Understanding the building blocks of matter with laws and formulas
Before learning about atoms and molecules, we need to understand some important laws that scientists discovered about how substances combine:
In a chemical reaction, mass is neither created nor destroyed.
This means: Total mass of reactants = Total mass of products
Imagine you have 10 LEGO bricks. You build a house with them. You still have 10 bricks, just arranged differently! Similarly, in chemical reactions, atoms just rearrange - no atom is lost or gained.
Take 2g of hydrogen and 16g of oxygen. When they react to form
water:
2g Hydrogen + 16g Oxygen = 18g Water
The total mass before reaction (18g) equals the total mass after
reaction (18g). Nothing is lost!
In a chemical compound, elements are always present in a fixed ratio by mass.
This means: No matter where you get a compound from, it will always have the same composition.
Water (H₂O) always has hydrogen and oxygen in the ratio 1:8 by
mass.
• Water from river: 1g H + 8g O
• Water from ocean: 1g H + 8g O
• Water from rain: 1g H + 8g O
• Water from laboratory: 1g H + 8g O
Always the same ratio! That's why it's called "constant
proportions".
An atom is the smallest particle of an element that can take part in a chemical reaction. It may or may not exist independently.
Think of atoms as the alphabets of chemistry! Just like words are made from letters (A, B, C...), all matter is made from atoms. And just like combining letters makes words, combining atoms makes molecules!
• Atoms are extremely small - about 10⁻¹⁰ meters in size
• If we line up 10 million atoms, the line would be only 1 mm
long!
• Atoms cannot be seen even with powerful microscopes
• Atoms of the same element are identical
• Atoms cannot be created or destroyed in chemical reactions
A molecule is the smallest particle of an element or compound that can exist independently and shows all the properties of that substance.
If atoms are like single LEGO bricks, molecules are like complete LEGO models! Just like you connect bricks to build something useful, atoms join together to form molecules that can exist on their own.
Formed by atoms of the same element joining together.
Examples:
• Hydrogen molecule (H₂) - 2 hydrogen atoms
• Oxygen molecule (O₂) - 2 oxygen atoms
• Nitrogen molecule (N₂) - 2 nitrogen atoms
• Ozone (O₃) - 3 oxygen atoms
• Phosphorus (P₄) - 4 phosphorus atoms
• Sulphur (S₈) - 8 sulphur atoms
Formed by atoms of different elements joining together.
Examples:
• Water (H₂O) - 2 hydrogen + 1 oxygen
• Carbon dioxide (CO₂) - 1 carbon + 2 oxygen
• Ammonia (NH₃) - 1 nitrogen + 3 hydrogen
• Methane (CH₄) - 1 carbon + 4 hydrogen
• Glucose (C₆H₁₂O₆) - 6 carbon + 12 hydrogen + 6 oxygen
The number of atoms present in one molecule of an element is called its atomicity.
| Type | Atomicity | Examples |
|---|---|---|
| Monoatomic | 1 | Helium (He), Neon (Ne), Argon (Ar) - Noble gases |
| Diatomic | 2 | Hydrogen (H₂), Oxygen (O₂), Nitrogen (N₂), Chlorine (Cl₂) |
| Triatomic | 3 | Ozone (O₃) |
| Tetraatomic | 4 | Phosphorus (P₄) |
| Polyatomic | Many | Sulphur (S₈) |
A chemical formula is a symbolic representation of a molecule. It shows which elements are present and how many atoms of each element are in one molecule.
H₂O (Water)
• H = Hydrogen
• ₂ = 2 atoms of hydrogen
• O = Oxygen (no number means 1 atom)
So, H₂O means 2 hydrogen atoms + 1 oxygen atom
CO₂ (Carbon Dioxide)
• C = 1 Carbon atom
• O₂ = 2 Oxygen atoms
H₂SO₄ (Sulphuric Acid)
• H₂ = 2 Hydrogen atoms
• S = 1 Sulphur atom
• O₄ = 4 Oxygen atoms
Step 1: Write symbols
Al (Aluminium) and O (Oxygen)
Step 2: Write valencies
Al has valency 3, O has valency 2
Al³⁺ O²⁻
Step 3: Cross-multiply
Al gets 2, O gets 3
Step 4: Write as subscripts
Al₂O₃
Answer: The formula is Al₂O₃ (reads as Aluminium
Two Oxygen Three)
An ion is an atom or group of atoms that has gained or lost electrons, giving it a positive or negative charge.
Cation: Positively charged ion (loses electrons).
Example: Na⁺, Ca²⁺, Al³⁺
Anion: Negatively charged ion (gains electrons).
Example: Cl⁻, O²⁻, N³⁻
Think of ions like magnets! When an atom loses an electron, it becomes positive (like the north pole). When it gains an electron, it becomes negative (like the south pole). Opposite charges attract, so positive and negative ions stick together to form compounds!
| Positive Ions (Cations) | Negative Ions (Anions) |
|---|---|
| Sodium (Na⁺) | Chloride (Cl⁻) |
| Potassium (K⁺) | Bromide (Br⁻) |
| Silver (Ag⁺) | Oxide (O²⁻) |
| Calcium (Ca²⁺) | Sulphide (S²⁻) |
| Magnesium (Mg²⁺) | Nitride (N³⁻) |
| Aluminium (Al³⁺) | Hydroxide (OH⁻) |
| Ammonium (NH₄⁺) | Nitrate (NO₃⁻) |
| - | Sulphate (SO₄²⁻) |
| - | Carbonate (CO₃²⁻) |
Atomic mass is the average mass of an atom of an element compared to 1/12th mass of a carbon-12 atom. It has no unit (it's a relative mass).
