Cheat Sheet: Atoms and Molecules

Table of Contents
1. Laws of Chemical Combination
2. Dalton's Atomic Theory
3. Atom
4. Symbols of Elements
5. Atomic Mass
6. Molecules
7. Ions
8. Valency
9. Writing Chemical Formulae
10. Molecular Mass
11. Formula Unit Mass
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1. Laws of Chemical Combination

Law of Conservation of Mass

Statement:
Mass can neither be created nor destroyed in a chemical reaction. The total mass of reactants is always equal to the total mass of products.

Example

Products formed:

Total mass of reactants = 11.3 g
Total mass of products = 11.3 g

Law of Constant Proportions (Law of Definite Proportions)

Statement:
In a chemical compound, elements are always present in the same fixed proportion by mass, regardless of the source of the compound.

Examples

2. Dalton's Atomic Theory

Postulates

1. All matter is made up of very small particles called atoms.
2. Atoms are indivisible particles that cannot be created or destroyed during chemical reactions.
3. Atoms of the same element have identical mass and chemical properties.
4. Atoms of different elements have different masses and chemical properties.
5. Atoms combine in simple whole-number ratios to form compounds.
6. The number and type of atoms in a compound are fixed.

3. Atom

Definition:
The smallest particle of an element that participates in chemical reactions.

Size of Atom

Atoms are extremely small. Their size is measured in nanometres.

1 nm = $10^\{-9\}$10^-9 m

Relative Sizes

4. Symbols of Elements

Rules

• First letter is always capital.
• Second letter, if present, is lowercase.
• Derived from English or Latin names.

Examples

5. Atomic Mass

Definition:
The relative atomic mass of an element is the average mass of its atoms compared with one-twelfth the mass of a carbon-12 atom.

Atomic Mass Unit (u)

1 u = $\frac{1}{12}\backslash frac\{1\}\{12\}$ mass of one carbon-12 atom.

Atomic Mass of Some Elements

6. Molecules

Definition:
A molecule is a group of two or more atoms chemically bonded together. It is the smallest particle of a substance that can exist independently and retain its chemical properties.

Molecules of Elements

Atomicity:
The number of atoms present in a molecule.

Molecules of Compounds

7. Ions

Definition:
Ions are charged particles formed when atoms lose or gain electrons.

Types of Ions

Polyatomic Ions

8. Valency

Definition:
The combining capacity of an atom is known as its valency.

Valency determines how atoms combine to form compounds.

9. Writing Chemical Formulae

Steps

1. Write the symbols of the elements.
2. Write their valencies.
3. Criss-cross the valencies.
4. Simplify the ratio if needed.
5. Use brackets for polyatomic ions if more than one is present.

Examples

10. Molecular Mass

Definition:
The molecular mass of a substance is the sum of the atomic masses of all atoms in one molecule of the substance.

Example

Water (H₂O)

Atomic mass of H = 1
Atomic mass of O = 16

Molecular mass = $2\; \times \; 1\; +\; 16$2 × 1 + 16 = 18 u

11. Formula Unit Mass

Used for ionic compounds.

Definition:
The sum of atomic masses of all atoms in the formula unit of a compound.

Example: Sodium chloride (NaCl)

Atomic mass of Na = 23
Atomic mass of Cl = 35.5

Formula unit mass = 23 + 35.5 = 58.5u