Atoms, Elements, and the Periodic Table: Structure and Properties

Atoms & Elements

Structure of the Atom

The atom is the fundamental unit of matter, composed of a nucleus containing protons and neutrons, surrounded by an electron cloud. The arrangement and behavior of these subatomic particles determine the chemical properties of each element.

• Protons: Positively charged particles found in the nucleus; define the atomic number.

• Neutrons: Neutral particles found in the nucleus; contribute to atomic mass.

• Electrons: Negatively charged particles found in the electron cloud; occupy specific energy levels.

• Atomic Number (Z): Number of protons in the nucleus; unique for each element.

• Mass Number (A): Total number of protons and neutrons in the nucleus.

Isotopes and Atomic Mass

Isotopes are atoms of the same element with different numbers of neutrons, resulting in different mass numbers. The atomic mass listed on the periodic table is a weighted average of all naturally occurring isotopes.

• Isotopic Notation: Element name followed by mass number (e.g., phosphorus-31).

• Atomic Weight: Weighted average of isotopic masses, based on relative abundance.

Periodic Table

Organization and Classification

The periodic table arranges elements by increasing atomic number and groups them by similar chemical properties. It is divided into periods (rows) and groups (columns).

• Periods: Horizontal rows; indicate repeating patterns in properties.

• Groups: Vertical columns; elements in the same group have similar chemical behaviors.

• Main Group Elements: Groups 1, 2, and 13–18; include metals, nonmetals, and metalloids.

• Transition Elements: Groups 3–12; all are metals.

• Inner-Transition Elements: Lanthanides and Actinides; many are radioactive or rare earth metals.

Types of Elements

• Metals: Shiny, malleable, good conductors of heat and electricity.

• Nonmetals: Dull, poor conductors of heat and electricity.

• Metalloids: Possess properties of both metals and nonmetals.

Subatomic Particles

Recap and Location

Subatomic particles are distributed within the atom as follows:

• Nucleus: Contains protons and neutrons.

• Electron Cloud: Contains electrons.

Bohr Model and Atomic Orbitals

Bohr Model

The Bohr Model describes electrons as orbiting the nucleus in fixed energy levels or shells. These energy levels are quantized, meaning electrons can only occupy specific levels.

• Energy Levels: Concentric layers around the nucleus; closest layer has lowest energy.

• Valence Electrons: Electrons in the outermost shell; involved in chemical reactions.

Atomic Orbitals and Quantum Numbers

Atomic orbitals are regions where electrons are most likely to be found. Their size, shape, and orientation are described by quantum numbers:

• Principal Quantum Number (n): Specifies size and energy level; n = 1 to 7.

• Angular Momentum Quantum Number (l): Specifies shape; l = 0 (s), 1 (p), 2 (d), 3 (f).

• Magnetic Quantum Number (ml): Specifies orientation; ml = -l to +l.

• Spin Quantum Number (ms): Specifies electron spin; ms = +½ or –½.

Subshells and Orbital Diagrams

Orbitals with the same value of l form subshells (s, p, d, f). The number of subshells in a shell equals the principal quantum number. Orbital diagrams visually represent electron configurations and obey Hund’s Rule and the Pauli Exclusion Principle.

• Hund’s Rule: Each orbital in a subshell is singly occupied before any is doubly occupied.

• Pauli Exclusion Principle: No two electrons in the same atom can have the same set of all four quantum numbers.

Electron Configurations

Writing Electron Configurations

Electron configurations show the arrangement of electrons in an atom’s orbitals. They are written using numbers (energy level), letters (subshell), and superscripts (number of electrons).

• Example: Sodium (Na): 1s22s22p63s1

• Abbreviated Configuration: Uses noble gas core in brackets (e.g., [Ne]3s1 for Na).

Orbital Filling Order

Electrons fill orbitals in order of increasing energy, following the Aufbau principle. Subshell energies can overlap (e.g., 4s is lower than 3d).

Ions

Formation and Electron Configuration

Ions are formed when atoms gain or lose electrons to achieve a full valence shell. Cations are positively charged (electrons lost), and anions are negatively charged (electrons gained).

• Cations: Electrons removed from highest energy orbitals first.

• Anions: Electrons added to highest energy orbitals to form an octet.

Periodic Properties

Atomic and Ionic Radius

Atomic radius is the distance from the nucleus to the valence electrons. It decreases across a period and increases down a group. Ionic radius follows similar trends, but cations are smaller and anions are larger than their neutral atoms.

Ionization Energy

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

• Trend: Higher ionization energy means electrons are harder to remove.

• Metals: Low ionization energy.

• Nonmetals: High ionization energy.

Summary Table: Classification of Elements

Type	Groups	Properties
Representative Elements	1, 2, 13–18	Include metals, nonmetals, metalloids
Transition Elements	3–12	All metals
Inner-Transition Elements	Lanthanides, Actinides	Many are radioactive or rare earth metals

Additional info:

• Quantum numbers fully describe the state of an electron in an atom.

• Electron configurations and periodic properties are foundational for understanding chemical bonding and reactivity.