SSS 1: PARTICULATE NATURE OF MATTER (I)

Matter is anything that has mass and occupies space. It is made up of discrete particles, the main ones are: atoms, molecules and ions.

                          Atoms

Atom is the smallest particle of an element which can take part in a chemical reaction.

The symbol of an element represents one atom e.g. Na represents one atom of sodium, O is an atom of oxygen, Cl is an atom of chlorine etc

                          Molecules

Molecule is the smallest particle of a substance that can exist on its own and still retain the chemical properties of that substance be it an element or compound. Most atoms cannot exist alone, they bond with other atoms to form molecules. Molecules may be made up of atoms of the same or different elements e. g. Cl2, H2O etc

                           Atomicity

Atomicity of an element is the number of atoms in each molecule of an element.

                  Types of Atomicity

Atomicity refers to the number of atoms present in a molecule. The main types of atomicity are:

1. Monatomic

2. Diatomic

3. Polyatomic

1. Monatomic

Monatomic substances consist of a single atom that exists independently. These are usually noble gases and some metals.

Examples: Helium (He), Neon (Ne), Argon (Ar), Calcium (Ca), Sodium (Na).

2. Diatomic

Diatomic substances are made up of two atoms of the same element bonded together.

Examples: Oxygen (O₂), Hydrogen (H₂), Nitrogen (N₂), Chlorine (Cl₂).

3. Polyatomic

Polyatomic substances contain three or more atoms of the same element in each molecule.

Examples: Ozone (O₃), Phosphorus (P₄), Sulphur (S₈).

                           

                               Ions

An ion is any atom or group of atoms which possess an electric charge.

                       Types of ions

The two types of ions are:

I. Cations

II. Anions

                     Cations:

Cations are ions that carry a positive charge. Examples include H⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, etc.

Anions:

Anions are ions that carry a negative charge. Examples include F⁻, Cl⁻, O²⁻, CO₃²⁻, SO₄²⁻, etc.

Valency

Valency is the combining power of an element

OR 

Valency is the number of electrons an atom can lose, gain, or share in order to achieve a stable outer shell.

Valency determines how an element combines with other elements to form compounds.

Elements can have different valencies as shown below:

I. Monovalent Elements (Valency = 1)

Examples: Potassium (K), Sodium (Na), Hydrogen (H), Fluorine (F), Chlorine (Cl).

II. Divalent Elements (Valency = 2)

Examples: Calcium (Ca), Barium (Ba), Magnesium (Mg), Oxygen (O), Zinc (Zn).

III. Trivalent Elements (Valency = 3)

Examples: Aluminium (Al), Iron (Fe³⁺), Boron (B), Nitrogen (N).

IV. Tetravalent Elements (Valency = 4)

Examples: Carbon (C), Silicon (Si), Lead (Pb⁴⁺).

V. Pentavalent Elements (Valency = 5)

Examples: Phosphorus (P), Arsenic (As), Antimony (Sb).

               Variable Valency

Elements that can exhibit more than one valency.

Examples:

•Iron (Fe) = 2 or 3 → Fe²⁺ (ferrous), Fe³⁺ (ferric)

•Copper (Cu) = 1 or 2 → Cu⁺ (cuprous), Cu²⁺ (cupric)

•Lead (Pb) = 2 or 4

•Nitrogen and Phosporus = 3 or 5

•Sulphur (S) = 2, 4 or 6

                        Radicals

A radical is a group of atoms that acts as a single unit and carries either a positive or negative charge during a chemical reaction.

Radicals behave like individual atoms and take part in forming compounds.

Examples of Common Radicals and Their Valencies

Names of radicals and their Valencies 

              Chemical formula

Chemical formula: This is the formula that shows the elements in a particular chemical substance and the number of each atom present.

