Molecule Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Molecule.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

The smallest unit of a covalent substance, consisting of two or more atoms held together by chemical bonds (typically covalent), acting as a single distinct unit.

Atoms stuck together. Water (H2O\text{H}_2\text{O}) is one molecule with 3 atoms.

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Molecule asks what the sample is, what property is being used, and whether a new substance is formed.

Common stuck point: Students often know a formula related to molecule but skip the recognition step: Am I classifying matter or using properties, state, particle behavior, or mixture evidence to describe a sample? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I classifying matter or using properties, state, particle behavior, or mixture evidence to describe a sample?

Worked Examples

Example 1

easy
Define a molecule and explain the difference between a molecule of an element and a molecule of a compound.

Answer

Molecule of element: O2. Molecule of compound: H2O.\text{Molecule of element: O}_2\text{. Molecule of compound: H}_2\text{O.}

First step

1
A molecule is a group of two or more atoms held together by covalent bonds, acting as a single unit.

Full solution

  1. 2
    A molecule of an element contains only one type of atom: e.g., O2\text{O}_2 (two oxygen atoms), N2\text{N}_2 (two nitrogen atoms).
  2. 3
    A molecule of a compound contains two or more different types of atoms: e.g., H2O\text{H}_2\text{O} (hydrogen and oxygen), CO2\text{CO}_2 (carbon and oxygen).
The term molecule specifically refers to covalently bonded groups of atoms. Ionic compounds like NaCl form crystal lattices rather than discrete molecules, so they are described by formula units instead.

Example 2

medium
The molecular formula of glucose is C6H12O6\text{C}_6\text{H}_{12}\text{O}_6. How many total atoms are in one molecule of glucose? How many molecules are in one mole of glucose?

Example 3

medium
How many atoms are in one molecule of C6H12O6\text{C}_6\text{H}_{12}\text{O}_6 (glucose)? How many molecules are in 1 mole of glucose?

Example 4

medium
A student labels each gas in a container as a molecule or not. The gases are Ne\text{Ne}, O2\text{O}_2, CO2\text{CO}_2, and He\text{He}. Sort them.

Example 5

medium
Why do chemists call H2O\text{H}_2\text{O} a molecule but call Na2O\text{Na}_2\text{O} a formula unit?

Example 6

medium
A bottle is labeled 'pure N2\text{N}_2 gas.' Are the smallest particles in the bottle atoms, molecules, or formula units?

Example 7

hard
Compare diamond and a sample of CO2\text{CO}_2. Both contain carbon. Which sample is composed of molecules?

Example 8

hard
A label says 'one molecule of KOH\text{KOH}.' Is the label scientifically accurate? Justify.

Example 9

challenge
Classify each sample as (i) a discrete molecule, (ii) a noble-gas atom, (iii) an ionic formula unit, or (iv) a network solid: S8\text{S}_8, CaF2\text{CaF}_2, SiO2\text{SiO}_2 (quartz), Kr.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

medium
Explain why O2\text{O}_2 and O3\text{O}_3 (ozone) are both molecules of the same element but have different properties.

Example 2

hard
A molecule of aspirin has the formula C9H8O4\text{C}_9\text{H}_8\text{O}_4. Calculate the total number of atoms in one molecule, the molar mass, and the number of hydrogen atoms in 0.500.50 mol of aspirin.

Example 3

easy
How many atoms are in one molecule of water, H2O\text{H}_2\text{O}?

Example 4

easy
Is O2\text{O}_2 (oxygen gas) a molecule? Explain in one line.

Example 5

easy
Is a single helium atom a molecule?

Example 6

easy
How many atoms are in one molecule of carbon dioxide, CO2\text{CO}_2?

Example 7

easy
What type of bond typically holds the atoms in a molecule together?

Example 8

easy
Is N2\text{N}_2 a molecule of an element or a compound?

Example 9

easy
Does NaCl\text{NaCl} exist as discrete molecules?

Example 10

easy
How many atoms total are in one molecule of methane, CH4\text{CH}_4?

