Atoms, Elements, and the Periodic Table: Foundations for Organic Chemistry

Atoms and Subatomic Particles

The Structure of the Atom

The atom is the fundamental unit of matter, composed of a nucleus containing protons and neutrons, surrounded by electrons in an electron cloud. The nucleus is extremely small compared to the overall size of the atom, but it contains most of the atom's mass.

• Protons: Positively charged particles found in the nucleus.

• Neutrons: Neutral particles found in the nucleus.

• Electrons: Negatively charged particles that orbit the nucleus in the electron cloud.

• Electron Cloud: Much larger than the nucleus, not to scale in most diagrams.

Nuclear Force: Holds protons and neutrons together in the nucleus. Electrostatic Force: Repulsion between like charges (protons).

Key Concept: For a stable nucleus, the nuclear force must be stronger than the electrostatic force.

Subatomic Particle Properties

Subatomic particles differ in mass and charge. The atomic mass unit (amu) is used to express their masses.

• 1 amu = $1.66 \times 10^{-27}$ kg

• Proton: 1.67262 x 10-27 kg, +1 charge

• Neutron: 1.67493 x 10-27 kg, 0 charge

• Electron: 0.00091 x 10-27 kg, -1 charge

Isotopes and Atomic Mass

Isotopes

Isotopes are atoms of the same element (same number of protons) but different numbers of neutrons. The atomic number (Z) identifies the element, while the mass number (A) is the sum of protons and neutrons.

• Atomic Number (Z): Number of protons

• Mass Number (A): Number of protons + neutrons

• Isotope Notation: $\text{Element}^{A}_{Z}$

• Number of Neutrons: $\text{Mass Number} - \text{Atomic Number}$

Atomic Mass

The atomic mass of an element is the weighted average of all its isotopes, expressed in amu, Da (Dalton), or kg.

• Most Abundant Isotope: The isotope whose mass number is closest to the atomic mass listed on the periodic table.

The Periodic Table

History and Organization

The periodic table organizes elements by increasing atomic number and recurring chemical properties (periodic law). Element symbols are usually derived from their English or Latin names.

Classifications

Elements are classified as metals, nonmetals, or metalloids based on their physical and chemical properties.

• Metals: Largest classification; typically lustrous, conductive, malleable, and opaque.

• Nonmetals: Second largest; typically brittle, poor conductors, non-ductile, and non-malleable.

• Metalloids: Have properties of both metals and nonmetals; found along the 'staircase' border.

Groups and Periods

The periodic table is organized into horizontal rows (periods) and vertical columns (groups or families). There are 18 groups, each with unique properties. Elements in the same group have similar chemical properties due to similar electron arrangements.

Representative Elements and Transition Metals

• Transition Metals: Groups 3-12, known for variable oxidation states.

• Representative Elements: Groups 1-2 and 13-18, also called main group elements.

Elemental Forms

• Monoatomic: Stable as single atoms (e.g., noble gases).

• Diatomic: Stable as pairs (e.g., H2, O2, N2).

• Polyatomic: Stable in groups larger than two (e.g., S8).

Phases of Elements

At room temperature and pressure, elements exist as solids, liquids, or gases.

• Solids: Fixed shape and volume

• Liquids: Fixed volume, variable shape

• Gases: Variable shape and volume

Atomic Theory and Models

Dalton's Atomic Theory

John Dalton proposed that matter is composed of indivisible atoms, which combine in simple ratios to form compounds. Modern atomic theory recognizes subatomic particles and isotopes.

Rutherford's Gold Foil Experiment

Ernest Rutherford's experiment demonstrated the existence of a dense, positively charged nucleus at the center of the atom, surrounded by electrons.

Electromagnetic Spectrum and Atomic Models

Electromagnetic Spectrum

The electromagnetic spectrum includes all wavelengths and frequencies of electromagnetic radiation, from radio waves to gamma rays. Energy increases as wavelength decreases and frequency increases.

• Photon: A quantum of electromagnetic energy.

• Visible Light: The portion of the spectrum visible to the human eye.

Bohr Model

In the Bohr Model, electrons occupy discrete energy levels (shells) around the nucleus. Electrons can absorb or emit energy to move between shells.

• Absorption: Electron moves to a higher shell (higher n).

• Emission: Electron moves to a lower shell (lower n).

• Energy difference: $\Delta E = E_{\text{photon}}$

Electron Arrangements and Valence Electrons

Electron Arrangement

The electron arrangement describes the distribution of electrons among energy levels. As n increases, the size and energy of the orbital increase.

Valence Electrons

Valence electrons are the outermost electrons involved in chemical bonding. For main group elements, the number of valence electrons equals the group number.

• Inner Core Electrons: All electrons not in the outer shell.

Lewis Dot Symbols

Lewis Dot Symbols represent valence electrons as dots around the element symbol. Transition metals may have unpredictable patterns.

Periodic Trends

Metallic Character

Metallic character describes how easily an element loses electrons. It decreases across a period (left to right) and increases down a group.

• Metals: Tend to lose electrons easily.

• Nonmetals: Tend to gain electrons.

Atomic Radius

Atomic radius is the distance from the nucleus to the outermost electron shell. It increases down a group and decreases across a period.

Ionization Energy

Ionization energy is the energy required to remove an electron from an atom. It increases across a period and decreases down a group.

• High IE: Electron is not easily lost (e.g., noble gases).

• Low IE: Electron is easily lost (e.g., alkali metals).

Summary Table: Subatomic Particles

Subatomic Particle	Actual Mass (kg)	Relative Mass (amu)	Relative Charge
Neutron	1.67493 x 10-27	1	0
Proton	1.67262 x 10-27	1	+1
Electron	0.00091 x 10-27	~0	-1

Summary Table: Periodic Table Classifications

Classification	Physical Properties
Metals	Lustrous, conductive, malleable, opaque
Nonmetals	Brittle, poor conductors, non-ductile, non-malleable
Metalloids	Intermediate properties, semi-conductors