Unit 2: Atomic Structure

Atom

The smallest unit of matter.

Element

A pure substance made up of atoms that all contain the exact same number of protons.

Proton

A positively charged particle located in the nucleus with a mass of $11\,AMU$ and symbol $p$.

Neutron

A particle with no charge (neutral) located in the nucleus with a mass of $1\,AMU$ and symbol $n$.

Electron

A negatively charged particle located outside the nucleus in orbitals, with a mass of $\frac{1}{1836}\,AMU$ and symbol $e^-$.

Nucleons

The particles located inside an atom's nucleus, specifically protons and neutrons.

Bohr Model

An atomic model developed by Niels Bohr stating that electrons travel around the nucleus in fixed energy level orbits.

Bohr Model Shell Capacities

The maximum number of electrons in the first three energy levels: the first shell holds $2$, while the second and third shells hold $8$ each.

Quantum Mechanical Model

An atomic model based on the Schrödinger equation describing electrons moving in a 3D cloud where they are located $90\%$ of the time.

Atomic Number

The number of protons in an atom's nucleus, which identifies the element and never changes.

Mass Number

The total number of protons plus neutrons in an atom's nucleus, rounded to a whole number in $AMU$.

Isotope

An atom of an element that has the same atomic number but a different number of neutrons, resulting in a different mass number.

Atomic Mass

The weighted average mass of all naturally occurring isotopes of an element.

Calculating Atomic Mass

$$\text{Atomic Mass} = (\text{fractional abundance}_1 \times \text{mass}_1) + (\text{fractional abundance}_2 \times \text{mass}_2) + \dots$$

Ion

An atom that has an overall net charge because its number of electrons does not equal its number of protons.

Cation

A positively charged ion formed when an atom loses electrons.

Anion

A negatively charged ion formed when an atom gains electrons.