Atoms and Their 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 defined energy levels.

Energy Levels and Electron Arrangement

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 (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 will have 2 in the first level and 6 in the second. It is not fully stable (octet rule not satisfied).

Electron Configuration Examples:

• 10 electrons: 2 in the first level, 8 in the second (stable, as the second shell is full).

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

• About 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 is unique for each element and determines the element's identity.

• 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 (in a neutral atom)

• Number of neutrons = atomic mass - atomic number

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

Ions and Isotopes

Ions

Ions are atoms or molecules that have gained or lost electrons, resulting in a net charge.

• Cation: Positively charged ion (lost electrons)

• Anion: Negatively charged ion (gained electrons)

• The number of protons does not change when an atom becomes an ion.

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

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons.

• Example: Carbon-12 (6 protons, 6 neutrons), Carbon-13 (6 protons, 7 neutrons), Carbon-14 (6 protons, 8 neutrons)

Chemical Bonds and Compounds

Compounds and Molecules

• 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

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, CH4).

Ionic Bonds

Ionic bonds form when electrons are transferred from one atom to another, creating 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 like oxygen or nitrogen) and another electronegative atom. These bonds are important in holding water molecules together and stabilizing large biological molecules.

Properties of Water

Polarity and Hydrogen Bonding

Water is a polar molecule due to the unequal sharing of electrons between oxygen and hydrogen. This results in a partial negative charge on the oxygen atom and a partial positive charge on the hydrogen atoms.

• Polarity allows water molecules to form up to four hydrogen bonds with neighboring water molecules.

Cohesion and Adhesion

• Cohesion: Attraction between molecules of the same substance (e.g., water molecules sticking together).

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

Other Properties

• 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 from surfaces (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, Acids, and Bases

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).

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 < 7)

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

• Neutral: Equal amounts of H+ and OH- ions (pH = 7, e.g., pure water)

• Examples: Soda (pH 3), Nair (pH 13), pure water (pH 7)

Chemical Equations

Structure and Interpretation

Chemical equations represent chemical reactions, showing reactants and products.

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

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

• Coefficients: Numbers placed before compounds to indicate the number of molecules or atoms involved.

• Subscripts: Numbers written below and to the right of element symbols, indicating the number of atoms in a molecule.

Example Equation:

• 6CO2: 6 molecules of carbon dioxide, each with 1 carbon and 2 oxygen atoms (total 6 C and 12 O atoms).

Table: Summary of Subatomic Particles

Table: Types of Chemical Bonds

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