• Hydrogen (H) = 1 u
• Carbon (C) = 12 u
• Nitrogen (N) = 14 u
• Oxygen (O) = 16 u
• Sodium (Na) = 23 u
• Sulphur (S) = 32 u
• Chlorine (Cl) = 35.5 u
(u = unified mass or atomic mass unit)
Molecular mass is the sum of atomic masses of all atoms present in one molecule of a substance.
Formula: Molecular Mass = Sum of (Number of atoms × Atomic mass) of each element
H₂O has:
• 2 atoms of Hydrogen (H = 1)
• 1 atom of Oxygen (O = 16)
Molecular mass = (2 × 1) + (1 × 16)
= 2 + 16
= 18 u
Answer: Molecular mass of water = 18 u
CO₂ has:
• 1 atom of Carbon (C = 12)
• 2 atoms of Oxygen (O = 16)
Molecular mass = (1 × 12) + (2 × 16)
= 12 + 32
= 44 u
Answer: Molecular mass of CO₂ = 44 u
NaCl has:
• 1 atom of Sodium (Na = 23)
• 1 atom of Chlorine (Cl = 35.5)
Formula unit mass = (1 × 23) + (1 × 35.5)
= 23 + 35.5
= 58.5 u
Answer: Formula unit mass of NaCl = 58.5 u
(We say "formula unit mass" for ionic compounds instead of
"molecular mass")
A mole is a unit that represents a specific number of particles (atoms, molecules, ions, etc.). One mole contains 6.022 × 10²³ particles.
This number is called Avogadro's Number or Avogadro's Constant.
Just like:
• 1 dozen = 12 items
• 1 gross = 144 items
• 1 pair = 2 items
Similarly:
• 1 mole = 6.022 × 10²³ particles
We use "mole" because atoms are so tiny that counting them
individually is impossible. Mole helps us count atoms in bulk,
just like we count eggs in dozens!
This number was chosen so that the mass of 1 mole of atoms (in
grams) equals the atomic mass (in u).
For example:
• Atomic mass of Carbon = 12 u
• Mass of 1 mole of Carbon atoms = 12 grams
• Atomic mass of Oxygen = 16 u
• Mass of 1 mole of Oxygen atoms = 16 grams
Molar mass is the mass of 1 mole of a substance. It is expressed in grams per mole (g/mol).
Important: Molar mass (in grams) = Atomic/Molecular mass (in u)
• Molar mass of H₂O = 18 g/mol
(1 mole of water weighs 18 grams)
• Molar mass of CO₂ = 44 g/mol
(1 mole of CO₂ weighs 44 grams)
• Molar mass of NaCl = 58.5 g/mol
(1 mole of salt weighs 58.5 grams)
Number of moles = Given mass / Molar mass
Use this when you know the mass and want to find moles
Mass = Number of moles × Molar mass
Use this when you know moles and want to find mass
Number of particles = Moles × 6.022 × 10²³
Use this to find actual number of atoms/molecules
Moles = Number of particles / 6.022 × 10²³
Use this when you know particle count
Question: Calculate the number of moles in 36g of
water.
Solution:
Given mass of water = 36g
Molar mass of H₂O = 18 g/mol
Number of moles = Given mass / Molar mass
= 36 / 18
= 2 moles
Answer: 36g of water contains 2 moles
Question: Calculate the mass of 0.5 mole of
CO₂.
Solution:
Number of moles = 0.5
Molar mass of CO₂ = 44 g/mol
Mass = Number of moles × Molar mass
= 0.5 × 44
= 22 grams
Answer: 0.5 mole of CO₂ weighs 22 grams
Question: How many molecules are present in 18g
of water?
Solution:
Step 1: Find number of moles
Given mass = 18g
Molar mass of H₂O = 18 g/mol
Number of moles = 18/18 = 1 mole
Step 2: Find number of molecules
1 mole contains 6.022 × 10²³ molecules
Answer: 18g of water contains 6.022 × 10²³
molecules
Question: Calculate the mass of one atom of
carbon.
Solution:
Molar mass of Carbon = 12 g/mol
This means 1 mole (6.022 × 10²³ atoms) weighs 12 grams
Mass of 1 atom = 12 / (6.022 × 10²³)
= 1.99 × 10⁻²³ grams
Answer: Mass of one carbon atom = 1.99 × 10⁻²³ g
❌ Don't confuse atom with molecule
✅ Atom is single, molecule is combination
❌ Don't forget to multiply by number of atoms
✅ In H₂O, hydrogen contributes 2×1 = 2 to molecular mass
❌ Don't mix up molar mass units
✅ Always use g/mol for molar mass, u for atomic mass
❌ Don't forget Avogadro's number
✅ 1 mole always = 6.022 × 10²³ particles