The representative elements are the A elements found in groups IA – VIIIA.
Representative elements have their valence electrons in orbitals of either s or p shells. The roman numeral and the letter designation determine the electron configuration. As an example, an element in Group VA has five valence electrons with the configuration s2p3.
Non-representative elements include the transition elements, which have valence electrons in the s and d shells, and the lanthanide and actinide series, which have valence electrons in the s and f shells. They have unexpected electron configurations. As an example, chromium and copper are 4s13d5 and 4s13d10 respectively.
MCAT Official Prep (AAMC)
• Representative elements include the s and p block
• Non-representative elements exist in the d and block
• They often have unexpected electron configurations
electron configuration: the distribution of electrons of an atom or molecule in atomic or molecular orbitals
roman numeral: any of the symbols used in a system of numerical notation based on the ancient Roman system
valence electron: an outer shell electron that can participate in chemical bonds
transition elements: any of the set of metallic elements occupying a central block (Groups IVB–VIII, IB, and IIB, or 4–12) in the periodic table
actinide: a series of elements with atomic numbers from 89 to 103, actinium through lawrencium
lanthanide: consists of the 14 elements, with atomic numbers 58 through 71, that follow lanthanum on the periodic table of elements.