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 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.

Energy Levels and Electron Configuration

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

• The first energy level can hold up to 2 electrons.

• The second energy level can hold up to 8 electrons.

• The third energy level can hold 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: 2 in the first level, 6 in the second. The second level is not full, so the atom is not fully stable.

Electron Configuration Examples:

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

• 16 electrons: 2 in the first, 8 in the second, 6 in the third (not a full third shell, so 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 about 96% of the mass of a human body.

Atomic Number and Atomic Mass

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

• Atomic Mass (A): The total number of protons and neutrons in the nucleus.

Determining Subatomic Particles:

• Number of protons = atomic number

• Number of electrons = atomic number (for neutral atoms)

• Number of neutrons = atomic mass - atomic number

• Example: Carbon (atomic number 6, atomic mass 12): 6 protons, 6 electrons, 6 neutrons.

Ions and Isotopes

• Ion: An atom or molecule with a net electric charge due to the loss or gain of electrons.

• Number of protons does not change; only electrons change.

• Example: Na+ has 11 protons and 10 electrons.

• Isotope: Atoms of the same element with different numbers of neutrons.

• Examples: 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

• Formed 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

• Formed when one atom donates an electron to another, creating oppositely charged ions that attract each other.

• Example: Na+ + Cl- → NaCl

Hydrogen Bonds

• Weak bonds that form between a hydrogen atom (already covalently bonded to a highly electronegative atom) and another electronegative atom.

• Important in holding water molecules together and stabilizing large biological molecules like proteins and DNA.

Properties of Water

Polarity and Hydrogen Bonding

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

• Oxygen has a higher electronegativity, resulting in a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms.

• This polarity allows water molecules to form hydrogen bonds with each other.

Key Properties of Water

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

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

• 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." In aqueous solutions, water is always the solvent.

Solutions

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

• Solute: The substance that is dissolved (e.g., iced tea mix).

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

Acids, Bases, and pH

pH Scale

• The pH scale measures the concentration of hydrogen ions (H+) in a solution.

• Ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral.

• Acid: Substance that increases H+ concentration (pH < 7).

• Base: Substance that increases OH- concentration (pH > 7).

• Examples: Pure water (pH 7), soda (pH 3), hair remover (pH 13).

Acid-Base Reactions

• When HCl is added to water:

• When NaOH is added to water:

Chemical Equations

Writing and Interpreting Chemical Equations

• Chemical equations represent the reactants and products in a chemical reaction.

• Reactants: Substances that undergo change.

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

• Chemical Equation Example:

• Coefficients: Numbers in front of compounds/elements indicating the number of molecules or atoms involved (e.g., 6CO2 means 6 molecules of CO2).

• Subscripts: Numbers within chemical formulas indicating the number of atoms of each element in a molecule (e.g., H2O has 2 hydrogen atoms and 1 oxygen atom).

Table: Comparison of Bonds

Additional info: Some explanations and examples have been expanded for clarity and completeness, following standard General Chemistry curriculum.