Most atoms follow the octet rule meaning they need electrons to fill their valence shells. One important exception is H which only needs electrons to fill its valence shell. full they blank

The octet rule means that atoms need a total of 8 valence electrons to fill their valence shell. Hydrogen only needs two to fill its valence shell.

Explanation:

All atoms except for hydrogen can hold a total of 8 valence electrons in their outer energy levels (2 in the s level, 6 in the p level). Hydrogen only has two electrons total, so it does not require more than the s level and two electrons to have a full valence.

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considering the question as per the attachment

here we can see that the radon nuclide has 222 as the atomic mass number and 86 as the atomic number.

when it undergoes nuclear fission it gives polonium which has atomic mass number as 218 and atomic number will be 84 along with a helium nucleus which has 4 atomic mass number and 2 as its atomic number.

222 = 218 + 4 (atomic mass number)

86 = ? + 2; therefore ? = 86-2 = 84.

the atomic mass number of polonium will be 84.