AP BIOLOGY

Detailed explanation-1: -Shell (electron): A grouping of electrons in an atom according to energy. The farther a shell is from the nucleus, the larger it is, the more electrons it can hold, and the higher the energies of those electrons. The first shell (closest to the nucleus) can hold two electrons. The second shell can hold 8 electrons.

Detailed explanation-2: -Similarly, neon has a complete outer 2n shell containing eight electrons. These electron configurations make helium and neon very stable.

Detailed explanation-3: -Each shell can contain only a fixed number of electrons: the first shell can hold up to two electrons, the second shell can hold up to eight (2 + 6) electrons, the third shell can hold up to 18 (2 + 6 + 10) and so on.

Detailed explanation-4: -The third period contains only eight elements even though the electron capacity of the third shell is 18 because when the other shells get filled and the resultant number of electrons becomes eighteen, it gets added up and settles in the third electron shell and three shells are acquired by the fourth period.

Detailed explanation-5: -So the two have to go to the next energy level so that the electronic configuration becomes 2, 8, 8, 2. Now, there are only two electrons in the valence shell and hence when you give energy, 2 electrons in the valence shell move out of the atom, thereby making the atom stable.