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Changes of State
The transition of matter from one state to another, e.g., melting, freezing, condensation, vaporization, sublimation.
Atomic Number (Z)
The number of protons in an atom's nucleus, determining the element.
Mass Number (A)
The sum of protons and neutrons in the nucleus of an atom.
Isotopes
Atoms of the same element with different numbers of neutrons.
Mass Spectrometry
A technique used to determine the relative masses of atoms and the isotopic composition of elements.
Electron Arrangement
The distribution of electrons in atomic orbitals around the nucleus.
Aufbau Principle
Electrons fill orbitals starting with the lowest energy levels.
Pauli Exclusion Principle
No two electrons in an atom can have the same set of quantum numbers.
Hund's Rule
Electrons fill degenerate orbitals singly before pairing up.
The Mole Concept
A unit for counting entities in chemistry, one mole equals 6.022 × 10²³ particles.
Molar Mass
The mass of one mole of a substance, typically measured in grams per mole.
Empirical Formula
The simplest whole number ratio of elements in a compound.
Molecular Formula`
The actual number of atoms of each element in a compound.
Molar Volume of Gas
The volume occupied by one mole of an ideal gas at standard temperature and pressure (STP).
Ideal Gas Law
(PV = nRT)The equation relating pressure, volume, temperature, and the number of moles of an ideal gas.
Ideal Gas Model
A model that assumes gas particles do not interact and occupy no volume.
Real GasesGases
that deviate from ideal behavior, particularly under high pressure and low temperature.
Molar Volume
The volume occupied by one mole of an ideal gas at STP, approximately 22.7 dm³/mol.
Periodic Law
The properties of elements repeat in a regular pattern when arranged by increasing atomic number.
Groups
The vertical columns of the periodic table, where elements have similar chemical properties.
Periods
The horizontal rows of the periodic table, where elements have the same number of electron shells.
Electronegativity
The tendency of an atom to attract electrons in a bond.
Trends in Electronegativity Across a Period and Down a Group and Why
Electronegativity increases across a period (from left to right) and decreases down a group due to increasing nuclear charge and electron shielding.
Electron Affinity
The energy change when an electron is added to an atom in the gas phase.
Trends in Electron Affinity Across a Period and Down a Group and Why
Electron affinity becomes more negative across a period and less negative down a group as atoms become more eager to gain electrons across a period and less so down a group due to electron shielding.
Ionization Energy
The energy required to remove an electron from an atom in the gas phase.
Trends in Ionization Energy Across a Period and Down a Group and Why
Ionization energy increases across a period (left to right) and decreases down a group due to increased nuclear charge across a period and greater distance between nucleus and electrons down a group.
Atomic Radius
The distance from the nucleus of an atom to its outermost electron.
Trends in Atomic Radius Across a Period and Down a Group and Why
Atomic radius decreases across a period due to increased nuclear charge, and increases down a group due to additional electron shells.
Metallic Character
The tendency of an element to lose electrons and form positive ions (cations).
Trends in Metallic Character Across a Period and Down a Group
Metallic character decreases across a period and increases down a group due to the decrease in ionization energy across a period and increase in size and shielding down a group.
Reactivity Trends
The tendency of elements to react with others.
Trends in Reactivity Across a Period and Down a Group
Reactivity increases down Group 1 (alkali metals) and decreases down Group 7 (halogens) due to the ease of electron loss in alkali metals and electron gain in halogens.
Properties of Metals
Good conductors of heat and electricity, malleable, ductile, shiny, high melting and boiling points, form positive ions.
Properties of Non-Metals
Poor conductors of heat and electricity, brittle (in solid form), dull, low melting and boiling points, form negative ions.
