Unit 1: Basic Concepts in Chemistry

Matter, Elements, Atoms, and Molecules

Matter: Anything that has mass and occupies space. It exists in three primary states: solid, liquid, and gas.

Element: A pure substance that cannot be broken down into simpler substances by chemical means. It is made of only one type of atom (e.g., Iron (Fe), Oxygen (O), Gold (Au)).

Atom: The smallest particle of an element that retains the chemical properties of that element. It is the basic building block of all matter.

Molecule: A group of two or more atoms held together by chemical bonds. It is the smallest particle of a substance that can exist independently (e.g., Water (H₂O), Oxygen (O₂), Nitrogen (N₂)).

Metal and Non-Metals

Elements are broadly classified as metals and non-metals.

The Structure of the Atom

An atom is composed of a central, dense nucleus and electrons that orbit it.

• Nucleus: Contains protons and neutrons.
  • Protons: Positively charged particles (+1).
  • Neutrons: Neutral particles (no charge).
• Electrons: Negatively charged particles (-1) that move in specific energy levels or shells around the nucleus.

In a neutral atom, the number of electrons is equal to the number of protons.

Chemical Bonding

Chemical bonds are the forces that hold atoms together to form molecules or compounds. Atoms bond to achieve a stable electron configuration, usually a full outer shell (the "octet rule").

Ionic Bonding

• How it forms: Involves the complete transfer of one or more electrons from a metal atom to a non-metal atom.
• Particles: This transfer creates ions: a positive cation (the metal that lost electrons) and a negative anion (the non-metal that gained electrons).
• The bond: The bond is the strong electrostatic attraction between these oppositely charged ions.
• Example: Sodium Chloride (NaCl)
  Sodium (Na) has 1 valence electron. Chlorine (Cl) has 7.
  Na gives its 1 electron to Cl.
  Na becomes a positive ion (Na⁺) and Cl becomes a negative ion (Cl⁻).
  The attraction between Na⁺ and Cl⁻ forms the ionic bond.

Covalent Bonding

• How it forms: Involves the sharing of one or more pairs of electrons between two non-metal atoms.
• The bond: The shared pair of electrons is attracted to the nuclei of both atoms, holding them together.
• Examples:
  • Methane (CH₄): Carbon shares one electron with each of four hydrogen atoms.
  • Oxygen (O₂): Two oxygen atoms share two pairs of electrons (a double bond).
  • Nitrogen (N₂): Two nitrogen atoms share three pairs of electrons (a triple bond).

Co-ordinate Bonding (or Dative Bonding)

• How it forms: A special type of covalent bond where both shared electrons are donated by only one of the atoms (the donor). The other atom (the acceptor) provides an empty orbital.
• Example: Ammonium Ion (NH₄⁺)
  Ammonia (NH₃) has a lone pair of electrons on the nitrogen.
  A hydrogen ion (H⁺) has an empty orbital.
  The nitrogen in NH₃ donates its entire lone pair to the H⁺ to form a co-ordinate bond, creating the NH₄⁺ ion.

Lewis Structural Representation

A Lewis structure is a simple diagram that shows the valence electrons of atoms in a molecule as dots. Shared pairs (bonds) are shown as lines, and unshared electrons are shown as lone pairs (dots).

Example: Water (H₂O)

• Oxygen has 6 valence electrons. Each Hydrogen has 1.
• Oxygen shares one electron with each H, forming two single bonds.
• Oxygen is left with two pairs of unshared electrons (lone pairs).

Example: Carbon Dioxide (CO₂)

• Carbon has 4 valence electrons. Each Oxygen has 6.
• Carbon forms two double bonds with the two oxygen atoms to satisfy the octet rule for all atoms.

Melting and Boiling Points

Melting Point: The temperature at which a solid turns into a liquid.
Boiling Point: The temperature at which a liquid turns into a gas.

These points are a measure of the strength of the forces holding the particles (atoms, ions, or molecules) together.

• Ionic Compounds (e.g., NaCl): Have very strong electrostatic forces between ions. This requires a huge amount of energy to break, so they have very high melting and boiling points.
• Covalent Compounds (e.g., H₂O, CH₄): The bonds *inside* the molecule are strong (covalent), but the forces *between* the molecules (intermolecular forces) are weak.
  • Simple Molecules (e.g., CH₄): Have very weak intermolecular forces, so they have low melting and boiling points (methane is a gas).
  • Water (H₂O): Has a special, stronger intermolecular force called hydrogen bonding, giving it a higher boiling point than expected, but still much lower than ionic compounds.

Scientific Notation

A method for writing very large or very small numbers in a compact form. It is written as the product of a number between 1 and 10 and a power of 10.

Format: M x 10ⁿ

• M is a number ≥ 1 and < 10.
• n is an integer (positive or negative).

Examples:
Speed of light = 300,000,000 m/s = 3.0 x 10⁸ m/s
Diameter of a hydrogen atom = 0.0000000001 m = 1.0 x 10⁻¹⁰ m

Chemical Reactions in Atmosphere

Acid Rain

• Cause: The burning of fossil fuels (coal, oil) releases sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) into the atmosphere.
• Reactions:
  • These gases react with water, oxygen, and other substances to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃).
  • SO₂ + H₂O → H₂SO₃ (Sulfurous acid)
  • 2SO₂ + O₂ → 2SO₃ (Sulfur trioxide)
  • SO₃ + H₂O → H₂SO₄ (Sulfuric acid)
• Effect: The acids fall to the earth as acid rain, which damages forests, lakes (killing fish), and buildings (corroding stone).

Greenhouse Effect and Global Warming

• Greenhouse Gases: Gases in the atmosphere that trap heat, primarily carbon dioxide (CO₂) and methane (CH₄).
• Mechanism: 1. The sun's high-energy (short-wavelength) radiation passes through the atmosphere and warms the Earth. 2. The Earth radiates this heat back as low-energy (long-wavelength) infrared radiation. 3. Greenhouse gases (like CO₂) absorb this infrared radiation and re-emit it, trapping the heat in the atmosphere, just like the glass of a greenhouse.
• Global Warming: The burning of fossil fuels has dramatically increased the concentration of CO₂ in the atmosphere. This enhanced greenhouse effect is causing the Earth's average temperature to rise, an effect known as global warming.