The Structure of the Atom

Atomic Models

Dalton’s Atomic Theory

• States that all matter is composed of tiny indivisible particles called atoms
• Atoms of the same element are identical in all respects, including mass and chemical properties
• Atoms of different elements have different masses and chemical properties
• Atoms combine in simple whole-number ratios to form compounds
• Chemical reactions involve the rearrangement of atoms, not their destruction or creation

Thomson’s Atomic Model

• Proposed that atoms are uniform, positively charged spheres with electrons embedded within them
• Also known as the “plum pudding model” because of its resemblance to a plum pudding with embedded raisins

Rutherford’s Atomic Model

• Discovered the atomic nucleus through the gold foil experiment
• Proposed that atoms have a small, dense, positively charged nucleus surrounded by a cloud of electrons
• The nucleus contains most of the atom’s mass, while electrons occupy most of the atom’s volume

Bohr’s Atomic Model

• Introduced the idea of quantized energy levels for electrons
• Electrons can only occupy certain specific energy levels, which are determined by their quantum numbers
• Electrons can move from one energy level to another by absorbing or emitting photons of light with a specific wavelength

Quantum Mechanical Model of the Atom

Quantum Numbers

• Principal quantum number (n): describes the electron’s energy level
• Azimuthal quantum number (l): describes the electron’s angular momentum and shape of its orbital
• Magnetic quantum number (ml): describes the electron’s spin orientation Spin quantum number (ms): describes the electron’s intrinsic spin

Quantum States and Orbitals

• Quantum states are the specific combinations of quantum numbers that describe the state of an electron
• Orbitals are the regions around the nucleus where electrons are most likely to be found

Electron Configurations

Aufbau Principle

• Electrons fill atomic orbitals in the order of increasing energy levels
• The lowest-energy orbitals are filled first, followed by higher-energy orbitals

Pauli Exclusion Principle

• No two electrons in an atom can have the same set of quantum numbers
• Each orbital can hold a maximum of two electrons with opposite spins

Hund’s Rule

• For a given set of degenerate orbitals (orbitals with the same energy), electrons occupy the orbitals with the maximum number of unpaired spins

Periodic Properties

Periodic Trends in Atomic Properties

• Atomic size: decreases across a period (left to right) and increases down a group (top to bottom)
• Ionization energy: increases across a period and decreases down a group
• Electron affinity: increases across a period and decreases down a group
• Electronegativity: increases across a period and decreases down a group

Isotopes, Isobars, and Isotopes

• Isotopes: atoms of the same element that have different numbers of neutrons
• Isobars: atoms of different elements that have the same mass number
• Isotopes: atoms of different elements that have the same number of protons

Applications of Atomic Structure

Chemical Bonding

• Chemical bonding involves the interactions between atoms to form molecules and compounds
• The properties of chemical bonds depend on the electronic configurations of the atoms involved

Spectroscopy

• Spectroscopy is the study of the interaction of light with matter
• Different substances absorb and emit light at characteristic wavelengths, which can be used to identify and analyze materials

Radioactivity

• Radioactivity is the process by which unstable atomic nuclei emit radiation to become more stable
• Radioactive isotopes have important uses in medicine, industry, and research

Nuclear Reactions

• Nuclear reactions are reactions that involve changes in the structure of atomic nuclei
• Nuclear reactions can release large amounts of energy and have many important applications, including nuclear power and nuclear medicine

References:

• NCERT Chemistry textbooks (Class 11 and 12)