Introduction to the Periodic Table
High-Yield Summary
- Mendeleev (1869) organized the first periodic table by atomic weight; Moseley (early 1900s) showed atomic number is the better organizing principle — the periodic law.
- Periods (rows) correspond to valence-electron energy levels; groups (columns) share valence electron configuration, which is why elements in a group share chemical behavior.
- Group 1 (alkali metals, 1 valence electron) are highly reactive, especially with water — reactivity increases down the group. Group 18 (noble gases, full valence shell) are stable and unreactive.
- Metals (left/center): lustrous, malleable, ductile, good conductors — from delocalized valence electrons (metallic bonding). Readily form cations.
- Nonmetals (upper right): brittle, non-lustrous, poor conductors; tend to gain rather than lose electrons.
- Hydrogen sits in Group 1 (1 valence electron) but behaves like a nonmetal (covalent bonding, diatomic H₂, forms both H⁺ and H⁻).
- Metalloids (B, Si, Ge, As, Sb, Te) sit on the metal/nonmetal stair-step line with intermediate properties — silicon is a semiconductor.
Key Terms
- Periodic law
- The chemical and physical properties of elements are a periodic function of their atomic number.
- Period vs. group
- Period = row (shares valence energy level). Group = column (shares valence electron configuration).
- Metallic bonding
- Valence electrons are delocalized and free to move throughout a metal's structure, rather than tied to one atom.
- Transition metals
- Central block (Groups 3–12) elements prone to multiple oxidation states, via d-orbital electrons participating in bonding.
- Metalloids (semimetals)
- Boron, silicon, germanium, arsenic, antimony, tellurium — properties intermediate between metals and nonmetals.
Metals vs. Nonmetals
| Property | Metals vs. Nonmetals |
|---|---|
| Location | Left/center of table vs. upper right |
| Luster | Lustrous (shiny) vs. lacks metallic luster |
| Malleability/ductility | Malleable and ductile vs. typically brittle as solids |
| Conductivity | Good conductors of heat/electricity vs. poor conductors |
| Electron tendency | Readily lose electrons (form cations) vs. tend to gain electrons |
Common MCAT Trap
- Hydrogen sits in Group 1 by electron count (1 valence electron) but behaves chemically like a nonmetal — don't assume it acts like an alkali metal.
- Reactivity trends run opposite for the two ends of the table's reactive groups: alkali metals get MORE reactive down the group, but this isn't true of all trends — check the specific property.
- Carbon shows nonmetals can vary wildly: graphite conducts electricity, diamond is an insulator — same element, very different behavior.
Quick Recall
Who established that atomic number, not atomic weight, is the better organizing principle for the periodic table?
Why do elements in the same group share similar chemical properties?
Why is hydrogen placed in Group 1 despite behaving like a nonmetal?
Name the six elements commonly classified as metalloids.