Glossary
Atoms
The fundamental building blocks of matter, consisting of a nucleus surrounded by electrons.
Example:
A single particle of gold is an atom of gold, retaining all the properties of the element.
Aufbau Principle
A rule stating that electrons fill atomic orbitals in order of increasing energy, starting from the lowest available energy level.
Example:
Following the Aufbau Principle, electrons will fill the 1s orbital before moving to the 2s orbital.
Bohr Model
A simplified model of the atom where electrons orbit the nucleus in fixed, quantized energy levels or shells.
Example:
The Bohr Model successfully explained the line spectra of hydrogen by proposing electrons exist in discrete energy levels.
Charge
A fundamental property of matter that causes it to experience a force when placed in an electromagnetic field. Protons have a positive charge, and electrons have a negative charge.
Example:
The magnitude of the charge on an ion, like Na⁺ or Cl⁻, determines how strongly it will interact with other charged species.
Chemical Reactions
Processes that involve the rearrangement of atoms to form new substances, without changing the identity of the atoms themselves.
Example:
The burning of methane (CH₄ + 2O₂ → CO₂ + 2H₂O) is a chemical reaction where atoms are rearranged.
Compound
A substance formed when two or more different elements are chemically bonded together in fixed, whole-number ratios.
Example:
Water (H₂O) is a compound formed from hydrogen and oxygen atoms in a 2:1 ratio.
Core Electrons
Electrons that are not in the outermost energy shell of an atom. They are typically inner-shell electrons and do not participate in chemical bonding.
Example:
In sodium (1s²2s²2p⁶3s¹), the 1s², 2s², and 2p⁶ electrons are core electrons.
Coulomb's Law
A law describing the electrostatic force between two charged particles, stating that the force is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.
Example:
According to Coulomb's Law, a proton and an electron will experience a strong attractive force, which weakens rapidly as they move further apart.
Dalton's Atomic Theory
A foundational scientific theory proposing that matter is composed of indivisible atoms, which combine in simple ratios to form compounds and are rearranged in chemical reactions.
Example:
The law of definite proportions, stating that a compound always contains the same elements in the same proportions by mass, is consistent with Dalton's Atomic Theory.
Electric Force
The attractive or repulsive force between charged particles, as described by Coulomb's Law.
Example:
The electric force between the positively charged nucleus and the negatively charged electrons holds the atom together.
Electron Configuration
A notation that describes the distribution of electrons among the atomic orbitals of an atom.
Example:
The electron configuration for carbon is 1s²2s²2p², showing how its six electrons are arranged.
Electrons
Negatively charged subatomic particles that orbit the nucleus in specific energy levels. They determine an atom's chemical properties and bonding behavior.
Example:
When sodium loses an electron, it forms a positive ion, Na⁺.
Element
A pure substance consisting of only one type of atom, defined by its unique number of protons (atomic number).
Example:
Gold (Au) is an element because all its atoms have 79 protons.
Energy Level
A specific, quantized path or shell around the nucleus where electrons can reside, each associated with a distinct amount of energy.
Example:
Electrons in the first energy level (n=1) are closest to the nucleus and have the lowest energy.
Hund's Rule
A rule stating that when filling degenerate orbitals (orbitals of the same energy), electrons will occupy each orbital singly with parallel spins before pairing up in any one orbital.
Example:
When filling the 2p orbitals, Hund's Rule dictates that each of the three p orbitals gets one electron before any orbital gets a second electron.
Neutrons
Neutral (no charge) subatomic particles located in the nucleus of an atom. They contribute to the atom's mass but not its charge.
Example:
Isotopes of an element have the same number of protons but different numbers of neutrons.
Noble Gas Shortcut
A shorthand notation for electron configurations where the electron configuration of the preceding noble gas is used in brackets to represent the core electrons.
Example:
Instead of writing 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁵ for Bromine, the noble gas shortcut allows you to write [Ar]4s²3d¹⁰4p⁵.
Nucleus
The dense, central part of an atom containing protons and neutrons. It holds most of the atom's mass.
Example:
The atomic nucleus is incredibly tiny compared to the overall size of the atom, yet it contains almost all its mass.
Orbital Diagram
A visual representation of electron configuration using boxes or lines to represent orbitals and arrows to represent electrons, showing their spin.
Example:
An orbital diagram for nitrogen shows three unpaired electrons in the 2p subshell, consistent with Hund's Rule.
Orbitals
Regions around the nucleus where electrons are most likely to be found. They describe the probable location and energy of an electron.
Example:
The 1s orbital is spherical and closest to the nucleus, holding up to two electrons.
Pauli Exclusion Principle
A rule stating that no two electrons in the same atom can have the same set of four quantum numbers, meaning an atomic orbital can hold a maximum of two electrons, which must have opposite spins.
Example:
According to the Pauli Exclusion Principle, if one electron in an orbital has an 'up' spin, the second electron in that same orbital must have a 'down' spin.
Protons
Positively charged subatomic particles located in the nucleus of an atom. Their number defines the atomic number and the element.
Example:
An atom with 6 protons is always carbon, regardless of its number of neutrons or electrons.
Subatomic Particles
Particles smaller than an atom, primarily protons, neutrons, and electrons, which compose atoms.
Example:
Protons and neutrons are found in the nucleus, while electrons orbit around it.
Subshell
A subdivision within an electron shell, designated by letters (s, p, d, f), each capable of holding a specific maximum number of electrons.
Example:
The 2p subshell can hold up to 6 electrons and consists of three p orbitals.
Valence Electrons
Electrons located in the outermost energy shell of an atom. They are primarily responsible for an atom's chemical reactivity and bonding.
Example:
Oxygen has six valence electrons, which explains why it often forms two bonds to achieve a stable octet.