Atomic Structure & Periodic Table – Comprehensive Study Notes

Sub-Atomic Particles

Protons
- Carry a positive electrical charge (+).
- Located inside the nucleus.
- Mass ≈ 1\;\text{amu} (atomic mass unit).
Neutrons
- Electrically neutral (0 charge).
- Also reside in the nucleus.
- Mass ≈ 1\;\text{amu}.
Electrons
- Possess a negative charge (−).
- Occupy regions surrounding the nucleus (shells/orbitals).
- About \tfrac{1}{2{,}000} the mass of a proton (≈ 0.000549\;\text{amu}).
- Their tiny mass is ignored when computing atomic mass.

Identity of an atom is set exclusively by its proton count (atomic number).
• Carbon → 6 protons.
• Oxygen → 8 protons.
• Hydrogen → 1 proton.
Atomic Number (Z)
Z = \text{number of protons}
Appears as the smaller of the two numbers in a periodic-table box.

Definition: Atoms of the same element (same Z) that contain different numbers of neutrons.
Naming convention: Element-Mass (e.g. Carbon-12, Carbon-13, Carbon-14).
Examples using carbon:
• Carbon-12 → 6p, 6n (mass = 12\;\text{amu}).
• Carbon-13 → 6p, 7n (mass = 13\;\text{amu}).
• Carbon-14 → 6p, 8n (mass = 14\;\text{amu}).
Car analogy ("Citrona" models C, CX, CXL): different trim packages (neutron counts) yet always the same make (element).

Atomic Mass / Mass Number (A)
A = \text{protons} + \text{neutrons}
• Electrons are negligible.
Computing neutrons:
\text{Neutrons} = A - Z
Example (Oxygen): A = 16,\; Z = 8 \Rightarrow \text{Neutrons} = 8.
PAN MAN mnemonic
• PAN → P = Protons, A = Atomic-number, N = Number.
• MAN → M = Mass-number, A = (placeholder E“of”), N = Neutrons.
(Helps separate atomic number from mass number.)

Ion = atom (or group) that has gained or lost electrons.
Cation
• Formed by electron loss.
• Carries a positive charge.
• Mnemonic: "Ca+ion has a + (cat paws are positive)."
• Example: \text{Na} \Rightarrow \text{Na}^+ (11p, 10e)
– Original Na: 11p, 11e, 12n → neutral.
– After losing 1 e⁻ → net +1.
Anion
• Formed by electron gain.
• Carries a negative charge.
• Mnemonic: "An-ion is a neg-ion."
• Example 1: Chloride \text{Cl} + e^- \Rightarrow \text{Cl}^- (17p, 18e, 18n).
• Example 2: Oxide \text{O} + 2e^- \Rightarrow \text{O}^{2-} (8p, 10e).

Ordered by increasing atomic number (left→right, top→bottom).
Columns = Groups (vertical).
Rows = Periods (horizontal).
First two columns (Groups 1 & 2) = Alkali metals and Alkaline-earth metals — termed the "active metals."
Center block = Transition/reactive metals.
Far right block = Noble gases + other non-metals (very stable).
"Outlier" strip at the bottom houses certain transition elements/metalloids pulled out to simplify table shape.
Each element’s box displays:
1. Atomic number (top).
2. Element symbol.
3. Average atomic mass (bottom).

Proton ≈ 1\;\text{amu}.
Neutron ≈ 1\;\text{amu}.
Electron ≈ 0.000549\;\text{amu}.
When weighing an atom, proton + neutron contributions dominate; electron mass is like ignoring the weight of a ring while weighing yourself.

Bohr Shells / Principal Energy Levels (n)
• n = 1 → K shell.
• n = 2 → L shell.
• n = 3 → M shell.
• n = 4 → N shell.

Principal level	Subshells present
n=1	s
n=2	s,\;p
n=3	s,\;p,\;d
n=4	s,\;p,\;d,\;f
Orbital = 3-D region inside a subshell where an electron is most likely found.
• Each orbital holds up to 2 e⁻.
• Number of orbitals depends on subshell type: s\,(1),\;p\,(3),\;d\,(5),\;f\,(7).

Practical / Exam Connections

Being able to assign Z, A, proton, neutron, and electron counts is a favorite test item (ATI TEAS, etc.).
Remember the formulas and mnemonics (PAN MAN, cat-ions, neg-ions) for quick recall.
Expect periodic-table questions: locating groups/periods, knowing which region contains active vs reactive metals.
Electron-configuration questions often begin with identifying the correct shell/subshell sequence (e.g. 1s, 2s, 2p, 3s …).

Isotopes (e.g. \text{C}^{14}) underpin radiometric dating and medical imaging.
Ion concepts explain electrolyte behavior in physiology (Na⁺, Cl⁻).
Understanding electron shells is foundational for bonding, reactivity, and material design.