Atomic Structure

Pure Substance

Matter with a constant composition and distinct chemical properties that cannot be separated by physical means.

Element

A pure substance composed of only one type of atom that cannot be broken down chemically.

Compound

A pure substance composed of two or more different elements chemically bonded in a fixed ratio.

Mixture

A physical combination of two or more substances that retain their individual identities and can be separated physically.

Atom

The basic unit of a chemical element, consisting of a dense nucleus surrounded by an electron cloud.

Nucleus

The dense, positively charged center of an atom containing protons and neutrons.

Proton

A subatomic particle located in the nucleus with a positive charge (+1) and a mass of approximately 1 amu.

Neutron

A subatomic particle located in the nucleus with a neutral charge (0) and a mass of approximately 1 amu.

Electron

A subatomic particle orbiting the nucleus with a negative charge (-1) and negligible mass.

Atomic Number (Z)

The number of protons in the nucleus of an atom, which uniquely identifies the element.

Mass Number (A)

The total number of protons and neutrons in an atom's nucleus.

Isotope

Atoms of the same element with the same number of protons but different numbers of neutrons (different mass numbers).

Average Atomic Mass

The weighted average mass of all naturally occurring isotopes of an element, based on their relative abundances.

Relative Abundance

The percentage or fraction of a specific isotope found naturally on Earth for a given element.

Coulomb's Law

A fundamental law stating that the force between charges is proportional to the product of their magnitudes and inversely proportional to the square of the distance between them (F ∝ q₁q₂/r²).

Mole (mol)

The SI unit for amount of substance, representing exactly 6.022 × 10²³ elementary entities.

Avogadro's Number

The number of particles in exactly one mole of a substance, equal to 6.022 × 10²³ particles/mol.

Molar Mass

The mass in grams of one mole of a chemical substance, numerically equal to its atomic or formula mass in amu.

Grams to Moles

Conversion achieved by dividing the given mass of a substance by its molar mass (mol = g / MM).

Moles to Grams

Conversion achieved by multiplying the given moles of a substance by its molar mass (g = mol × MM).

Particles to Moles

Conversion achieved by dividing the number of particles by Avogadro's number (mol = particles / N_A).

Moles to Particles

Conversion achieved by multiplying the moles of a substance by Avogadro's number (particles = mol × N_A).

Percent Composition

The percent by mass of each element in a compound, calculated as (mass of element / total molar mass) × 100%.

Empirical Formula

The simplest chemical formula showing the lowest whole-number ratio of atoms of each element in a compound.

Molecular Formula

The actual chemical formula showing the exact number of atoms of each element in a molecule of a compound.

Ground State

The lowest energy, most stable configuration of electrons in an atom.

Excited State

A high-energy state achieved when an electron absorbs energy and jumps to a higher energy level.

Photon

A quantum of electromagnetic radiation released or absorbed when an electron transitions between energy levels.

Energy Level (Shell)

The quantized region around a nucleus designated by the principal quantum number (n) where electrons reside.

Valence Electron

An electron in the outermost energy level of an atom that participates in chemical bonding.

Core Electron

An electron in an inner energy level that does not participate in chemical bonding and shields valence electrons.

Orbital

A three-dimensional region of space around the nucleus where there is a high probability (90%) of finding an electron.

Electron Configuration

A notation that describes the distribution of electrons among the various orbitals and subshells of an atom.

Aufbau Principle

The rule stating that electrons fill the lowest available energy orbitals first before moving to higher ones.

Hund's Rule

The rule stating that electrons occupy degenerate orbitals singly and with parallel spins before pairing up.

Pauli Exclusion Principle

The rule stating that no two electrons in an atom can have the exact same four quantum numbers; an orbital holds max 2 electrons with opposite spins.

Ion

An atom or molecule that has gained or lost one or more electrons, resulting in a net electrical charge.

Cation

A positively charged ion formed when an atom loses one or more valence electrons.

Anion

A negatively charged ion formed when an atom gains one or more valence electrons.

Photoelectron Spectroscopy (PES)

An experimental technique that measures the binding energy of electrons in a sample by bombarding it with high-energy photons.

Binding Energy

The energy required to remove an electron from an atom; high binding energy corresponds to electrons closer to the nucleus.