Electron shells fill in the order 2, 8, 18, 18, 32, 32

2 electrons

Octet rule and 18-electron rule.

The 18 electron rule (also known as the Effective Atomic Number Rule or EAN rule) was originally proposed by N.V. Sigwick when extending the octet rule proposed by G.N. Lewis in applying it to organometallic compounds. The idea behind both of these rules is that in a compound the sum total of all of the electrons would have the configuration of a noble gas.

With the octet rule there is the basic assumption that the central atom in the compound is 8 (the maximum capacity of the s and p orbitals). The 18 electron rule is based on a similar idea as the octet rule, however it takes into consideration the d orbital too - 2 (s) + 6 (p) + 10 (d) = 18.

the transion metal from carbonyals 18 electron rule