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 the fundamental building blocks of matter.

• 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 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 2 electrons.

• The second energy level can hold 8 electrons.

• The third energy level can hold 18 electrons (but is often stable with 8 in basic chemistry).

For an atom to be stable, its outermost energy level (valence shell) should be full.

• 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 stable.

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

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 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 (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 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 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 atoms transfer electrons, resulting in 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 uneven distribution 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 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 molecules of different 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 that is dissolved (e.g., iced tea mix).

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

• In biological systems, water is almost always the solvent.

Acids, Bases, and pH

pH Scale and Definitions

• pH measures the concentration of hydrogen ions (H+) versus hydroxide ions (OH-) 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 (pH < 7).

• Base: Substance that increases OH- concentration (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 atoms (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 Bond Types

Additional info: The above notes expand on the original content with academic context, definitions, and examples to ensure completeness and clarity for GOB Chemistry students.