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 found in the nucleus with no electric charge.

• 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 must be full or satisfy the octet rule (having 8 electrons in the outer shell for most main-group elements).

• Example: An atom with 8 electrons will have 2 in the first energy level and 6 in the second. It is not fully stable unless the outer shell is complete.

• Electron Configuration: The arrangement of electrons in the energy levels of an atom. For example, an atom with 10 electrons (Neon) has the configuration 2,8 and is 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. The majority of the human body 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 is unique for each element and 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 has atomic number 6 and atomic mass 12, so it has 6 protons, 6 electrons, and 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 in ions; only electrons change.

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

• Isotope: Atoms of the same element with different numbers of neutrons. For example, Carbon-12, Carbon-13, and Carbon-14 have 6 protons but 6, 7, and 8 neutrons, respectively.

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: A bond formed when two atoms share electrons. Example: H2O.

• 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: A bond formed when one atom donates an electron to another, resulting in oppositely charged ions that attract each other (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.

Properties of Water

Polarity and Hydrogen Bonding

• Water (H2O) is a polar molecule due to the uneven distribution of electrons between oxygen and hydrogen atoms.

• 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.

Unique 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 solutions, water is almost 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 and Definitions

• pH measures 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.

• Acids have more H+ ions and a pH below 7 (e.g., HCl in water forms H+ and Cl-).

• Bases have more OH- ions and a pH above 7 (e.g., NaOH in water forms Na+ and OH-).

• Pure water has a pH of 7 (equal amounts of H+ and OH-).

Chemical Equations

Structure and Interpretation

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

• Reactants: Substances that undergo change (left side of the equation).

• Products: Substances formed as a result of the reaction (right side).

• Chemical Coefficient: The number before a formula, indicating the number of molecules or atoms involved.

• Subscript: The small number after an element symbol, indicating the number of atoms of that element in a molecule.

Example Equation:

• 6CO2: 6 molecules of carbon dioxide

• 6H2O: 6 molecules of water

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

Counting Atoms: Multiply the coefficient by the subscript to find the total number of each atom.

• In 6CO2: 6 × 1 = 6 C atoms, 6 × 2 = 12 O atoms