Learning Objective: To understand how electrons are configured in atoms.
Rule 1: For neutral atoms (ie not ions) the number of electrons equals the number of protons. This results in equal numbers of + proton and – electron charges for a net charge of 0.
Rule 2: Electrons are held in very specific ways in shells and each shell is composed of orbital orbitals. Each orbital can contain up to 2 electrons (these electrons are paired spin up and spin down – we’ll discuss spin later).
Rule 3: Each row of the periodic table describes the outer shell or valence electrons. The shells are defined by “principal quantum numbers” as n=1, n=2, n=3 etc. The first row of the periodic table is n=1, the second row n=2 etc.
The following table shows the shell and the number of electrons it can hold.
n=1 2 electrons
n=2 8 electrons
n=3 18 electrons
n=4 32 electrons
What do orbitals look like?
Do now: We’ll replay the video. Draw the 1S, then separately 1S and 2S, and 1S, 2S and 2P orbitals.
There’s a subtlety here in that the order in which the shells are filled is mixed so the 4s orbital fills before the 3d orbital is filled We’ll learn more about this when we study the Aufbrau Principle.