Chemistry Basics - Atoms, Elements and Compounds

Master the fundamental concepts of chemistry for RRB exam preparation with clear explanations and practical examples.

Atomic Structure

Basic Components

Atoms

• Definition: Smallest unit of an element
• Composition: Protons, neutrons, electrons
• Size: Approximately 10{�p meters

Subatomic Particles

• Protons: Positive charge, mass = 1 amu
• Neutrons: No charge, mass = 1 amu
• Electrons: Negative charge, mass H 0 (1/1836 of proton)

Atomic Number and Mass Number

• Atomic Number (Z): Number of protons
• Mass Number (A): Protons + neutrons
• Formula: A = Z + (number of neutrons)

Practice Problems

Question 1

An atom has 6 protons and 8 neutrons. What are its atomic number and mass number?

Question 2

If an element has atomic number 17 and mass number 35, how many neutrons does it have?

Question 3

Find the number of electrons in a neutral atom with atomic number 26.

Periodic Table

Organization

Periods and Groups

• Periods: Horizontal rows (1-7)
• Groups: Vertical columns (1-18)
• Elements: 118 known elements

Classification

• Metals: Left side and center
• Non-metals: Right side
• Metalloids: Staircase between metals and non-metals

Important Groups

Group 1: Alkali Metals

• Properties: Soft, reactive, 1 valence electron
• Examples: Lithium, Sodium, Potassium
• Reactivity: Increases down the group

Group 17: Halogens

• Properties: Highly reactive, 7 valence electrons
• Examples: Fluorine, Chlorine, Bromine
• State at room temp: F (gas), Cl (gas), Br (liquid)

Group 18: Noble Gases

• Properties: Inert, 8 valence electrons (except Helium)
• Examples: Helium, Neon, Argon
• Uses: Lighting, preservation

Periodic Trends

Atomic Size

• Decreases across a period
• Increases down a group

Ionization Energy

• Energy required to remove an electron
• Increases across a period
• Decreases down a group

Electronegativity

• Ability to attract electrons
• Increases across a period
• Decreases down a group

Practice Problems

Question 4

Which element is more reactive: Sodium or Magnesium? Why?

Question 5

Arrange these elements in increasing atomic size: F, Cl, Br, I.

Question 6

Why are noble gases chemically inert?

Chemical Bonding

Types of Bonds

Ionic Bond

• Transfer of electrons
• Metal + non-metal
• Forms ions
• Example: NaCl (Sodium Chloride)

Covalent Bond

• Sharing of electrons
• Non-metal + non-metal
• Forms molecules
• Example: H�O (Water), CO� (Carbon Dioxide)

Metallic Bond

• Sea of electrons
• Metal atoms
• Conducts electricity
• Example: Iron, Copper

Chemical Formulas

Writing Formulas

• Balance positive and negative charges
• Use smallest whole numbers
• Example: Mg�z + Cl{ � MgCl�

Naming Compounds

• Ionic: Metal name + non-metal name + “-ide”
• Covalent: Prefixes + names

Practice Problems

Question 7

Write the chemical formula for Calcium Oxide.

Question 8

Name the compound: Na�SO�

Question 9

What type of bond exists between Hydrogen and Oxygen in water?

Chemical Reactions

Types of Reactions

Synthesis (Combination)

• A + B � AB
• Example: 2H� + O� � 2H�O

Decomposition

• AB � A + B
• Example: 2H�O � 2H� + O�

Single Replacement

• A + BC � AC + B
• Example: Zn + CuSO� � ZnSO� + Cu

Double Replacement

• AB + CD � AD + CB
• Example: AgNO� + NaCl � AgCl + NaNO�

Balancing Equations

Steps to Balance

1. Write correct formulas
2. Count atoms on both sides
3. Add coefficients to balance
4. Check all atoms are balanced

Example

Unbalanced: H� + O� � H�O Balanced: 2H� + O� � 2H�O

Practice Problems

Question 10

Balance the equation: Fe + O� � Fe�O�

Question 11

Identify the type of reaction: 2Na + Cl� � 2NaCl

Question 12

Complete and balance: CaCO� � CaO + ?

