Atomic Structure (1)

MCAT General Chemistry Review 2025-2026

Proton

has a positive charge and mass around 1 amu

Neutron

has no charge and mass around 1 amu

Electron

has a negative charge and negligible mass

Nucleus

contains the protons and neutrons, while the electrons move around it

Atomic Number

is the number of protons in a given element

Mass Number

is the sum of an element’s protons and neutrons

Atomic Mass

is essentially equal to the mass number, the sum of an element’s protons and neutrons

Isotopes

are atoms of a given element (same atomic number) that have different mass numbers. They differ in the number of neutrons

Atomic Weight

is the weighted average of the naturally occurring isotopes of an element. The periodic table lists this

Rutherford

first postulated that the atom had a dense, positively charged nucleus that made up only a small fraction of the volume of the atom

Bohr Model of the Atom

a dense, positively charged nucleus is surrounded by electrons revolving around the nucleus in orbits with distinct energy levels

Quantum

the energy different between energy levels; first described by Planck

Quantization

means that there is not an infinite range of energy levels available to an electron; electrons can exist only at certain energy levels

Increases

the energy of an electron _________ the farther it is from the nucleus

Atomic Absorption Spectrum

this of an element is unique; for an electron to jump from a lower energy level to a higher one, it must absorb an amount of energy precisely equal to the energy difference between the two levels

Atomic Emission Spectrum

when electron return from the excited state to the ground state, they emit an amount of energy that is equal to the energy difference between the two levels; every element has a characteristic __________, and sometimes the electromagnetic energy emitted corresponds to a frequency in the visible light range

Quantum Mechanical Model

posits that electrons do not travel in defined orbits but rather are localized in orbitals

Orbital

is a region of space around the nucleus defined by the probability of finding an electron in that region of space

Heisenberg Uncertainty Principle

states that it is impossible to know both an electron’s position and its momentum exactly at the same time

Four Quantum Numbers

there are __________; these numbers completely describe any electron in an atom

Principal Quantum Number (n)

describes the average energy of a shell

Azimuthal Quantum Number (l)

describes the subshells within a given principal energy level (s, p, d, and f)

Magnetic Quantum Number (ml)

specifies the particular orbital within a subshell where an electron is likely to be found at a given moment in time

Spin Quantum Number (ms)

indicates the spin direction (± 1/2) of an electron in an orbital

Electron Configuration

uses spectroscopic notation to designate the location of electrons

Spectroscopic Notation

combining the n and l values as a number and letter, respectively

n+1 Rule

electrons fill the principal energy levels and subshells according to increasing energy, which can be determined by this

Hund’s Rule

electrons fill orbitals according to this, which states that subshells with multiple orbitals (p, d, and f) fill electrons so that every orbital in a subshell gets one electron before any of them gets a second

Paramagnetic

materials have unpaired electrons that align with magnetic fields, attracting the material to a magnet

Diamagnetic

materials have all paired electrons, which cannot easily be realigned; they are repelled by magnets

Valence Electrons

are those electrons in the outermost shell available for interaction (bonding) with other atoms

s- and/or p-

for the representative elements (those in Groups 1, 2, and 13-18), the valence electrons are found in __________ orbitals

s- and either d- or f-

for the transition elements, the valence electrons are found in ________ orbitals

Octet

many atoms interact with other atoms to form bonds that complete this in the valence shell