Electron Structure

Shells are energy levels.
• Atomic Orbitals: It is region around a nucleus that can hold up to two electrons, with opposite
spin. Must have opposite spin due to repulsion between two negative electrons.
• Four Different Atomic Orbitals: s-, p-, d- and f-. The greater the shell number n, the greater the
radius of its orbital. Must write in lower case and write superscript accurately.
• S- Orbitals: The electron cloud is within the shape of a sphere.
• P- Orbitals: Electron cloud is in shape of dumb- bell. There are three p- orbitals at right angles to
one another, called px, py and pz.
• d- Orbitals and f- Orbitals: From n = 3 there are five d- orbitals. From n = 4 there are seven forbitals.
• Sub- Shell: Within a shell, orbitals of the same type are grouped together as sub- shells. The
number of orbitals increases with each new type of orbital so number of electrons in sub shell
• A 3p orbital would be 2, not 6. The 3rd shell would be 18. The 3s sub shell would be 2.
• Filling Orbitals: Orbitals fill in order of increasing energy. 3d sub- shell is at a higher energy level
than 4s sub- shell so 4s sub- shell fills first like 3p, 4s, 3d.
• Electrons in Box Modal: Two arrows represents two electron, indicating opposite spin down. One
box represents one orbital.
• Energy of Orbital: Energy is equal in orbitals. One electron occupies each
orbital before pairing starts to prevent further repulsion between paired
electrons. In this example there are one completely filled orbitals.
• Watch out for ionic charges on elements, don’t just see the element and go
straight in.
• Electron Configuration: List number of electrons occupying orbital in superscript e.g. 1s2
, 2s2
, 2p6…
3d10, 4p6
. 3d10 is before 4s2 as electron configuration shows shell order NOT order of filling or how
it appears on periodic table. All numbers are together.
• Shorthand Electron Configuration: Use previous noble gas in periodic table e.g. for Krypton [Ar]
• Periodic Table: Periodic table can be divided into blocks corresponding to highest energy subshell. All the s- and p- blocks correspond with period number. The d- block are p minus 1.

Electron Configuration of Ions: When forming ions, the highest energy sub- shells loses or gains
electrons first. Once 4s is filled, 3d energy level falls below 4s energy level so 4s sub- shell empties
before 3d sub-shell as in order of electron configuration.