Covalency has to do with how polar/non-polar the bond is. The more equal a partnership, the more covalent the bond is. Thus: compare electronegativities. Metal/non-metal pairs form ionic (extreme of polar covalent to the point of being NOT covalent) bonds.
Try selecting various energy levels at the top of the applet. We have only talked about being in a single energy level at a time - thus deselect an energy level before selecting another one.
Note that this is how you would calculate ENC - Although the nucleus gets MORE positive down the PT, the ENC actually decreases! Not just the compensation part, but actually in it's entirety.
Graduate student in Chemistry (Physical/Theoretical and Education). Live in Madison, WI. Graduated from Hope College in Holland, MI. Born and raised in Kalamazoo/Portage, MI.