Atoms, Elements, and Molecules

Everything around us is made of tiny particles called atoms, which combine to form elements and molecules.

Reading is good — doing is better. Practice Atoms, Elements, and Molecules as an interactive lesson.

Definition

An atom is the smallest particle of matter that still has the properties of that substance. An element is a pure substance made of only one kind of atom. A molecule is two or more atoms bonded together. Think of atoms as LEGO bricks, elements as a bag of all the same-colored bricks, and molecules as structures you build by snapping different bricks together.

Remember the rule

Atoms → Elements → Molecules: one atom type alone = element; two or more atoms bonded = molecule; two or more DIFFERENT atom types bonded = compound.

Key words

Atom: The tiniest building block of matter; too small to see with the naked eye.
Element: A pure substance made of only one type of atom, like gold, oxygen, or carbon.
Molecule: Two or more atoms bonded together, like water (H2O) which is 2 hydrogen atoms plus 1 oxygen atom.
Compound: A molecule made of two or more DIFFERENT kinds of atoms bonded together, like water or table salt.
Chemical Symbol: A 1- or 2-letter shorthand for an element; O = oxygen, H = hydrogen, Fe = iron.
Periodic Table: The organized chart that lists all known elements by their properties and atomic number.
Proton: A positively charged particle inside the nucleus (center) of an atom; its count tells you which element it is.
Atomic Number: The number of protons in one atom of an element; every element has its own unique atomic number.

Worked examples

Is pure gold (Au) an element, a molecule, or a compound?

→ Pure gold is an ELEMENT because it is made of only one kind of atom — gold atoms (Au). There is nothing else mixed in. · Gold's chemical symbol Au comes from its Latin name, Aurum.

Water is written as H2O. How many atoms are in one water molecule, and is water an element or a compound?

→ One water molecule has 3 atoms total: 2 hydrogen (H) atoms and 1 oxygen (O) atom. Because it contains TWO different kinds of atoms bonded together, water is a COMPOUND (and also a molecule). · The small '2' after H tells you there are 2 hydrogen atoms; no number after O means just 1 oxygen atom.

Oxygen gas in the air is written O2. Is O2 an element or a compound?

→ O2 is still an ELEMENT because both atoms are the same kind — both are oxygen. It is also a molecule because two atoms are bonded together. A molecule does NOT have to be a compound. · This surprises many students: you can have a molecule that is not a compound if both atoms are the same element.

Table salt is NaCl. Name the elements it contains and tell whether it is a compound.

→ NaCl contains sodium (Na) and chlorine (Cl) — two DIFFERENT elements. Because different atom types are bonded together, table salt is a COMPOUND. The ratio is 1 sodium atom to 1 chlorine atom. · On its own, sodium is a dangerous metal and chlorine is a toxic gas, but bonded together they make safe table salt — compounds can have very different properties from their elements!

Carbon dioxide is CO2. Count the total atoms in one molecule and identify the elements.

→ One CO2 molecule has 3 atoms: 1 carbon (C) and 2 oxygen (O). The elements are carbon and oxygen. Because two different element types are present, CO2 is a compound. · This is the gas humans breathe out and plants use for photosynthesis.

A student says hydrogen gas (H2) is a compound because it has two atoms. Is the student correct?

→ No, the student is incorrect. H2 is a MOLECULE but NOT a compound, because both atoms are the same element (hydrogen). A compound requires at least two DIFFERENT kinds of atoms. · Molecule and compound are not the same word — always check if the atoms are the same or different.

Common mistakes

Confusing 'molecule' and 'compound' — a molecule is any two or more bonded atoms, but a compound is only when those atoms are from different elements.
Thinking an element cannot be a molecule — O2 and H2 are both elements AND molecules because their atoms are bonded pairs of the same type.
Misreading chemical formulas — forgetting that a small subscript number (like the 2 in H2O) multiplies only the atom right before it, not the whole formula.
Mixing up atomic number and atomic mass — atomic number counts protons and identifies the element; atomic mass is the average mass of the atom and is usually a larger decimal number.
Believing atoms are visible under a regular microscope — atoms are far too small; you need special equipment like an electron microscope to image them.

FAQs

What makes one element different from another?

The number of protons in the nucleus. Carbon always has 6 protons, oxygen always has 8, and gold always has 79. Change the proton count and you have a completely different element.

Can atoms be broken apart?

Yes, but only with extreme energy (like in a nuclear reaction). When atoms are broken apart in everyday chemistry class, we are actually just rearranging or breaking the bonds BETWEEN atoms, not splitting the atoms themselves.

How many elements are there?

As of today there are 118 known elements, all listed on the Periodic Table. About 94 of them occur naturally on Earth; the rest have been made by scientists in laboratories.

Why do chemical symbols sometimes not match the element name, like Fe for iron?

Many symbols come from the element's Latin or Greek name. Iron's Latin name is Ferrum (Fe), sodium's is Natrium (Na), and gold's is Aurum (Au). These names were used by early scientists before English became common in science.

Is air an element, a molecule, or a mixture?

Air is a MIXTURE — it contains many different gases (mostly nitrogen N2 and oxygen O2, plus small amounts of argon, carbon dioxide, and others) that are NOT chemically bonded to each other, just mixed together.

What is the difference between an atom and a molecule in simple terms?

One atom is a single particle of one element, like one oxygen atom (O). A molecule is when two or more atoms are chemically bonded, like two oxygen atoms bonded together (O2) or two hydrogens and one oxygen bonded into water (H2O).

Want the whole picture for your child?

Every K–6 subject, an AI tutor that teaches step by step, unlimited practice, and a reward world.

Start a 3-day free trial