Atoms, Elements, and Molecules

• Define the key terms atom, element, molecule, and compound.

• Differentiate between an element and a compound based on their composition.

• Use the periodic table to determine the composition of an atom.

• Identify substances as elements, molecules, or compounds based on their chemical makeup.

Key Vocabulary

Atom

The smallest unit of matter, the atom is the basic building block of a chemical element.

Element

An element is a pure substance made of only one type of atom that cannot be broken down.

Molecule

A molecule is a group of two or more atoms that are chemically bonded together.

Compound

A compound is a substance made of two or more different elements chemically bonded together.

The Atom: Building Block of Matter

• An atom is the smallest unit of an element that has its chemical properties.

• Its nucleus has positive protons and neutral neutrons, circled by negative electrons.

• An element’s atomic number is equal to its total number of protons.

• Mass number is protons plus neutrons; you can find neutrons by subtracting numbers.

Elements, Molecules, and Compounds

Element

• An element is a pure substance made of only one type of atom.

• It cannot be broken down into simpler substances by chemical means.

• Examples are Gold (Au), Iron (Fe), and Oxygen gas (O₂).

Molecule

• A molecule is formed when two or more atoms are chemically bonded.

• It is the smallest unit of a pure substance.

• Examples include Hydrogen gas (H₂) and water (H₂O).

Compound

• A compound is formed from two or more different elements chemically bonded.

• Water (H₂O) is a compound of hydrogen and oxygen atoms.

• Examples are salt (NaCl) and glucose (C₆H₁₂O₆).

• Atoms are the basic units of matter, made of protons, neutrons, and electrons.

• An element is a pure substance made of only one type of atom.

• Molecules are bonded atoms; compounds are molecules made from different elements.

• An element's atomic number is the number of protons in its atoms.