AP Chemistry Unit 2: The Atom Vocabulary

atomism

the theory that all matter is composed of small invisible particles called atoms.

Democritus

(5th century BCE) the creator of atomism.

solid sphere model

the theory that atoms are invisible, and compounds are combinations of different types of atoms.

John Dalton

(1803) the creator of the solid sphere model.

plum pudding model

the theory that the atom is a sphere, but the positive and negative charges are embedded within it.

J. J. Thomson

(1897) the creator of the plum pudding model.

nuclear model

the theory that electrons orbit in a set and in predictable paths around fixed, positively charged nucleus.

Ernest Rutherford

(1911) the creator of the nuclear model and the gold foil experiment.

planetary model

the theory that electrons are arranged in concentric specific circular orbits around the nucleus.

Niels Bohr

(1913) the creator of the planetary model.

quantum model

the theory that the locations of electrons can only be described as being part of a “cloud” around the nucleus.

Erwin Schrödinger

(1926) the creator of the quantum model.

nucleus

the center of the atom, which contains protons and neutrons and almost all of the mass in an atom.

proton

a positively-charged subatomic particle.

neutron

a neutral subatomic particle.

electron

a negatively-charged subatomic particle much smaller than protons and neutrons in size and mass, located outside the nucleus in orbitals.

atomic number

the number of protons in an atom.

isotope

an atom with the same number of protons as a number atom but a different number of neutrons.

mass number

the sum of the numbers of protons and neutrons in a nucleus.

atomic mass

the weighted average mass of the isotopes of that element.

electron configuration

a diagram that shows how the electrons of an atom are arranged in lowest-to-highest energy levels.

Aufbau Principle

the principle that in the ground state of an atom or ion, electrons first fill sublevels of the lowest available energy, then fill sublevels of higher energy.

noble gas notation

a notation used to abbreviate electron configurations.

Lewis electron-dot diagram

a diagram that shows the outermost, highest-energy electrons of an atom.

orbital diagram

a diagram that represents the valence shell of an atom using boxes for the orbitals and arrows for the electrons.

Hund’s Rule

the rule that states that the p, d, or f orbitals in a sublevel must be filled with one electron each, all with the same directional spin, before any electron can pair.

principal quantum number (n)

the principal energy level where the electron is located.

Azimuthal/angular quantum number (l)

an integer from 0 to n–1 the represents the sublevel where the electron is located.

magnetic quantum number (m_l)

an integer (inclusive of 0) from –l to +l which represents the orbital within a sublevel where the electron is located.

spin quantum number (m_s)

a number that is either +1/2 or –1/2 and represents the spin of an electron.

Pauli Exclusion Principle

the principle that states that no two electrons in the same atom may have the same four quantum numbers.

excited state

the state of an atom when an electron goes from a lower level to a higher level.

bright line spectrum

a diagram created when electrons from a higher level move to a lower level and release energy in the form of light.

octet rule

the rule that elements prefer to have either eight or zero electrons in their valence shell.

ion

an atom that has a net electric charge that is not zero because it has either gained or lost electrons in order to achieve the octet rule.