Shells , Subshells and Orbitals

• electrons shell with the lowest energy level is closest to the nucleus
• fill electron shells across a period, and a shell is affedf each time in a group.
• subshells contain separate energy levels of similar energy: s, p, d, f, which can only hold s:2 e-, p=6 e-, d= 10 e-, f=14 e-
• orbitals are where electrons sit in pairs (half # of orbitals as electrons)
• the Pauli exclusion principle: each orbital holds two electrons with opposite spins, either up or down (half arrow)
• Aufbau principle: electrons fill the lowest energy orbital first (2s, then 2p etc)
• Hund's Rule: every orbital in a subshell is singly occupied before it is paired.
• when writing final electron configurations and diagrams, shells should be grouped together (2s and 2p, 3s and 3p and 3d etc.)
• EXCEPTIONS TO GROUPED SHELLS: copper fills the 3d orbital before the 4s, as it is much more stable. Chromium has exactly half of 3d to create more stabilisation, so fill halfway before 4s
• condensing electron configurations: most filled noble gas before the atom, eg copper condensed = [Ar] 3d10 4s1