Atomic Structure

Subatomic Particles and Atomic Organization

Atoms are the fundamental units of matter, composed of three types of subatomic particles. The arrangement and properties of these particles determine the chemical behavior of elements.

• Protons: Positively charged particles located in the nucleus.

• Neutrons: Neutral particles also found in the nucleus.

• Electrons: Negatively charged particles found in orbitals surrounding the nucleus.

The nucleus contains protons and neutrons, while electrons occupy regions called orbitals outside the nucleus.

Atomic Number and Mass Number

• Atomic Number (Z): The number of protons in an atom's nucleus; defines the element.

• Mass Number (A): The sum of protons and neutrons in the nucleus.

Atoms with the same atomic number have the same chemical properties and belong to the same element.

Electron Arrangement

• Electrons occupy orbitals, which are grouped into electron shells (energy levels).

• Each orbital can hold up to two electrons.

• Electron shells are numbered (1, 2, 3, ...), with lower numbers closer to the nucleus.

• The first shell has one orbital (up to 2 electrons); the second shell has four orbitals (up to 8 electrons).

• Electrons fill the innermost shells first before occupying outer shells.

Chemical Bonding

Types of Chemical Bonds

Atoms form chemical bonds to achieve stable electron configurations. The main types of bonds are covalent and ionic bonds.

• Covalent Bonds: Atoms share pairs of valence electrons to fill their outermost shells.

• Ionic Bonds: Electrons are transferred from one atom to another, resulting in oppositely charged ions that attract each other.

Electronegativity and Bond Polarity

• Electronegativity: The ability of an atom to attract shared electrons in a covalent bond.

• If atoms have similar electronegativity, electrons are shared equally (nonpolar covalent bond).

• If one atom is more electronegative, electrons are shared unequally (polar covalent bond), resulting in partial charges (δ+ and δ−).

• If the difference is very large, electrons are fully transferred, forming ionic bonds.

Ion Formation

• Cation: An atom that loses an electron and becomes positively charged.

• Anion: An atom that gains an electron and becomes negatively charged.

Electron-Sharing Continuum

The degree of electron sharing in chemical bonds forms a continuum:

• Equal sharing: Nonpolar covalent bonds (e.g., H2).

• Unequal sharing: Polar covalent bonds (e.g., H2O).

• Complete transfer: Ionic bonds (e.g., NaCl).

Water: Structure and Properties

Polarity of Water

Water (H2O) is a polar molecule due to the difference in electronegativity between oxygen and hydrogen. Oxygen attracts electrons more strongly, resulting in partial negative (δ−) and partial positive (δ+) charges.

• Polar covalent bonds within the molecule.

• Hydrogen bonds form between water molecules due to polarity.

Hydrogen Bonding

• Hydrogen bonds: Weak interactions between the partial positive charge of hydrogen in one molecule and the partial negative charge of oxygen in another.

• Responsible for many of water's unique properties.

Properties of Water

• Universal Solvent: Water dissolves many substances, especially ions and polar molecules, due to its polarity and ability to form hydrogen bonds.

• Cohesion: Water molecules stick to each other via hydrogen bonds, resulting in high surface tension.

• Adhesion: Water molecules stick to other polar or charged surfaces.

• High Specific Heat: Water can absorb or release large amounts of heat with little temperature change, moderating Earth's climate and body temperature.

• High Heat of Vaporization: Water requires significant energy to change from liquid to gas, aiding in cooling mechanisms like sweating.

• Expansion Upon Freezing: Water expands as it freezes, making ice less dense than liquid water (ice floats).

Table: Specific Heats of Some Liquids

Carbon: The Backbone of Life

Importance and Versatility of Carbon

Carbon is the most versatile element in biological molecules due to its four valence electrons, allowing it to form up to four covalent bonds. This enables a vast diversity of molecular shapes and sizes.

• Can form single, double, or triple bonds.

• Can create chains, branched structures, and rings.

• Forms the backbone of organic molecules.

Major Elements in Living Organisms

• Oxygen (65%)

• Carbon (18.5%)

• Hydrogen (9.5%)

• Nitrogen (3.3%)

• Phosphorus and Sulfur (2%)

Variations in Carbon Skeletons

• Chains (e.g., octane)

• Rings (e.g., cyclohexane, benzene)

• Branching (e.g., isobutane)

• Double bonds (e.g., 2-butene)

Functional Groups: Determinants of Chemical Behavior

Major Functional Groups in Organic Molecules

Functional groups are specific groups of atoms attached to carbon skeletons that confer distinct chemical properties to molecules.

• Amino group (–NH2): Acts as a base; can pick up a proton (H+).

• Carboxyl group (–COOH): Acts as an acid; can donate a proton.

• Carbonyl group (–C=O): Found in aldehydes and ketones; can link molecules into more complex compounds.

• Hydroxyl group (–OH): Polar; can form hydrogen bonds; acts as a weak acid.

• Phosphate group (–PO42−): Contributes negative charge; involved in energy transfer (e.g., ATP).

• Sulfhydryl group (–SH): Can form disulfide bonds, stabilizing protein structure.

Table: Major Functional Groups

Summary

• Atoms are composed of protons, neutrons, and electrons; their arrangement determines chemical properties.

• Chemical bonds (covalent and ionic) result from interactions between atoms' electrons.

• Water's unique properties arise from its polarity and hydrogen bonding, making it essential for life.

• Carbon's versatility allows for the diversity of organic molecules.

• Functional groups attached to carbon skeletons determine the chemical behavior of organic molecules.