Atoms and Atomic 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 composed of subatomic particles arranged in a specific structure.

• 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 Configuration

Electrons occupy specific energy levels (also called shells) around the nucleus. Each energy level can hold a 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 will have 2 in the first level and 6 in the second. It is not fully stable (octet not complete).

• Example: Draw the electron configuration for an atom with 10 electrons: 2 in the first level, 8 in the second (stable configuration).

Elements and the Periodic Table

Definition and Classification

An element is a pure 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 organisms.

• Four elements—carbon (C), hydrogen (H), nitrogen (N), and oxygen (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.

• Protons do not change in ions; only electrons are gained or lost.

• 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).

Types of Chemical Bonds

• Covalent Bond: Formed when two atoms share electrons. Found in many organic compounds.

• 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 Bond: Formed when one atom donates an electron to another, creating oppositely charged ions that attract (e.g., Na+ + Cl- → NaCl).

• Hydrogen Bond: A weak bond between a hydrogen atom (already covalently bonded to a highly electronegative atom) and another electronegative atom. Important in water and biological molecules.

Water: Structure, Properties, and Importance

Polarity and Hydrogen Bonding

• Water (H2O) 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.

• Hydrogen bonds form between the slightly positive hydrogen of one water molecule and the slightly negative oxygen of another.

Properties of Water

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

• Adhesion: Attraction between water molecules and other substances (causes meniscus and capillary action).

• 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

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

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

• Solvent: The substance doing the dissolving (e.g., water; water is often called the universal solvent).

Acids, Bases, and pH

pH Scale and Definitions

• pH: A measure of the concentration of hydrogen ions (H+) in a solution.

• The pH scale ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral.

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

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

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

Chemical Equations

Structure and Interpretation

• Chemical Equation: Represents a chemical reaction using symbols and formulas.

• Reactants: Substances that undergo change.

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

• Chemical Equation Example:

• Coefficients: Numbers in front of formulas indicating the number of molecules or moles (e.g., 6CO2 means 6 molecules of CO2).

• Subscripts: Numbers within formulas indicating the number of atoms in a molecule (e.g., H2O has 2 hydrogen atoms).

Table: Comparison of Subatomic Particles

Additional info: The above notes expand on the original content with standard academic definitions, examples, and context suitable for GOB Chemistry students.