Atomic Structure: Nucleus, Protons, and Electrons

• Atomic structure involves the arrangement of electrons around a central nucleus consisting of protons and neutrons
• Understanding atomic structure is fundamental to comprehending chemical properties and behaviors of elements

Basic Constituents

• Atoms consist of three primary subatomic particles: protons, neutrons, and electrons
• Protons are positively charged particles located in the nucleus
• The number of protons (atomic number) defines the element
• Neutrons are neutral particles located in the nucleus
• Neutrons contribute to the mass of the atom but not its charge
• Electrons are negatively charged particles orbiting the nucleus
• The number of electrons in a neutral atom equals the number of protons

Nucleus

• The nucleus is the dense, central region of an atom
• It contains protons and neutrons, collectively called nucleons
• The strong nuclear force holds the nucleons together, overcoming the electrostatic repulsion between protons
• Most of the atom's mass is concentrated in the nucleus

Atomic Number and Mass Number

• Atomic number (Z) is the number of protons in the nucleus of an atom
• It uniquely identifies an element
• Mass number (A) is the total number of protons and neutrons in the nucleus
• The number of neutrons (N) can be calculated as N = A - Z

Isotopes

• Isotopes are atoms of the same element with different numbers of neutrons
• Isotopes have the same atomic number but different mass numbers
• Most elements exist as a mixture of isotopes
• Isotopes of an element have similar chemical properties but different physical properties (e.g., mass, radioactivity)

Atomic Mass

• Atomic mass is the weighted average mass of all isotopes of an element, taking into account their relative abundance
• Measured in atomic mass units (amu) or Daltons (Da)
• Carbon-12 is used as the standard, with one atom defined as exactly 12 amu

Electrons and Orbitals

• Electrons occupy specific energy levels or shells around the nucleus
• These energy levels are quantized, meaning electrons can only exist at discrete energy values
• Electrons do not orbit the nucleus in fixed paths, but rather exist in regions of space called orbitals
• Orbitals are mathematical functions that describe the probability of finding an electron in a specific region of space

Electron Configuration

• Electron configuration describes the arrangement of electrons in the different energy levels and sublevels within an atom
• The Aufbau principle, Hund's rule, and the Pauli exclusion principle govern the filling of orbitals
• The Aufbau principle fills electrons from lowest to highest energy levels
• Hund's rule states that electrons individually occupy each orbital within a subshell before doubling up in any one orbital
• Pauli exclusion principle states that no two electrons in an atom can have the same set of four quantum numbers (each orbital can hold a maximum of two electrons, which must have opposite spins)

Shells and Subshells

• Electron shells are the main energy levels around the nucleus, denoted by the principal quantum number n (n = 1, 2, 3, ...)
• Higher values of n correspond to higher energy levels and greater distances from the nucleus
• Each shell consists of one or more subshells, denoted by the azimuthal quantum number l (l = 0, 1, 2, ..., n-1)
• Subshells are labeled as s (l=0), p (l=1), d (l=2), and f (l=3)
• Each subshell contains one or more orbitals
• The s subshell has 1 orbital, the p subshell has 3 orbitals, the d subshell has 5 orbitals, and the f subshell has 7 orbitals

Quantum Numbers

• Quantum numbers describe the properties of atomic orbitals and the electrons within them
• Principal quantum number (n): specifies the energy level or shell (n = 1, 2, 3, ...)
• Azimuthal quantum number (l): specifies the shape of the orbital and the subshell (l = 0, 1, 2, ..., n-1)
• Magnetic quantum number (ml): specifies the orientation of the orbital in space (ml = -l, -l+1, ..., 0, ..., l-1, l)
• Spin quantum number (ms): specifies the intrinsic angular momentum of the electron (+1/2 or -1/2)

Valence Electrons

• Valence electrons are the electrons in the outermost shell of an atom
• These electrons are involved in chemical bonding and determine the chemical properties of the element
• Atoms with similar numbers of valence electrons exhibit similar chemical behavior
• The number of valence electrons can be determined from the element's group in the periodic table

Ion Formation

• Ions are formed when atoms gain or lose electrons
• Cations are positively charged ions formed when an atom loses electrons
• Anions are negatively charged ions formed when an atom gains electrons
• Ion formation alters the electron configuration and affects the stability of the atom

Periodic Table

• The periodic table organizes elements based on their atomic number and recurring chemical properties
• Elements in the same group (vertical column) have similar valence electron configurations and exhibit similar chemical behavior
• Periods (horizontal rows) represent elements with the same number of electron shells
• The periodic table provides valuable information about the electronic structure and properties of elements