Atoms and Atomic Structure

Definition of an Atom

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

• Element: A pure substance consisting of only one type of atom.

Subatomic Particles and Atomic Arrangement

Atoms are composed of three main subatomic particles:

• Proton: A positively charged particle located inside the nucleus.

• Neutron: A neutral particle (no charge) also found in the nucleus.

• Electron: A negatively charged particle that orbits the nucleus in energy levels.

The nucleus is the dense central core of the atom, containing protons and neutrons.

Energy Levels (Electron Shells)

Electrons occupy specific regions around the nucleus called energy levels or shells. Each energy level can hold a certain 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 satisfy the octet rule (8 electrons for most main-group elements).

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

Electron Configuration Examples

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

• 16 electrons: 2 in the first shell, 8 in the second shell, 6 in the third shell (not fully stable, as the third shell can hold up to 18 electrons).

Elements and the Periodic Table

Definition and Occurrence

An element is a substance that cannot be broken down into simpler substances by chemical means. There are about 90 naturally occurring elements, all listed in the Periodic Table.

• Only about 25 elements are essential for living things.

• 96% of the mass of a human is composed of carbon (C), hydrogen (H), nitrogen (N), and oxygen (O).

Atomic Number and Atomic Mass

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

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

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

Determining Subatomic Particles

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

• Electrons (e-): Equal to the atomic number in a neutral atom; different in ions.

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

Ions

Ions are atoms or molecules with a net electric charge due to the loss or gain of electrons.

• Cation: Positively charged ion (lost electrons).

• Anion: Negatively charged ion (gained electrons).

• The number of protons does not change in 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

How Elements Combine

• Compound: A substance made of two or more different elements chemically bonded together (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, ethane).

Ionic Bonds

Ionic bonds form when atoms transfer electrons, resulting in 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) and another electronegative atom. They are important in holding water molecules together and stabilizing large biological molecules like proteins.

Properties of Water

Polarity of Water

Water (H2O) is a polar molecule due to the unequal sharing of electrons between oxygen and hydrogen. Oxygen is more electronegative, so it pulls electrons closer, giving it a partial negative charge and hydrogen a partial positive charge.

Hydrogen Bonding in Water

Hydrogen bonds cause water molecules to attract each other, leading to unique properties:

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

• Adhesion: Attraction between molecules of different substances (e.g., water and glass, causing a meniscus).

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

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

• Versatile Solvent: Water dissolves many substances due to its polarity.

Solutions

A solution consists of a solute (substance dissolved) and a solvent (substance doing the dissolving). Water is known as the "universal solvent."

• Example: Iced tea mix (solute) dissolved in water (solvent).

Acids, Bases, and pH

pH Scale

The pH scale measures the concentration of hydrogen ions (H+) versus hydroxide ions (OH-) in a solution. The scale ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral.

• Acid: Substance with more H+ ions (pH < 7).

• Base: Substance with more OH- ions (pH > 7).

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

Examples:

• Pure water: pH 7.0

• Soda: pH 3.0 (acidic)

• Hair remover (Nair): pH 13 (basic)

Chemical Equations

Structure of Chemical Equations

Chemical equations represent chemical reactions, showing reactants (starting substances) and products (substances formed).

• Reactants: Substances that undergo change.

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

• Coefficients: Numbers before compounds/elements indicating the number of molecules or atoms involved.

• Subscripts: Small numbers within formulas indicating the number of atoms of each element in a molecule.

Example Equation:

• 6CO2: 6 molecules of carbon dioxide

• 6H2O: 6 molecules of water

• C6H12O6: 1 molecule of glucose

• 6O2: 6 molecules of oxygen

Counting Atoms Example: In 6CO2, there are 6 × 1 = 6 carbon atoms and 6 × 2 = 12 oxygen atoms.

Summary Table: Subatomic Particles

Additional info: Some explanations and examples were expanded for clarity and completeness, such as the octet rule, the definition of cations/anions, and the detailed breakdown of chemical equations.