and is called a cation. An atom or molecule with extra electrons has a net negative charge and is called an anion.

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

The law of conservation of mass states that there is no change in total mass during a chemical reaction.

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

The molecular formula is a notation that indicates the type and number of atoms in a molecule. For example, the molecular formula of carbon dioxide is CO2.


A molecule is a collection of chemically bound atoms with characteristic composition and structure. Making or breaking bonds in a molecule changes it into a new molecule.

Monatomic Ion

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


A neutron is an elementary particle found in the atomic nucleus. Neutrons are neutral in charge, but do have a significant mass.


The nucleus is a very small, dense, positively charged center of an atom containing protons and neutrons.


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

The periodic table is an arrangement of the elements according to increasing atomic number that shows relationships between element properties.

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 proton is an elementary particle found in the atomic nucleus with a positive charge.

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.

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