C. Matter and Energy · Pure Substances and Mixtures

Elements and Compounds: Atoms and Molecules

C2.8 C2.2 A3.3 A1.5About 45 minutes

Learning goals

I can describe pure substances as either elements or compounds.
I can explain that elements are made of one kind of atom and compounds are made of two or more kinds of atoms joined together.
I can read simple chemical symbols and formulas, like O, Fe and H₂O.
I can explain how a compound is different from a mixture.

Words to know

atom
The smallest particle of an element that still has the properties of that element.

element
A pure substance made of only one kind of atom. Examples: oxygen, iron, gold.

compound
A pure substance made of two or more kinds of atoms chemically joined together in a fixed ratio. Examples: water, carbon dioxide, table salt.

molecule
A group of two or more atoms joined together. A water molecule is 2 hydrogen atoms and 1 oxygen atom.

chemical symbol
A one- or two-letter short form for an element. The first letter is a capital: H, O, Fe, Au.

chemical formula
Symbols and small numbers that show which atoms, and how many of each, are in a compound. Water is H₂O.

periodic table
A chart that organizes all the known elements by their properties.

Remember from Grade 6

In Grade 6 you learned that everything is made of tiny atoms, with positively charged protons and negatively charged electrons. Now you will see how atoms build the pure substances you met last lesson.

Building with atoms

Think of the letters of the alphabet. With just 26 letters, you can write every word in the dictionary. Atoms are like that. There are 118 known elements, so there are 118 kinds of atoms, but they combine to make millions of different substances.

Pure substances come in two kinds:

An element is made of only one kind of atom. It can't be broken down into simpler substances. Examples: oxygen, iron, copper, gold, carbon, helium.
A compound is made of two or more kinds of atoms that are chemically joined in a fixed ratio. Examples: water (hydrogen and oxygen), carbon dioxide (carbon and oxygen), table salt (sodium and chlorine), sugar (carbon, hydrogen and oxygen).

A group of atoms joined together is called a molecule. Some elements form molecules too: the oxygen we breathe is made of pairs of oxygen atoms (O₂). It is still an element, because it has only one kind of atom.

Left: copper, an element made of single copper atoms. Middle: oxygen gas, an element made of molecules of 2 oxygen atoms. Right: water, a compound; each molecule has 1 large oxygen atom (green) and 2 small hydrogen atoms (yellow).

Symbols and formulas

Scientists everywhere use the same short forms.

A chemical symbol stands for one element. It is one capital letter, sometimes followed by a small letter: H (hydrogen), O (oxygen), C (carbon), N (nitrogen), Na (sodium), Cl (chlorine), Cu (copper), Fe (iron), Au (gold), Ag (silver), Al (aluminium). Some symbols come from Latin names: ferrum = iron, aurum = gold.
A chemical formula shows the atoms in a compound. The small number after a symbol tells how many of that atom. No number means 1.

H₂O → 2 hydrogen atoms + 1 oxygen atom. CO₂ → 1 carbon atom + 2 oxygen atoms. NaCl → 1 sodium + 1 chlorine (in a ratio of 1 to 1).

	Element	Compound	Mixture
Kinds of particles	One kind of atom	One kind of molecule (or unit), made of 2+ kinds of atoms	Two or more kinds of particles
Pure substance?	Yes	Yes	No
Fixed recipe?	Yes	Yes: always the same ratio (water is always H₂O)	No: you can add more or less of each part
How to separate	Can't be broken down further	Only by a chemical change	By physical methods (filtering, evaporating...)
Properties	Its own	New properties, different from its elements	Parts keep their own properties
Examples	Gold, oxygen, helium	Water, salt, carbon dioxide	Air, salt water, granola

Elements, compounds and mixtures compared.

A map of all matter. Elements and compounds are both pure substances; solutions and heterogeneous mixtures are both mixtures. The bottom row gives an example of each.

Worked example

Let's compare a compound with its elements

Table salt is the compound sodium chloride (NaCl). Compare its properties with the properties of the elements in it.

Sodium (Na) is a soft, shiny metal. It reacts violently with water, so it is stored under oil in labs.
Chlorine (Cl) is a poisonous, greenish-yellow gas.
When sodium and chlorine atoms chemically join, they make sodium chloride: white crystals that we sprinkle on food.
The compound has completely new properties. It is not just a mix of the two elements. That's the big difference between a compound and a mixture.

Sodium chloride is safe to eat, while sodium and chlorine on their own are dangerous. A compound has properties that are different from the elements that make it.

Worked example

Let's read formulas

How many atoms of each kind are in one molecule of CH₄ (methane, natural gas) and one molecule of C₁₂H₂₂O₁₁ (table sugar)?

