Language of Chemistry

1. Introduction & Symbols

Chemistry is the branch of science that deals with the study of the composition, physical and chemical properties of various forms of matter. To understand chemical reactions easily, we use a specific Language of Chemistry consisting of symbols, formulae, and equations.

Evolution of Symbols

• Alchemists: Used pictorial symbols for elements.
• John Dalton (1807): Proposed circular symbols to represent elements (e.g., a circle with a dot for Hydrogen).
• J.J. Berzelius (1814): Suggested using letters of the alphabet to represent elements. This laid the foundation for the modern system.

Modern System of Naming Elements (IUPAC)

The International Union of Pure and Applied Chemistry (IUPAC) approves the names and symbols of elements. The rules are:

• Rule 1: The first letter of the English name of the element is used (e.g., H for Hydrogen, O for Oxygen). It is always written in a capital letter.
• Rule 2: If two or more elements start with the same letter, a two-letter symbol is used. The first letter is capital, and the second is small (e.g., Ca for Calcium, Co for Cobalt, C for Carbon).
• Rule 3: Symbols derived from their Latin names (e.g., Sodium/Natrium is Na, Potassium/Kalium is K, Iron/Ferrum is Fe, Copper/Cuprum is Cu).

2. Valency

Hydrogen is considered the standard for valency because its combining capacity is taken as 1. Thus, valency can also be defined as the number of hydrogen atoms that combine with or displace one atom of that element.

Modern Electronic Concept of Valency

• Atoms with 1, 2, or 3 valence electrons (metals) tend to lose them to achieve stability. Their valency is positive (e.g., Na⁺¹, Mg⁺², Al⁺³).
• Atoms with 5, 6, or 7 valence electrons (non-metals) tend to gain electrons (3, 2, or 1 respectively) to complete their octet. Their valency is negative (e.g., N^-3, O^-2, Cl^-1).
• Atoms with 4 valence electrons usually share them (e.g., Carbon).

Variable Valency

Certain elements exhibit more than one valency. This is called variable valency.

Naming conventions for variable valency:

• The suffix '-ous' is used for the lower valency.
• The suffix '-ic' is used for the higher valency.

3. Radicals

When molecules of an inorganic compound are dissolved in water, they split into two electrically charged halves known as radicals. They are of two types:

• Basic Radicals (Cations): Positively charged radicals. Usually formed by metallic ions (e.g., Na⁺, Ca²⁺, Al³⁺). An exception is the Ammonium ion (NH₄⁺), which is a non-metallic cation.
• Acid Radicals (Anions): Negatively charged radicals. Usually formed by non-metallic ions (e.g., Cl^-, O^2-, SO₄^2-).

Classification Based on Number of Atoms

• Simple Radicals: Contain only one atom (e.g., Na⁺, Mg²⁺, Cl^-).
• Compound/Complex Radicals: Contain a group of two or more atoms of different elements (e.g., SO₄^2- [Sulphate], OH^- [Hydroxide]).

4. Chemical Formula

The chemical formula is the symbolic representation of a molecule of a substance (element or compound). It tells us the number of atoms of each element present in one molecule of the substance.

Steps to Write a Chemical Formula (Criss-Cross Method)

Follow these rules to correctly deduce the molecular formula of a compound:

1. Write the symbols of the radicals side by side. The basic radical (positive) is written on the left, and the acid radical (negative) is written on the right.
2. Write the valency of each radical at the top right corner of its symbol.
3. Divide the valency numbers by their highest common factor (HCF) to get a simple ratio. (Ignore the + or - signs).
4. Criss-Cross the valencies. Shift the valency of the basic radical to the lower right of the acid radical, and vice versa.
5. If the radical is a compound radical and its resulting subscript is greater than 1, enclose it in brackets (e.g., (SO₄)₃). The subscript '1' is never written.

5. Chemical Equations

• Reactants: The substances taking part in the reaction (written on the Left-Hand Side).
• Products: The new substances formed as a result of the reaction (written on the Right-Hand Side).
• An arrow (→) separates the reactants from the products, indicating the direction of the reaction.

Balancing a Chemical Equation

A balanced chemical equation is one in which the total number of atoms of each element on the reactant side is exactly equal to the total number of atoms of that element on the product side.