Example:

Write the chemical formula of the following compounds:

I. Calcium oxide 

II. Aluminium oxide 

III. Sodium trioxocarbonate (IV)

IV. Potassium trioxonitrate (V)

Solution:

I. CaO

II. Al₂O₃

III.Na₂CO₃

IV.KNO₃

           Chemical equations 

A chemical equation is a symbolic way of expressing a chemical reaction, showing how reactants are converted into products using their chemical formulae.

For example, when hydrogen reacts with oxygen, water is formed:

           H₂(g) + O₂(g) → 2H₂O(g)

             Reactant             Product 

The reactant side is the part of the chemical equation to the left of the ‘→’ symbol whereas the product side is the part to the right of the arrow symbol.

Examples: Balancing the Chemical Equations

1. Al + O₂ → Al₂O₃

Balanced equation: 4Al + 3O₂ → 2Al₂O₃

2. N₂ + H₂ → NH₃

Balanced equation: N₂ + 3H₂ → 2NH₃

React and balance the following equations:

1. FeCl₃(s) + NaOH →

Balanced equation: FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl

2. FeSO₄ + NaOH → 

Balanced equation: FeSO₄ + 2NaOH → Fe(OH)₂ + Na₂SO₄

3. Zn + HCl → 

Balanced equation:

Zn + 2HCl → ZnCl₂ + H₂

4. Mg + HCl →

Balanced equation:

Mg + 2HCl → MgCl₂ + H₂

5. Ca(OH)₂ + HNO₃ →

Balanced equation:

Ca(OH)₂ + 2HNO₃ → Ca(NO₃)₂ + 2H₂O

6. FeCl₂ + NaOH → 

Balanced equation:

FeCl₂ + 2NaOH → Fe(OH)₂ + 2NaCl

7. Al(s) + O₂(g) →

Balanced equation: 4Al + 3O₂ → 2Al₂O₃

         DALTON'S ATOMIC THEORY

Before the discovery of subatomic particles (protons, neutrons, and electrons), John Dalton (1808) proposed a theory to explain the nature and behavior of matter. His theory was based on experimental evidence and observations from chemical reactions and laws such as:

I. Law of Conservation of Mass

II. Law of Definite Proportions

III. Law of Multiple Proportions

Dalton’s ideas marked the beginning of modern atomic theory.

Dalton’s Atomic Theory states that:

1. All matter is made up of tiny, indivisible particles called atoms.

2. Atoms of a given element are identical in size, mass, and other properties.

3. Atoms of different elements differ in their masses and properties.

4. Atoms combine in simple whole-number ratios to form compounds.

5. Chemical reactions involve the rearrangement of atoms. Atoms are neither created nor destroyed during chemical reactions.

Modifications (Limitations) of Dalton’s Theory:

Later discoveries showed that some parts of Dalton’s theory were not completely correct.

 These modifications include:

1. Atoms are not indivisible.

Modification : Discovery of subatomic particles (protons, neutrons, and electrons) proved that atoms can be divided.

2. Atoms of the same element are not always identical.

Modification: Isotopes of an element have the same chemical properties but different masses.

Example: Carbon-12 and Carbon-14 are isotopes of carbon.

3. Atoms can be changed into other atoms.

Modification: In nuclear reactions, one element can be transformed into another (e.g., Uranium to Thorium).

4. Atoms do not always combine in simple whole-number ratios.

Modification: Some compounds (especially complex organic compounds) have non-integer ratios.

Modern Atomic Theory (Based on Dalton’s Work):

I. Atoms are divisible into subatomic particles (protons, neutrons, electrons).

II. Atoms of the same element may have different masses (isotopes).

III. Atoms of different elements combine chemically to form compounds.

IV. Atoms can change during nuclear reactions.

V. The number and arrangement of electrons determine the chemical behavior of an atom.

Assignment:

1. Why was Dalton’s Atomic Theory important in the development of modern chemistry?

2. State the law of conservation of mass and give one example of how Dalton’s theory explains the law of conservation of mass.

3. What discovery led to the modification that atoms are divisible?