Example 11

medium
A molecule of glucose is C6H12O6\text{C}_6\text{H}_{12}\text{O}_6. How many atoms are in one molecule?

Example 12

medium
Compare O2\text{O}_2 and O3\text{O}_3 (ozone). Are they the same molecule? Why or why not?

Example 13

medium
Why is H2O\text{H}_2\text{O} classified as a molecule but MgO\text{MgO} is not?

Example 14

medium
Ammonia is NH3\text{NH}_3. How many atoms are in 3 molecules of ammonia?

Example 15

medium
Is sugar dissolved in water a molecule, a compound, or a mixture?

Example 16

medium
Why can diatomic Cl2\text{Cl}_2 be a molecule but a single Cl atom cannot?

Example 17

medium
A formula unit is CaCl2\text{CaCl}_2. Explain why chemists avoid calling it a molecule.

Example 18

medium
Ethanol is C2H6O\text{C}_2\text{H}_6\text{O}. How many atoms are in one molecule?

Example 19

medium
Why is HCl\text{HCl} considered a molecule but KCl\text{KCl} is not?

Example 20

challenge
Hydrogen peroxide is H2O2\text{H}_2\text{O}_2 and water is H2O\text{H}_2\text{O}. Both contain only H and O. Explain why they are different molecules with different properties.

Example 21

challenge
Sulfuric acid is H2SO4\text{H}_2\text{SO}_4. How many atoms are in one molecule, and how many in 2 molecules?

Example 22

challenge
Classify each as molecular element, molecular compound, or neither: O2\text{O}_2, CO2\text{CO}_2, Ne, NaCl\text{NaCl}.

Example 23

easy
How many atoms are in one molecule of hydrogen gas, H2\text{H}_2?

Example 24

easy
Is argon (Ar) found in air a molecule or a single atom?

Example 25

easy
Classify F2\text{F}_2 as a molecule of an element or a compound.

Example 26

easy
Is MgO\text{MgO} (magnesium oxide) best described as a molecule? Why or why not?

Example 27

easy
How many atoms are in 4 molecules of carbon monoxide, CO\text{CO}?

Example 28

medium
Caffeine has the molecular formula C8H10N4O2\text{C}_8\text{H}_{10}\text{N}_4\text{O}_2. How many atoms are in one molecule of caffeine?

Example 29

medium
How many hydrogen atoms are in 5 molecules of ammonia, NH3\text{NH}_3?

Example 30

medium
Propane has formula C3H8\text{C}_3\text{H}_8. How many atoms are in 2 molecules of propane?

Example 31

medium
A student sees P4\text{P}_4 as a label for white phosphorus. Is P4\text{P}_4 a molecule? Explain.

Example 32

medium
Acetone is C3H6O\text{C}_3\text{H}_6\text{O}. In one mole of acetone, how many atoms of hydrogen are present?

Example 33

hard
A sample contains 1.20×10241.20 \times 10^{24} molecules of H2O\text{H}_2\text{O}. How many moles of water and how many total atoms are in the sample?

Example 34

hard
Ethylene glycol is C2H6O2\text{C}_2\text{H}_6\text{O}_2. What is the total number of atoms in 0.400.40 mol of ethylene glycol?

Example 35

hard
Vitamin C is C6H8O6\text{C}_6\text{H}_8\text{O}_6. How many molecules are present in a 250 mg250\,\text{mg} tablet? (Molar mass ≈176.12 g/mol\approx 176.12\,\text{g/mol})

Example 36

hard
A gas sample contains equal numbers of CH4\text{CH}_4 and C2H6\text{C}_2\text{H}_6 molecules — 3.0×10223.0 \times 10^{22} of each. How many total atoms are in the sample?

Example 37

challenge
Buckminsterfullerene is C60\text{C}_{60}, a soccer-ball-shaped molecule. Calculate (a) the number of atoms in one molecule, and (b) the mass in grams of one molecule. (C = 12.01 g/mol12.01\,\text{g/mol})
← Back to Molecule Practice Mode

Background Knowledge

These ideas may be useful before you work through the harder examples.

atomchemical bond