Atoms and Their Structure

Definition and Components of the Atom

An atom is the smallest unit of an element that retains the properties of that element. Atoms are the fundamental building blocks of matter.

• Nucleus: The dense central core of the atom, containing protons and neutrons.

• Proton (p+): A positively charged particle located inside the nucleus.

• Neutron (n0): A particle with no charge, also found in the nucleus.

• Electron (e-): A negatively charged particle that orbits the nucleus in defined energy levels.

Energy Levels and Electron Arrangement

Electrons occupy specific energy levels (also called shells) around the nucleus. Each energy level can hold a maximum number of electrons:

• First energy level: up to 2 electrons

• Second energy level: up to 8 electrons

• Third energy level: up to 18 electrons

For an atom to be stable, its outermost energy level (valence shell) should be full or follow the octet rule (8 electrons).

Example: An atom with 8 electrons will have 2 in the first energy level and 6 in the second. It is not fully stable unless the second shell is complete.

Electron Configuration Examples

• 10 electrons: 2 in the first shell, 8 in the second (stable configuration).

• 16 electrons: 2 in the first shell, 8 in the second, 6 in the third (not fully stable).

Elements and the Periodic Table

Definition of Elements

An element is a pure substance that cannot be broken down into simpler substances by chemical means. Each element is defined by its number of protons.

• There are about 90 naturally occurring elements.

• All elements are listed in the Periodic Table.

• Only about 25 elements are essential for living things; C, H, N, and O make up most of the mass of living organisms.

Atomic Number and Atomic Mass

• Atomic Number: The number of protons in an atom; also equals the number of electrons in a neutral atom.

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

Example: Carbon has atomic number 6 (6 protons) and atomic mass 12 (6 protons + 6 neutrons).

Determining Subatomic Particles

• Protons (p+): Equal to atomic number.

• Electrons (e-): Equal to atomic number in a neutral atom; changes if the atom is an ion.

• Neutrons (n0): Atomic mass minus atomic number.

Ions and Isotopes

Ions

Ions are atoms or molecules that have gained or lost electrons, resulting in a net charge.

• Cation: Positively charged ion (lost electrons).

• Anion: Negatively charged ion (gained electrons).

• The number of protons does not change when forming ions.

Example: Na+ has 11 protons and 10 electrons.

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons.

• Example: Carbon-12 (6p, 6n), Carbon-13 (6p, 7n), Carbon-14 (6p, 8n)

Chemical Bonds and Compounds

Compounds and Molecules

• Compound: A substance made of two or more different elements chemically bonded (e.g., NaCl, H2O).

• Molecule: A group of atoms held together by covalent bonds (e.g., O2).

Covalent Bonds

Covalent bonds form when two atoms share electrons.

• Polar Covalent Bond: Electrons are shared unequally, resulting in partial charges (e.g., H2O).

• Nonpolar Covalent Bond: Electrons are shared equally (e.g., H2, CH4).

Ionic Bonds

Ionic bonds form when electrons are transferred from one atom to another, creating oppositely charged ions that attract each other.

• Example: Na+ + Cl- → NaCl

Hydrogen Bonds

Hydrogen bonds are weak attractions between a hydrogen atom (already covalently bonded to a highly electronegative atom like oxygen or nitrogen) and another electronegative atom. These bonds are important in holding water molecules together and stabilizing large biological molecules.

Properties of Water

Polarity and Hydrogen Bonding

Water is a polar molecule due to the unequal sharing of electrons between oxygen and hydrogen. This results in a partial negative charge on oxygen and a partial positive charge on hydrogen, allowing water molecules to form hydrogen bonds with each other.

Key Properties of Water

• Cohesion: Attraction between molecules of the same substance (e.g., water droplets).

• Adhesion: Attraction between molecules of different substances (e.g., water climbing up plant roots).

• High Specific Heat Capacity: Water can absorb large amounts of heat before changing temperature, due to hydrogen bonding.

• Evaporative Cooling: As water evaporates, it removes heat, helping organisms regulate temperature (e.g., sweating).

• Versatile Solvent: Water dissolves many substances due to its polarity, making it the 'universal solvent.'

Solutions

• Solution: A homogeneous mixture of two or more substances.

• Solute: The substance that is dissolved (e.g., salt, sugar).

• Solvent: The substance that does the dissolving (e.g., water).

• In aqueous solutions, water is always the solvent.

Acids, Bases, and pH

pH Scale

The pH scale measures the concentration of hydrogen ions (H+) in a solution, ranging from 0 (most acidic) to 14 (most basic).

• Acid: Substance with more H+ ions; pH less than 7.

• Base: Substance with more OH- ions; pH greater than 7.

• Neutral: Equal H+ and OH- ions; pH = 7 (e.g., pure water).

Examples:

• Pure water: pH 7

• Soda: pH 3 (acidic)

• Hair remover: pH 13 (basic)

Chemical Equations

Structure of Chemical Equations

Chemical equations represent the reactants and products in a chemical reaction. The general format is:

• Chemical formulas show the types and numbers of atoms involved.

• Coefficients indicate the number of molecules or moles of a substance (e.g., 6CO2 means 6 molecules of CO2).

• Subscripts indicate the number of atoms in a molecule (e.g., H2O has 2 hydrogen atoms and 1 oxygen atom).

Example:

Here, glucose and oxygen are reactants; carbon dioxide, water, and energy are products.

Table: Comparison of Bond Types