5.
a 120.0 g sample of thallium-208 decays. how much of the sample will remain after 3 half lifest

15.0 g

We are given;

We are required to calculate the remaining mass after three half lives

N = N₀ × 0.5^n

where, N is the remaining mass, N₀ is the original mass and n is the number of half lives.

Therefore;

N = 120.0 g × 0.5^3

  = 15.0 g

Thus, the remaining mass of Thallium-208 is 15.0 g

Alternatively;

120.0 g → 60.0 g (120 × 0.5) → 30.0 g (60 × 0.5) → 15.0 g (30 × 0.5)

try answer.com that works for me

answer:

the answer is 6 valence electrons.

explanation:

valence is the # of electrons on the outer most shell of an atom.

oxygen has 6.

although, the valency of oxygen is 2, because it only needs to make 2 covalent bonds, or gain 2 electrons, to achieve the stable octet configuration of a noble gas.

that's it!

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~ trueboss72