Atoms and Atomic Structure

Definition and Structure of the Atom

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

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

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

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

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

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

Energy Levels and Electron Configuration

Electrons travel around the nucleus in specific regions called energy levels or shells. Each energy level can hold a certain maximum number of electrons:

• First energy level: can hold up to 2 electrons

• Second energy level: can hold up to 8 electrons

• Third energy level: can hold up to 18 electrons

All energy levels being used must be filled for an element to be stable.

Example: An atom with 8 electrons will have 2 in the first level and 6 in the second level.

Electron Configuration Examples:

• 10 electrons: 2 in the first level, 8 in the second level (stable)

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

Elements and the Periodic Table

Definition of Elements

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 on the Periodic Table.

• Only 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 is also equal to the number of electrons in a neutral atom.

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

Formula:

• Number of neutrons = Atomic mass - Atomic number

Example: Carbon has atomic number 6 and atomic mass 12, so it has 6 neutrons.

Ions and Isotopes

Ions

Ions are charged particles formed when atoms gain or lose electrons. The number of protons does not change.

• Cation: Positively charged ion (lost electrons)

• Anion: Negatively charged ion (gained electrons)

Example: has 11 protons and 10 electrons.

Isotopes

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

• Carbon-12: 6 protons, 6 neutrons

• Carbon-13: 6 protons, 7 neutrons

• Carbon-14: 6 protons, 8 neutrons

How Elements Combine: Compounds and Molecules

Compounds

A compound is a substance made of two or more different elements bonded together.

• Example: Salt (NaCl), Water (H2O)

Molecules

A molecule is a group of atoms held together by covalent bonds.

• Example: Oxygen molecule (O2)

Chemical Bonds

Covalent Bonds

Covalent bonds form when two atoms share electrons.

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

• Nonpolar Covalent Bond: Electrons are shared equally (e.g., hydrogen gas).

Example: Water (H2O) is a polar molecule.

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 in one molecule and an electronegative atom (like oxygen) in another molecule. They are important in holding water molecules together and stabilizing large biological molecules.

Properties of Water

Polarity

Water is a polar molecule due to the uneven distribution of charge. Oxygen attracts electrons more strongly than hydrogen, giving the oxygen side a slight negative charge and the hydrogen side a slight positive charge.

Cohesion and Adhesion

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

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

High Specific Heat Capacity

Water can absorb large amounts of heat due to its many hydrogen bonds. This property helps regulate temperature in living organisms.

Evaporative Cooling

When water evaporates, it removes heat, cooling the surface (e.g., sweating).

Versatile Solvent

Water's polarity makes it an excellent solvent, able to dissolve many substances.

Solutions, Acids, and Bases

Solutions

A solution is a mixture of a solute (substance dissolved) and a solvent (substance doing the dissolving). Water is often the solvent in biological systems.

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

pH Scale

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

• Acidic: pH < 7 (more H+ ions)

• Neutral: pH = 7 (equal H+ and OH- ions)

• Basic: pH > 7 (more OH- ions)

Examples:

• Pure water: pH 7

• Soda: pH 3

• Hair remover: pH 13

Chemical Equations

Reactants and Products

Chemical equations show how substances combine and change during a chemical reaction. Reactants are the starting substances, and products are the substances formed.

Example:

• Reactants: C6H12O6 and O2

• Products: CO2, H2O, and energy

Chemical Equation Notation

• Coefficients: Numbers before elements or compounds, indicating how many molecules are involved.

• Subscripts: Numbers written below and to the right of element symbols, indicating the number of atoms in a molecule.

Example: In 6CO2, the coefficient 6 means there are 6 molecules of CO2; the subscript 2 means each molecule has 2 oxygen atoms.

*Additional info: Some details (such as the maximum number of electrons in the third energy level) were inferred based on standard chemistry knowledge. The notes are structured to cover all major introductory chemistry topics relevant to a General Chemistry college course.*