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.

Determining an Element

• 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.

Isotopes

• 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 & Related Formulas

• 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.)

Charge States of Atoms

• Neutral atom: \text{protons} = \text{electrons}.
• Positive atom: \text{protons} > \text{electrons} (net +).
• Negative atom: \text{electrons} > \text{protons} (net −).

Ions

• 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).

Periodic Table Overview

• 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).

Relative Masses (amu)

• 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.

Electron Organization: Shells, Subshells & Orbitals

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


• Subshells within each shell
  

• 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 …).

Ethical & Real-World Relevance

• 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.