Atoms and the Periodic Table: Structured Study Notes

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Atoms and the Periodic Table

Atomic Theory and the Structure of Atoms

The atomic theory forms the foundation of modern chemistry, describing the nature and behavior of atoms—the smallest units of matter that retain the identity of an element.

Atom: The smallest particle of an element that retains its chemical properties.
Subatomic particles: Atoms are composed of protons (positively charged), neutrons (neutral), and electrons (negatively charged).
Nucleus: The dense core of the atom containing protons and neutrons; electrons move rapidly in the space surrounding the nucleus.
Relative size: The nucleus is extremely small compared to the overall size of the atom.
Electrical charges: Opposite charges attract, so electrons are held near the nucleus, while like charges repel.

Elements and Atomic Number

Each element is defined by its atomic number, which is the number of protons in its nucleus. Atoms are electrically neutral, meaning the number of protons equals the number of electrons.

Atomic number (Z): Number of protons in the nucleus.
Mass number (A): Sum of protons and neutrons in the atom.
Neutral atom: Number of electrons equals the atomic number.
Element notation: A (mass number) is written as a superscript, Z (atomic number) as a subscript, before the atomic symbol.

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

Isotope notation for carbon-12

Isotopes and Atomic Weight

Isotopes are atoms of the same element with identical atomic numbers but 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: Mass number as superscript, atomic number as subscript, before the symbol.
Atomic weight calculation: Weighted average based on isotope masses and their relative abundances.

Example: Gallium has two isotopes: Ga-69 and Ga-71. The atomic weight is calculated as:

The Periodic Table

The periodic table organizes all known elements by increasing atomic number and groups elements with similar chemical properties together.

Groups (columns): Elements with similar chemical properties.
Periods (rows): Elements with increasing atomic number.
Classification: Metals (left side), Nonmetals (upper right), Metalloids (zigzag band between metals and nonmetals).

Periodic table with element classification

Characteristics of Different Groups

Elements are grouped based on recurring chemical behaviors, known as periodicity.

Group 1A (Alkali metals): Shiny, soft, highly reactive metals.
Group 2A (Alkaline earth metals): Lustrous, silvery, less reactive than alkali metals.
Group 7A (Halogens): Colorful, corrosive nonmetals, found in compounds.
Group 8A (Noble gases): Colorless, chemically 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: Energy levels around the nucleus, numbered 1, 2, 3, etc.
Subshells: Types within shells: s, p, d, f (in order of increasing energy).
Orbitals: Regions within subshells where electrons are most likely found. s = 1 orbital, p = 3, d = 5, f = 7.

Number of orbitals in each shell and subshell Shapes of s and p orbitals Electron distribution in atoms

Electron Configurations

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

Electrons occupy the lowest energy orbitals available.
Each orbital holds a maximum of two electrons with opposite spins.
Orbitals of equal energy are half-filled before any is completely filled.

Energy level diagram for orbitals Order of orbital filling Electron configuration notation and orbital diagrams Helium electron configuration and orbital diagram Electron configurations for B, C, N Electron configurations for O, F, Ne Phosphorus electron configuration and orbital diagram

Electron Configurations and the Periodic Table

The periodic table is divided into blocks (s, p, d, f) based on the subshell being filled. Elements in the same group have similar valence shell electron configurations, which determine their chemical properties.

Valence shell: The outermost electron shell.
Valence electrons: Electrons in the outermost shell, crucial for chemical reactivity.

Periodic table blocks by subshell Valence-shell electron configurations for main groups

Electron-Dot Symbols (Lewis Dot Symbols)

Electron-dot symbols represent the valence electrons of an atom as dots around the atomic symbol. These are useful for visualizing bonding and chemical reactivity.

Lewis dot symbol: Dots are placed around the element symbol to indicate the number of valence electrons.

Lewis dot symbol for a group 5A element

Summary Table: Electron Distribution in Atoms

The following table summarizes the electron distribution in the first four shells:

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 distribution in atoms table

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