Atoms and the Periodic Table: Structure, Properties, and Electron Configurations

Atoms and the Periodic Table

Atomic Theory and the Structure of Atoms

The foundation of chemistry is the atomic theory, which describes the nature and behavior of atoms—the smallest identifiable units of elements. Atoms are composed of subatomic particles: protons, neutrons, and electrons.

• Atom: The smallest particle of an element that retains its chemical identity.

• Proton: Positively charged particle found in the nucleus; mass ≈ 1 amu.

• Neutron: Electrically neutral particle in the nucleus; mass ≈ 1 amu.

• Electron: Negatively charged particle outside the nucleus; mass ≈ 1/1836 amu.

• Nucleus: Dense core of the atom containing protons and neutrons.

• Electrostatic Forces: Opposite charges attract, like charges repel, influencing atomic structure.

Example: The nucleus is to the atom as a pea is to a stadium in terms of relative size.

Elements and Atomic Number

Each element is defined by its atomic number (Z), which is the number of protons in its nucleus. The mass number (A) is the sum of protons and neutrons.

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

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

• Isotope Notation: (shows mass number as superscript, atomic number as subscript).

Example: Carbon-12 has 6 protons and 6 neutrons.

Isotopes and Atomic Weight

Isotopes are atoms of the same element (same Z) with different numbers of neutrons (different A). The atomic weight of an element is the weighted average of the masses of its naturally occurring isotopes.

• Isotope: Atoms with identical atomic numbers but different mass numbers.

• Atomic Weight: Weighted average mass of all isotopes of an element.

Formula: The average atomic weight is calculated as:

$\text{Atomic\ weight} = \sum (\text{isotopic\ mass} \times \text{fractional\ abundance})$

Example: Gallium has two isotopes: Ga-69 (68.9257 amu, 60.4%) and Ga-71 (70.9248 amu, 39.6%).

The Periodic Table

The periodic table organizes elements by increasing atomic number and groups them by similar chemical properties. Each element's box contains its symbol, atomic number, and atomic mass.

• Metals: Malleable, lustrous, good conductors; left side of the table.

• Nonmetals: Poor conductors; upper-right side.

• Metalloids: Properties intermediate between metals and nonmetals; found in a zigzag band.

• Groups (columns): Elements with similar chemical properties.

• Periods (rows): Horizontal rows; elements arranged by increasing atomic number.

Characteristics of Different Groups

Elements in the same group share similar chemical behaviors:

• Group 1A (Alkali metals): Shiny, soft, highly reactive, never found pure in nature.

• Group 2A (Alkaline earth metals): Lustrous, silvery, less reactive than 1A, not found pure in nature.

• Group 7A (Halogens): Colorful, corrosive nonmetals, found in compounds.

• Group 8A (Noble gases): Colorless, inert gases, very low reactivity.

Electronic Structure of Atoms

The arrangement of electrons in shells, subshells, and orbitals determines the properties of elements. The quantum mechanical model describes electrons as occupying quantized energy levels.

• Shells: Main energy levels around the nucleus (numbered 1, 2, 3, ...).

• Subshells: Subdivisions within shells (s, p, d, f).

• Orbitals: Regions within subshells where electrons are most likely found.

Table: Electron Distribution in Atoms

Shell Number	Subshell Designation	Number of Orbitals	Number of Electrons	Total Electron Capacity
1	s	1	2	2
2	s, p	1, 3	2, 6	8
3	s, p, d	1, 3, 5	2, 6, 10	18
4	s, p, d, f	1, 3, 5, 7	2, 6, 10, 14	32

Electron Configurations

The electron configuration of an atom describes the arrangement of electrons in shells and subshells. Three rules govern electron filling:

1. Electrons occupy the lowest energy orbitals available (Aufbau principle).

2. Each orbital holds a maximum of two electrons with opposite spins (Pauli exclusion principle).

3. Orbitals of equal energy are each half-filled before any is completely filled (Hund's rule).

Example: Magnesium (Z = 12): 1s2 2s2 2p6 3s2 or [Ne] 3s2

Table: Electron Configurations of the First 20 Elements

Electron Configuration Notation: Shell number and subshell letter, with superscript for number of electrons (e.g., 1s2 2s2 2p6).

Orbital Diagrams: Use arrows to represent electron spins in orbitals.

Electron Configurations and the Periodic Table

The periodic table is divided into blocks (s, p, d, f) based on the subshell being filled. The group number for main group elements indicates the number of valence electrons.

• s-block: Groups 1A and 2A (filling s orbitals)

• p-block: Groups 3A–8A (filling p orbitals)

• d-block: Transition metals (filling d orbitals)

• f-block: Inner transition metals (filling f orbitals)

Valence Shell: The outermost electron shell of an atom.

Valence Electrons: Electrons in the outermost shell, important for chemical reactivity.

Table: Valence-Shell Electron Configurations for Selected Groups

Electron-Dot Symbols (Lewis Symbols)

Electron-dot symbols represent the valence electrons of an atom as dots around the element symbol. These are useful for visualizing bonding and lone pairs in molecules.

Example: An element in group 5A has five valence electrons, shown as five dots around the symbol.