⚗️ Periodic Table
H He Li Be B C N O F Ne
First 10 Elements in Order
The first 10 elements — every chemistry student's starting point
Hydrogen, Helium, Lithium, Beryllium, Boron, Carbon, Nitrogen, Oxygen, Fluorine, Neon. Atomic numbers 1–10. Learn these cold — they appear on every general chemistry exam.
⚗️ Periodic Table
Groups go down, Periods go across
Groups vs Periods
Groups are columns, periods are rows — never mix them up
Groups (vertical columns) share similar chemical properties. Periods (horizontal rows) represent the same number of electron shells. Group 1 = alkali metals. Period 2 = Li through Ne.
⚗️ Periodic Table
Atomic radius: bigger going down-left
Periodic Trend — Atomic Radius
Atomic radius increases down and to the left on the table
Going down a group: more electron shells = larger atom. Going left across a period: fewer protons pulling electrons in = larger atom. Smallest elements are top-right. Largest are bottom-left.
⚗️ Periodic Table
Electronegativity: bigger going up-right
Periodic Trend — Electronegativity
Electronegativity is opposite of atomic radius
Electronegativity increases going up and to the right. Fluorine is the most electronegative element (3.98). Cesium and Francium are least electronegative. Think: top-right corner is "greedy" for electrons.
⚗️ Periodic Table
Alkali metals: soft, reactive, 1 valence electron
Group 1 Properties
Group 1 alkali metals — key properties for every exam
Li, Na, K, Rb, Cs, Fr. All have 1 valence electron, making them extremely reactive. They react violently with water. Reactivity increases down the group. Never found pure in nature.
Halogens
Halogens (Group 17): F Cl Br I At — very reactive, form salts with metals
Halogens
Group 17 elements — one electron short of a full outer shell
Fluorine, Chlorine, Bromine, Iodine, Astatine. All need one more electron → extremely reactive. React violently with alkali metals to form salts (NaCl = sodium chloride). Reactivity decreases going down the group. F is the most reactive element on the periodic table.
Noble Gases
Noble gases (Group 18): He Ne Ar Kr Xe Rn — full outer shells, almost no reactions
Noble Gases
Group 18 — the most stable elements, almost completely unreactive
Helium, Neon, Argon, Krypton, Xenon, Radon. Full valence shells → no need to gain or lose electrons → extremely stable and unreactive. Used in light bulbs, lasers, balloons. 'He Never Argues, Kinda Xenophobic Really' — He, Ne, Ar, Kr, Xe, Rn.
Transition Metals
Transition metals: variable oxidation states, colored compounds, good conductors
Transition Metals
The large middle block — Groups 3-12
Fill the d subshell. Variable oxidation states: iron can be Fe²⁺ or Fe³⁺. Form colored compounds (copper sulfate = blue, potassium dichromate = orange). Good electrical and heat conductors. Catalysts: iron in Haber process, platinum in catalytic converters.
Ionization Energy Trend
Ionization energy: energy to REMOVE an electron. Increases going up and right on the table.
Ionization Energy Trend
The energy needed to pull an electron away from an atom
High ionization energy = harder to remove electron = more stable. Increases across a period (more protons pulling electrons in). Decreases down a group (electrons further from nucleus). Noble gases have highest IE. Alkali metals have lowest. Opposite trend to atomic radius.
Electron Configuration Order
Electron configuration: fill orbitals in order — 1s 2s 2p 3s 3p 4s 3d...
Electron Configuration Order
The Aufbau principle — how electrons fill atomic orbitals
Fill lowest energy orbitals first. Order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p... Diagonal rule (Madelung rule) explains why 4s fills before 3d. Pauli exclusion: max 2 electrons per orbital. Hund's rule: one electron per orbital before pairing.
Periodic Table Regions
Metals vs Nonmetals: left of staircase = metals. Right = nonmetals. On the line = metalloids.
Periodic Table Regions
Three broad regions of the periodic table — and the staircase divides them
Metals (left): shiny, malleable, ductile, conduct heat and electricity. Nonmetals (right): dull, brittle, poor conductors. Metalloids/semimetals (on the staircase: B, Si, Ge, As, Sb, Te): properties of both — semiconductors. Silicon is the basis of computer chips.
Valence Electrons
Valence electrons = group number for main group elements. Determines reactivity.
Valence Electrons
The outer electrons that determine how an element reacts
Group 1: 1 valence electron. Group 2: 2. Groups 13-18: 3-8. Elements react to achieve 8 valence electrons (octet rule). Lose electrons → cation (metals). Gain electrons → anion (nonmetals). The valence electrons are shown in Lewis dot structures.