Atoms, Molecules, and Ions


An atom is the smallest particles of an element that retains the chemical properties of the element. So we're talking some super-tiny stuff here.  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 anion is a negatively charged ion. Non-Metals typically form anions.

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.


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