Atoms are the building blocks of matter, composed of protons, neutrons, and electrons. These subatomic particles have unique properties that determine an atom's structure, behavior, and identity.

The atomic number, equal to the number of protons, defines an element. Neutrons contribute to atomic mass, while electrons occupy shells around the nucleus. Understanding these components is crucial for grasping atomic physics.

Subatomic Particles and Their Properties

Protons, Neutrons, and Electrons

• The three primary subatomic particles are protons, neutrons, and electrons
• Protons have a positive charge, a mass of approximately 1 atomic mass unit (amu), and are located in the nucleus of an atom
• Neutrons have no charge, a mass of approximately 1 amu, and are also located in the nucleus of an atom
• Electrons have a negative charge, a mass of approximately 1/1836 amu, and are located in the electron shells surrounding the nucleus

Charges and Masses of Subatomic Particles

• The charges of protons and electrons are equal in magnitude but opposite in sign (protons +1, electrons -1)
• Neutrons are electrically neutral, with no net charge
• The mass of a proton is roughly equal to that of a neutron, while an electron's mass is much smaller (about 1/1836 of a proton or neutron)
• The atomic mass unit (amu) is a convenient unit for expressing the masses of subatomic particles (1 amu ≈ 1.66 × 10⁻²⁴ grams)

Atomic Structure: Nucleus and Shells

Nucleus: Protons and Neutrons

• An atom consists of a dense, positively charged nucleus surrounded by negatively charged electrons in electron shells or orbitals
• The nucleus contains protons and neutrons, which account for most of the atom's mass
• Protons and neutrons are held together in the nucleus by the strong nuclear force, overcoming the electrostatic repulsion between positively charged protons

Electron Shells and Energy Levels

• Electrons occupy specific energy levels or shells around the nucleus, with each shell having a maximum number of electrons it can hold (2n², where n is the shell number)
• The electron shells are labeled K, L, M, N, and so on, with K being the closest to the nucleus and having the lowest energy (n=1, 2, 3, 4, respectively)
• Electrons in the outermost shell, known as the valence shell, determine an atom's chemical properties and bonding behavior
• The arrangement of electrons in an atom's shells is called its electron configuration (e.g., 1s² 2s² 2p⁶ for neon)

Atomic Number and Its Significance

Definition and Relationship to Protons and Electrons

• The atomic number (Z) of an element is equal to the number of protons in the nucleus of its atoms
• In a neutral atom, the number of protons is equal to the number of electrons, as the positive charges of the protons balance the negative charges of the electrons
• The atomic number uniquely identifies an element and determines its position in the periodic table (e.g., hydrogen: Z=1, helium: Z=2, lithium: Z=3)

Ions and Isotopes

• Atoms of the same element always have the same number of protons, but can vary in the number of neutrons (isotopes) or electrons (ions)
• Ions are atoms with an unequal number of protons and electrons, resulting in a net positive (cations) or negative (anions) charge (e.g., Na⁺, Cl⁻)
• Isotopes are atoms of the same element with different numbers of neutrons, resulting in different mass numbers (e.g., carbon-12, carbon-13, carbon-14)

Neutrons and Atomic Stability

Mass Number and Isotopes

• The mass number (A) of an atom is the sum of the number of protons and neutrons in its nucleus
• Neutrons contribute to the mass of an atom but do not affect its atomic number or chemical properties
• Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons, resulting in different mass numbers (e.g., carbon-12, carbon-13, carbon-14)
• Isotopes of an element have similar chemical properties but may have different physical properties (e.g., radioactivity, half-life)

Nuclear Stability and Magic Numbers

• The stability of an atom is influenced by the ratio of protons to neutrons in its nucleus, with certain ratios being more stable than others
• Atoms with an unstable ratio of protons to neutrons may undergo radioactive decay to achieve a more stable configuration (e.g., beta decay, alpha decay)
• The number of neutrons can also affect the stability of an atom, with "magic numbers" of protons and neutrons (2, 8, 20, 28, 50, 82, 126) resulting in increased nuclear stability
• Nuclei with magic numbers of protons or neutrons are more stable due to the complete filling of nuclear energy levels, analogous to the stability of noble gases with complete electron shells