ATOMS AND MOLECULES REVISON NOTES

 

ATOMS AND MOLECULES

What is an Atom?

• An atom is the smallest particle of an element.

📚 Dalton’s Atomic Theory -Jhon Dalton gave atomic theory.

1. Matter is made of tiny particles called atoms.
2. Atoms of same element are same.
3. Atoms of different elements are different.
4. Atoms combine in simple whole numbers.
5. Atoms cannot be created or destroyed in a chemical reaction.

What is the Law of Conservation of Mass?

👨‍🔬 Given by: Antoine Lavoisier

📖 Statement

👉 Mass can neither be created nor destroyed in a chemical reaction.

✏️ Easy Meaning

• In a chemical reaction,
  Total mass of reactants = Total mass of products

OR

👉 Nothing disappears, nothing new is created — it only changes form.

🧠 Why Does This Happen?

• In a reaction, atoms only rearrange.
• Atoms are not destroyed.
• Atoms are not created.
• So total mass remains same.

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🧪 Example

Suppose:

10 g Hydrogen + 80 g Oxygen → Water

Total mass before reaction = 10 + 80 = 90 g

Mass of water formed = 90 g

✔ Mass before = Mass after

Law of Constant Proportions

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👨‍🔬 Given by: Joseph Proust

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📖 Statement

👉 In a chemical compound, the elements are always present in a fixed proportion by mass, irrespective of its source or method of preparation.

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✏️ Easy Meaning (Very Simple Words)

• A compound always has the same elements.
• These elements are always present in the same fixed ratio by mass.
• It does not matter:
• Where the compound comes from.
• How it is made.

👉 The ratio never changes.

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🧪 Example 1: Water (H₂O)

Water contains:Hydrogen (H) & Oxygen (O)

By mass:

• Hydrogen = 1 part
• Oxygen = 8 parts

So, ratio = 1 : 8

Even if water is:From river 💧,From sea 🌊,From rain ☔,Made in laboratory 🧪

👉 The ratio of H and O will always be 1 : 8

It never changes.

DIFFERENCE BETWEEN ATOMS AND MOLECULES

ATOMS	MOLECULES
1.Atoms are the smallest particles that can exist independently	1.Molecules are made up of atoms
2.atoms are composed of neutrons,protons and electrons.	2.Molecules are composed of atoms.
3.Atoms are more reactive.	3.molecules are less reactive.
4.Atoms cannot divide into their particles by chemical methods .	4.molecules can be divide in to their atoms by chemical methods.
5.Example. oxygen (O)	Water (H2O)

ATOMICITY- No of atoms in a molecule is known as atomicity.

Atomicity	Example
Monoatomic (1)	He
Diatomic (2)	O₂
Triatomic (3)	O₃
Polyatomic (more than 3)	P₄

Ions-charged particles are called ions.

They are of two types:-

Cations:-the positively charged ions are called cations. Examples: Na⁺, Ca²⁺

2.      Anion: A negatively charged ion. Examples: F⁻, Cl⁻.

Molecular Mass

• Definition: The total of the atomic masses of all atoms in a molecule is called the molecular mass.

• Calculation: Multiply the atomic mass of each element by the number of its atoms in the molecule, then add these values.

Formula Unit Mass

• The sum of all atoms' atomic masses in an ionic compound's formula unit.

• Calculation: Add the atomic masses of the elements in the formula unit. For example, for Sodium Chloride (NaCl): 

(1×23) + (1×35.5) = 58.5 u

• Valency: The combining capacity of an element, showing how atoms mix with others. For example, hydrogen (H⁺) has a valency of 1, and oxygen (O²⁻) has a valency of 2.

·        Polyatomic Ions: If more than one polyatomic ion is present, place them in brackets. For example, calcium nitrate is written as Ca(NO₃)₂.

Valency table

Some important chemical formulas

1.      sodium oxide – Na₂O

2.       aluminium chloride – AlCl₃

3.       sodium sulphide – Na₂S

4.       magnesium hydroxide – Mg (OH)₂

5.       Al₂(SO₄)₃ – Aluminium sulphate

6.       CaCl₂ – Calcium chloride

7.       K₂SO₄ – Potassium sulphate

8.       KNO₃ – Potassium nitrate

9.       CaCO₃ – Calcium carbonate(lime stone)

10.  Magnesium chloride – MgCl₂

11.   Calcium oxide – CaO

12.   Copper nitrate – Cu(NO₃)₂

Give the names of the elements present in the following compounds.

(a) Quick lime

(b) Hydrogen bromide

(c) Baking powder

(d) Potassium sulphate

Solution:

The following are the names of the elements present in the following compounds:

(a) Quick lime – Calcium and oxygen (CaO)

(b) Hydrogen bromide – Hydrogen and bromine (HBr)

(c) Baking powder – Sodium, Carbon, Hydrogen, Oxygen (NaHCO₃)

(d) Potassium sulphate – Sulphur, Oxygen, Potassium (K₂SO₄)