Chemistry

Components of an Atom

Atoms are the fundamental units of matter, composed of three main subatomic particles.

• Protons: Positively charged particles located in the nucleus.

• Neutrons: Neutral particles also found in the nucleus.

• Electrons: Negatively charged particles that orbit the nucleus in electron shells.

Example: A carbon atom has 6 protons, 6 neutrons, and 6 electrons.

Atomic Number and Atomic Mass

The atomic number and atomic mass are key properties that define elements.

• Atomic Number: The number of protons in the nucleus of an atom. It determines the element's identity.

• Atomic Mass (Mass Number): The sum of protons and neutrons in the nucleus.

Example: For carbon, atomic number = 6, atomic mass = 12.

Electron Energy Levels

Electrons occupy energy levels or shells around the nucleus. The energy of an electron increases with its distance from the nucleus.

• Electrons in higher shells (e.g., third shell) have more energy than those in lower shells (e.g., first shell).

Example: An electron in the third shell has higher energy than one in the first shell.

Types of Molecular Bonds

Molecular bonds are the forces that hold atoms together in molecules. The main types are covalent and ionic bonds.

• Covalent Bonds: Electrons are shared between atoms. These bonds can be very strong.

  • Nonpolar Covalent Bonds: Electrons are shared equally.

  • Polar Covalent Bonds: Electrons are not shared equally, resulting in a molecule with partial positive and negative charges.

• Ionic Bonds: Electrons are transferred from one atom to another, creating ions that attract each other.

Example: Water (H2O) is a polar covalent molecule; sodium chloride (NaCl) is an ionic compound.

Strength and Examples of Bonds

• Covalent bonds (e.g., in O2 or H2O) are generally stronger than ionic bonds in aqueous solutions.

• Ionic bonds (e.g., in NaCl) are strong in solid form but can dissociate in water.

Hydrogen Bonds

Hydrogen bonds form between a slightly positive hydrogen atom and a slightly negative atom (often oxygen or nitrogen) on another molecule.

• These are weaker than covalent and ionic bonds but are crucial for the properties of water and biological molecules.

Example: Hydrogen bonds hold water molecules together and stabilize DNA structure.

Chemical Equilibrium

Chemical equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction in a chemical process.

• Reactants: Substances present at the start of a reaction.

• Products: Substances formed as a result of the reaction.

Example: In the reaction , equilibrium is reached when the concentrations of A, B, C, and D remain constant over time.

Structure and Function of Molecules

The structure of a molecule determines its function in biological systems.

• Shape, charge distribution, and chemical properties influence how molecules interact.

Example: The double-helix structure of DNA allows it to store genetic information efficiently.

Water

Hydrogen Bonding in Water

Water molecules are polar and form hydrogen bonds with each other, leading to unique properties.

• Hydrogen bonds form between the slightly positive hydrogen of one water molecule and the slightly negative oxygen of another.

Example: This bonding explains water's high boiling point and surface tension.

Polarity of Water

Water is a polar molecule due to the unequal sharing of electrons between oxygen and hydrogen atoms.

• Oxygen is more electronegative, pulling electrons closer and creating a partial negative charge.

• Hydrogen atoms have a partial positive charge.

Cohesion and Adhesion

Water molecules stick together (cohesion) and to other substances (adhesion) due to hydrogen bonding.

• Cohesion: Responsible for surface tension.

• Adhesion: Allows water to climb up plant roots and stems (capillary action).

Calorie Definition

A calorie is a unit of energy. In biology, it is the amount of heat needed to raise the temperature of 1 gram of water by 1°C.

• Food calories are actually kilocalories (1 kcal = 1000 calories).

Properties of Water Essential for Life

Water's unique properties make it vital for life on Earth.

• High specific heat: Stabilizes temperatures.

• High heat of vaporization: Allows cooling through evaporation.

• Ice is less dense than liquid water: Enables aquatic life to survive under ice.

• Excellent solvent: Dissolves many substances, facilitating chemical reactions.

Molarity

Molarity is a measure of concentration, defined as the number of moles of solute per liter of solution.

• Formula:

Buffers

Buffers are substances that minimize changes in pH by accepting or donating hydrogen ions.

• They help maintain stable pH in biological systems.

Example: The bicarbonate buffer system in blood.

Ocean Acidification (Scientific Skills Exercise)

Rising carbon dioxide levels increase ocean acidity, affecting marine life.

• CO2 dissolves in seawater, forming carbonic acid, which lowers pH.

Example: Coral reefs are threatened by ocean acidification.

Dependent and Independent Variables

In scientific experiments:

• Independent variable: The factor that is changed or controlled by the researcher.

• Dependent variable: The factor that is measured or observed.

Example: In a study on CO2 and ocean pH, CO2 level is the independent variable; pH is the dependent variable.

Hypothesis Testing

Researchers determine if their hypothesis is supported or falsified based on experimental results.

• If results match predictions, the hypothesis is supported; otherwise, it is falsified.