Acids and Bases

Properties

Acids

• Taste: Sour
• Feel: Burning sensation
• Indicators: Turn blue litmus red
• pH: Less than 7
• Examples: HCl, H�SO�, HNO�

Bases

• Taste: Bitter
• Feel: Slippery
• Indicators: Turn red litmus blue
• pH: Greater than 7
• Examples: NaOH, KOH, Ca(OH)�

Neutralization Reaction

• Reaction: Acid + Base � Salt + Water
• Example: HCl + NaOH � NaCl + H�O

pH Scale

• Range: 0 to 14
• Neutral: pH = 7 (water)
• Acidic: pH < 7
• Basic: pH > 7

Practice Problems

Question 13

What is the pH of a solution that turns blue litmus red?

Question 14

Write the reaction between sulfuric acid and sodium hydroxide.

Question 15

Which of these is strongest acid: pH 2, pH 5, pH 7?

Metals and Non-metals

Properties of Metals

• Physical: Shiny, malleable, ductile, good conductors
• Chemical: Lose electrons easily, form basic oxides
• Examples: Iron, Copper, Aluminum, Gold

Properties of Non-metals

• Physical: Dull, brittle, poor conductors (exceptions)
• Chemical: Gain electrons, form acidic oxides
• Examples: Oxygen, Carbon, Sulfur, Chlorine

Metal Reactivity Series

Most Reactive to Least Reactive:

1. Potassium (K)
2. Sodium (Na)
3. Calcium (Ca)
4. Magnesium (Mg)
5. Aluminum (Al)
6. Zinc (Zn)
7. Iron (Fe)
8. Copper (Cu)
9. Silver (Ag)
10. Gold (Au)

Uses of Common Metals

• Iron: Construction, tools, machinery
• Aluminum: Aircraft, cans, foil
• Copper: Electrical wiring, pipes
• Gold: Jewelry, electronics

Practice Problems

Question 16

Which metal is more reactive: Iron or Copper?

Question 17

Why are metals good conductors of electricity?

Question 18

What happens when sodium reacts with water?

Organic Chemistry Basics

Hydrocarbons

Definition

• Compounds containing only carbon and hydrogen
• Main types: Alkanes, Alkenes, Alkynes

Alkanes (Saturated)

• Formula: C�H����
• Bonds: Single bonds only
• Examples: Methane (CH�), Ethane (C�H�)

Alkenes (Unsaturated)

• Formula: C�H��
• Bonds: At least one double bond
• Examples: Ethene (C�H�), Propene (C�H�)

Alkynes (Unsaturated)

• Formula: C�H����
• Bonds: At least one triple bond
• Examples: Ethyne (C�H�), Propyne (C�H�)

Functional Groups

• Alcohols: -OH group (e.g., ethanol C�H�OH)
• Carboxylic Acids: -COOH group (e.g., acetic acid CH�COOH)
• Esters: -COO- group (fruity smells)

Practice Problems

Question 19

Write the formula for propane (alkane with 3 carbons).

Question 20

Identify the functional group: CH�CH�OH

Question 21

What type of hydrocarbon is: C�H�?

Quick Reference

Important Formulas

• Atomic Mass Number: A = Z + N
• Moles: n = mass/molar mass
• Concentration: M = moles/volume (L)

Common Elements and Symbols

• Hydrogen: H
• Carbon: C
• Oxygen: O
• Nitrogen: N
• Sodium: Na
• Potassium: K
• Iron: Fe
• Copper: Cu

Acid Formulas

• Hydrochloric Acid: HCl
• Sulfuric Acid: H�SO�
• Nitric Acid: HNO�
• Acetic Acid: CH�COOH

Base Formulas

• Sodium Hydroxide: NaOH
• Potassium Hydroxide: KOH
• Calcium Hydroxide: Ca(OH)�

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