CH₄: C has no number, so 1 carbon. H₄ means 4 hydrogen. Total: 5 atoms.
C₁₂H₂₂O₁₁: 12 carbon, 22 hydrogen, 11 oxygen. Total: 12 + 22 + 11 = 45 atoms.
Both have more than one kind of atom, so both are compounds.

Methane: 1 C + 4 H (5 atoms). Sugar: 12 C + 22 H + 11 O (45 atoms). Both are compounds.

Just for fun

What did the chemist say when she found two pieces of gold?

"Au, yeah!" (Remember: Au is the symbol for gold, from the Latin aurum.)

Who discovered the elements?

People from many cultures worked with elements long before modern chemistry. Gold, copper, silver, iron and tin were used by ancient peoples around the world. Copper from around Lake Superior was worked by Indigenous peoples of the Great Lakes region thousands of years ago.

In 1869, the Russian chemist Dmitri Mendeleev arranged the known elements into a table by their properties, and left gaps for elements nobody had found yet. Later scientists discovered those elements, and they fit his predictions. Scientists such as Marie Curie, who was born in Poland and worked in France, discovered new elements (polonium and radium). Today's periodic table is the work of many scientists from many countries.

Hands-on activity

Build molecules with food

You need: two kinds of small food items, such as mini marshmallows and raisins (or grapes cut in half by a grown-up), plus toothpicks, and a clean plate. Toothpicks are sharp: point them away from your face and keep them away from little children.

Let marshmallow = oxygen (O), raisin = hydrogen (H).

Build one water molecule (H₂O): 1 marshmallow joined by toothpicks to 2 raisins.
Build one oxygen molecule (O₂): 2 marshmallows joined together.
Build one hydrogen molecule (H₂): 2 raisins joined together.
Sort your models: which are elements and which is a compound?
Now put 3 water molecules and 2 oxygen molecules together on the plate, without joining them. Is the plate a pure substance or a mixture?
Share it: label each model with its formula, and take a photo or draw it.

What to look for: O₂ and H₂ are elements (one kind of atom). H₂O is a compound (two kinds of atoms joined). The plate with water molecules and oxygen molecules side by side is a mixture, because it has two kinds of particles that are not joined together.

Safety: only eat food that hasn't touched the floor, and wash hands first. Marshmallows and raisins can be a choking hazard for small children.

Your turn

What is an element?

What is a compound?

Element or compound? Gold (Au).

Element or compound? Carbon dioxide (CO₂).

How many atoms of each element are in one molecule of H₂O?

Why is oxygen gas (O₂) an element even though its particles are molecules?

Give one way a compound is different from a mixture.

More practice: warm-up

What is the symbol for oxygen?

Which element has the symbol Fe?

Which element has the symbol Au?

How many known elements are there?

Element, compound or mixture? Salt water.

More practice: on your own

How many atoms of each element are in NH₃ (ammonia)?

How many atoms in total are in one molecule of C₆H₁₂O₆ (glucose)?

Water is made of hydrogen and oxygen. Hydrogen and oxygen are both gases at room temperature. What does this tell you about compounds?

A student mixes iron filings and sulfur powder. A magnet can still pull out the iron. Is this a compound or a mixture? Explain.

Why is CO a different substance from CO₂?

Challenge

Sort into element, compound or mixture, and explain each: (a) helium, (b) sugar, (c) sugar dissolved in tea, (d) aluminium foil.

Draw (or describe) a particle diagram of a mixture of two elements, and one of a compound.

Mendeleev left gaps in his periodic table. Explain why this was a strong scientific move.

Check your understanding

Quiz

An element is made of...

Which is a compound?

How many hydrogen atoms are in one molecule of CH₄?

Which statement about compounds is true?

The symbol Na stands for...

Who arranged the elements into an early periodic table in 1869?

For parents and teachers

Big idea: Ontario expectation C2.8 asks students to describe pure substances as elements and compounds made of atoms and combinations of atoms. Your child reads simple formulas and learns that a compound has new properties and a fixed recipe, unlike a mixture. The history section touches on contributions from different communities (A3.3).

How to help: Look at food labels and the periodic table online together. Ask: "How many atoms are in H₂O? Is that an element or a compound?" Build molecules with food or modelling clay.

Common difficulties:

Thinking every molecule is a compound. O₂ and H₂ are molecules but they are elements (one kind of atom).
Confusing compounds and mixtures. Compound = atoms chemically joined, fixed ratio, new properties. Mixture = just mixed, any amounts, parts keep their properties.
Reading formulas: no small number means 1 atom; the small number belongs to the symbol just before it.
Capital letters: Co (cobalt) is not CO (carbon monoxide).

Safety: toothpicks are sharp, and small foods can be a choking hazard for younger siblings.