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 three primary subatomic particles:

• Proton: Positively charged particle located in the nucleus.

• Neutron: Neutral particle also found in the nucleus.

• Electron: Negatively charged particle that orbits the nucleus in defined energy levels.

Atomic Structure and Energy Levels

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

• First energy level: 2 electrons

• Second energy level: 8 electrons

• Third energy level: 18 electrons

All occupied energy levels must be full for an atom to be stable (noble gas configuration).

Example: An atom with 8 electrons will have 2 in the first level and 6 in the second. It is not stable because the second level is not full.

Atomic Number and Atomic Mass

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

• Atomic Mass (A): Sum of protons and neutrons in the nucleus.

Calculating Subatomic Particles:

• Protons = Atomic number

• Electrons = Atomic number (unless ionized)

• Neutrons = Atomic mass - Atomic number

Ions and Isotopes

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

• Cation: Positively charged ion (loss of electrons).

• Anion: Negatively charged ion (gain of electrons).

• Isotope: Atoms of the same element with different numbers of neutrons (e.g., Carbon-12, Carbon-13, Carbon-14).

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. All known elements are organized in the Periodic Table based on their atomic number.

• There are 90 naturally occurring elements.

• Only 25 elements are essential for living organisms; C, H, N, and O make up 96% of human body mass.

Chemical Bonding

Types of Chemical Bonds

Atoms combine to form compounds and molecules through different types of chemical bonds:

• Covalent Bond: Atoms share pairs of electrons. Found in most organic compounds (e.g., H2O, O2).

• Polar Covalent Bond: Electrons are shared unequally, resulting in partial charges (e.g., H2O).

• Nonpolar Covalent Bond: Electrons are shared equally (e.g., H2, O2).

• Ionic Bond: Electrons are transferred from one atom to another, creating oppositely charged ions that attract (e.g., NaCl).

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

Examples and Applications

• Compound: Substance formed from two or more different elements (e.g., NaCl, H2O).

• Molecule: Group of atoms held together by covalent bonds (e.g., O2).

Water: Structure, Properties, and Importance

Structure and Polarity of Water

Water (H2O) is a polar molecule due to the unequal sharing of electrons between oxygen and hydrogen. Oxygen is more electronegative, resulting in a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms.

Hydrogen Bonding in Water

Hydrogen bonds form between the slightly positive hydrogen atom of one water molecule and the slightly negative oxygen atom of another. These bonds are responsible for many of water's unique properties.

Properties of Water

• Cohesion: Attraction between water molecules, leading to surface tension (e.g., water strider walking on water).

• Adhesion: Attraction between water molecules and other substances, causing phenomena like the meniscus in a graduated cylinder and capillary action.

• High Specific Heat Capacity: Water absorbs large amounts of heat before changing temperature, helping regulate temperature in organisms and environments.

• Evaporative Cooling: As water evaporates, it removes heat, cooling surfaces (e.g., sweating).

• Versatile Solvent: Water dissolves many substances due to its polarity, making it essential for biological processes.

Solutions

A solution consists of a solute (substance being dissolved) and a solvent (substance doing the dissolving). In biological systems, water is almost always the solvent.

• Example: Iced tea mix (solute) dissolved in water (solvent).

Acids, Bases, and pH

pH Scale and Definitions

The pH scale measures the concentration of hydrogen ions (H+) in a solution, ranging from 0 (most acidic) to 14 (most basic). Pure water has a neutral pH of 7.

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

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

Examples:

• Vinegar and lemon juice are acidic (low pH).

• Soda is acidic (pH ≈ 3).

• Hair remover (Nair) is basic (pH ≈ 13).

Chemical Equations and Stoichiometry

Writing and Interpreting Chemical Equations

Chemical equations represent the reactants and products in a chemical reaction. The coefficients indicate the number of molecules or moles, while subscripts indicate the number of atoms in each molecule.

• Example:

• Reactants: and

• Products: and

• Coefficients multiply all atoms in the formula that follows (e.g., means 6 carbon atoms and 12 oxygen atoms).

Subscripts indicate the number of atoms of each element in a molecule (e.g., means two oxygen atoms).

Summary Table: Key Atomic and Chemical Properties