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 atomic theory, which describes the nature and behavior of atoms—the smallest unit of an element that retains its identity. Atoms are composed of three fundamental subatomic particles: protons, neutrons, and electrons.

• Protons: Positively charged particles found in the nucleus.

• Neutrons: Electrically neutral particles with a mass similar to protons, also located in the nucleus.

• Electrons: Negatively charged particles with a much smaller mass, moving rapidly around the nucleus.

The nucleus is extremely small compared to the overall size of the atom, analogous to a pea in a stadium. Opposite charges attract, so electrons are held near the nucleus, while like charges repel, causing electrons to spread out as much as possible.

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. In a neutral atom, the number of electrons equals the number of protons.

• Atomic Number (Z): Number of protons; determines the element's identity.

• Mass Number (A): Number of protons plus neutrons.

• Neutral Atom: Number of electrons equals number of protons.

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

Isotopes and Atomic Weight

Isotopes are atoms of the same element (same atomic number) but with different mass numbers due to varying numbers of neutrons. The atomic weight of an element is the weighted average mass of its naturally occurring isotopes.

• Isotope Notation: $^A_ZX$ where A is mass number, Z is atomic number, and X is the element symbol.

• Atomic Weight Calculation: $\text{Atomic Weight} = \sum (\text{isotope mass} \times \text{fractional abundance})$

Example: Gallium has two isotopes, Ga-69 and Ga-71, with atomic weight calculated from their masses and abundances.

The Periodic Table

The periodic table organizes elements by increasing atomic number and groups elements with similar chemical properties into columns called groups and rows called periods. Elements are classified as metals, nonmetals, or metalloids based on their physical and chemical properties.

• Metals: Malleable, lustrous, good conductors; found on the left side.

• Nonmetals: Poor conductors; found on the upper-right side.

• Metalloids: Intermediate properties; located in a zigzag band.

Groups include main group elements (1A–8A), transition metals (1B–8B), and inner transition metals (lanthanides and actinides).

Characteristics of Different Groups

Elements in the same group exhibit similar chemical behavior. Notable groups include:

• Group 1A (Alkali Metals): Highly reactive, soft, shiny metals.

• Group 2A (Alkaline Earth Metals): Lustrous, silvery, less reactive than alkali metals.

• Group 7A (Halogens): Colorful, corrosive nonmetals, form salts.

• Group 8A (Noble Gases): Colorless, inert gases.

Electronic Structure of Atoms

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

• Shells: Principal energy levels (n = 1, 2, 3, ...).

• Subshells: s, p, d, f types within each shell.

• Orbitals: Regions where electrons are most likely found; s (1), p (3), d (5), f (7) orbitals per subshell.

Electron Configurations

Electron configuration describes the arrangement of electrons in an atom's orbitals. Three rules govern electron filling:

1. Electrons occupy the lowest energy orbitals first.

2. Each orbital holds a maximum of two electrons with opposite spins.

3. Orbitals of equal energy are half-filled before any is fully filled.

Electron configurations are written using shell and subshell notation, with superscripts indicating the number of electrons.

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

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 indicates the number of valence electrons for main group elements, which are crucial for chemical reactivity.

• s-block: Groups 1A and 2A

• p-block: Groups 3A–8A

• d-block: Transition metals

• f-block: Inner transition metals

Electron-Dot Symbols (Lewis Dot Symbols)

Electron-dot symbols (Lewis dot symbols) represent the valence electrons of an atom as dots around the element's symbol. The number of dots equals the number of valence electrons.

• Example: Group 5A elements have five valence electrons, shown as five dots around the symbol.

Summary 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