Atoms and Atomic Structure

Definition of an 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.

Subatomic Particles and Atomic Arrangement

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

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

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

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

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

Electron Energy Levels and Configuration

Energy Levels

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

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

Electron Configuration Examples

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

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

Elements and the Periodic Table

Definition of an Element

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 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: The total number 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 (unless the atom is an ion)

• Neutrons (n0): Atomic mass minus atomic number

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 (loss of electrons)

• Anion: Negatively charged ion (gain of electrons)

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 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. These are important in holding water molecules together and stabilizing large biological molecules.

Properties of Water

Polarity of Water

Water (H2O) is a polar molecule due to the uneven distribution of electrons. Oxygen has a stronger pull on electrons, making the oxygen side slightly negative and the hydrogen side slightly positive.

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 sticking together).

• Adhesion: Attraction between molecules of different substances (e.g., water climbing up plant roots or forming a meniscus in a cylinder).

• High Specific Heat Capacity: Water absorbs large amounts of heat before changing temperature, helping regulate temperature in organisms and environments.

• Evaporative Cooling: As water evaporates, it removes heat (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 being dissolved (e.g., iced tea mix).

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

In biological systems, water is almost always the 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 below 7)

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

• Neutral: Equal amounts of H+ and OH- ions (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 the reactants and products in a chemical reaction. The general format is:

• 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:

• 6CO2: 6 molecules of carbon dioxide

• 6H2O: 6 molecules of water

• Products: 1 molecule of glucose (C6H12O6) and 6 molecules of oxygen (O2)

Coefficients multiply all atoms in the formula that follows them. Subscripts only apply to the element they follow.

Summary Table: Subatomic Particles

Key Point: Protons and neutrons have similar mass and are found in the nucleus, while electrons are much lighter and orbit the nucleus.