Hit and Trial Method of Balancing

1. Count the number of atoms of each element on both sides of the skeletal equation.
2. Start balancing the element that appears least often. Typically, balance metals first, then non-metals, then Oxygen, and finally Hydrogen.
3. Multiply the formula of the substance by a suitable integer (coefficient) to equalize the atoms. Never change the chemical formula of a substance!
4. Recount to verify that all atoms are balanced.

Information Conveyed by a Balanced Equation

A balanced equation tells us:

• What substances are reacting and what substances are formed.
• The number of molecules of each substance taking part.
• The ratio of masses of the reactants and products.
• The relative volumes of gases (if gases are involved).

Limitations of a Chemical Equation & How to Overcome Them

A simple chemical equation does NOT tell us the physical state, concentration, temperature, pressure, or whether heat is absorbed or evolved. We make them more informative by adding:

• Physical states: (s) for solid, (l) for liquid, (g) for gas, (aq) for aqueous solution.
• Heat changes: Adding +Δ (heat evolved/exothermic) or -Δ (heat absorbed/endothermic).
• Conditions: Temperature, pressure, and catalyst are written above or below the arrow (→).
• Precipitate or Gas: A downward arrow (↓) for a precipitate and an upward arrow (↑) for gas evolved.

6. Relative Atomic and Molecular Masses

Atoms and molecules are extremely small, so their masses cannot be weighed directly. Instead, their masses are compared with a standard atom.

Relative Molecular Mass (RMM) or Molecular Weight

It is the number of times one molecule of a substance is heavier than 1/12th the mass of an atom of Carbon-12.
Calculation: It is calculated by adding the relative atomic masses of all the atoms present in one molecule of the substance.

Percentage Composition

Percentage composition is the percentage by weight of each element present in a compound.

Formula:

Percentage of element = [(Total weight of the element in one molecule) / (Gram Molecular Weight of the compound)] × 100

7. Practice Questions & Assignments

Test your understanding with these exam-oriented questions. Click on the questions to reveal the step-by-step answers.

Q1. Using the criss-cross method, write the chemical formulas for: (a) Magnesium Nitride (b) Calcium Bicarbonate.

(a) Magnesium Nitride:
Symbols: Mg and N. Valencies: Mg²⁺, N^3-.
Criss-crossing gives: Mg₃N₂

(b) Calcium Bicarbonate:
Symbols: Ca and HCO₃. Valencies: Ca²⁺, HCO₃^-.
Criss-crossing gives: Ca(HCO₃)₂

Q2. Balance the following chemical equations:
      (a) Fe + H₂O → Fe₃O₄ + H₂
      (b) Pb(NO₃)₂ → PbO + NO₂ + O₂

(a) First balance Oxygen (multiply H₂O by 4), then Hydrogen (multiply H₂ by 4), then Iron (multiply Fe by 3).
Balanced: 3Fe + 4H₂O → Fe₃O₄ + 4H₂

(b) This is a classic thermal decomposition reaction. Balance Lead, then Nitrogen, then Oxygen.
Balanced: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂

Q3. Calculate the relative molecular mass of washing soda (Na₂CO₃·10H₂O). [Given RAM: Na=23, C=12, O=16, H=1]

RMM of Na₂CO₃·10H₂O = (2 × Na) + (1 × C) + (3 × O) + 10 × [(2 × H) + (1 × O)]

= (2 × 23) + 12 + (3 × 16) + 10 × [2 + 16]

= 46 + 12 + 48 + 10[18]

= 106 + 180 = 286 amu

Q4. Calculate the percentage of Nitrogen in Urea (NH₂CONH₂). [Given RAM: N=14, H=1, C=12, O=16]

Step 1: Calculate RMM of Urea (NH₂CONH₂)
RMM = (2 × N) + (4 × H) + (1 × C) + (1 × O) = (2 × 14) + (4 × 1) + 12 + 16 = 28 + 4 + 12 + 16 = 60 amu

Step 2: Apply Percentage Formula
Mass of Nitrogen in 1 molecule = 28
% of N = (Mass of N / Total RMM) × 100
% of N = (28 / 60) × 100 = 46.67%

Q5. Name two metals that exhibit variable valency and give the names and formulas of their respective ions.

• Iron (Fe): Exhibits valencies of 2 and 3. Its ions are Ferrous (Fe²⁺) and Ferric (Fe³⁺).
• Copper (Cu): Exhibits valencies of 1 and 2. Its ions are Cuprous (Cu⁺) and Cupric (Cu²⁺).