Chemistry: Atomic Structure

• Chemistry is the study of matter and its properties as well as how matter changes

Atomic Structure

• The atom is the basic building block of all matter
• Atoms consist of protons, neutrons, and electrons
• Protons possess a positive charge, electrons a negative charge; neutrons are neutral
• Protons and neutrons reside in the atom's nucleus; electrons orbit the nucleus in shells
• The number of protons determines an element's identity, known as the atomic number (Z)
• An atom's mass number (A) is the sum of protons and neutrons in the nucleus
• Isotopes are variants of an element with differing numbers of neutrons, hence different mass numbers
• The atomic mass unit (amu) expresses atomic and molecular weights, defined as 1/12 of carbon-12's mass
• Ions are atoms or molecules with a net electrical charge due to electron gain or loss
• Cations are positively charged ions formed by electron loss
• Anions are negatively charged ions formed by electron gain

Electron Configuration

• Electrons occupy specific energy levels or shells around the nucleus
• The first shell holds up to 2 electrons
• The second shell holds up to 8 electrons
• The third shell can hold up to 18 electrons, though often 8 per the octet rule
• Electron configuration describes the arrangement of electrons in energy levels and sublevels
• The Aufbau principle dictates electrons fill the lowest energy levels first
• Hund's rule states that electrons individually occupy each orbital within a sublevel before pairing
• The Pauli exclusion principle prohibits two electrons in an atom from having the same four quantum numbers
• Valence electrons are in the outermost shell
• Valence electrons dictate an atom's chemical properties via chemical bonding
• Atoms gain, lose, or share electrons for a stable configuration: usually eight valence electrons (octet rule) or two (duet rule for hydrogen and helium)

The Periodic Table

• The periodic table organizes elements by increasing atomic number, grouping similar elements
• Elements are in horizontal rows (periods) and vertical columns (groups)
• Elements in the same group share valence electrons and chemical properties
• Group 1 elements (excluding hydrogen) are alkali metals and are highly reactive
• Group 2 elements are alkaline earth metals, reactive but less so than alkali metals
• Group 17 elements are halogens, highly reactive nonmetals
• Group 18 elements are noble gases, stable and unreactive due to a full valence shell
• Metals are lustrous, ductile, malleable, and good conductors
• Nonmetals are dull, brittle, and poor conductors
• Metalloids (semimetals) have properties of both metals and nonmetals
• Electronegativity measures an atom's attraction for electrons in a bond
• Electronegativity increases across a period and decreases down a group
• Ionization energy is the energy to remove an electron from an atom
• Ionization energy increases across a period, decreases down a group
• Atomic radius is the distance from the nucleus to the outermost shell
• Atomic radius decreases across a period, increases down a group
• Metallic character increases down a group, decreases across a period