a question about Barron's Biology book: third electron shell (M) ... THANKS!!!

[engstu214041]

<p>Hi, this is a paragraph from Barron’s SAT II Biology 13th edition, page 43</p>

<p>

Each shell holds a certain number of electrons. The innermost shell, designated as K, has 2; the next shell, L, can hold 8;** the third shell, M, also 8**; etc. When an atom has a shell that does not have a complete set of electrons, it is said to be structurally unbalanced, and will tend to interact with other atoms. The chemical activity of an atom, then, arises from the number of electrons in its outer shell

</p>

<p>I wonder why the third shell can hold 8 electrons. I thought M is the 3s 3p 3d, which can hold up to 18: 3s–2 3p–6 3d–10</p>

<p>2+6+10=18 ???</p>

<p>What do you think?</p>

[Weather11]

<p>1s2 =2
2s2 2p6 =8
3s2 3p6 =8
4s2 3d10 4p6 =18
etc</p>