Know how to solve a half-life problem. If you have 300 grams of a radioactive isotope with a half-life of 15 years: How much of the isotope will you have left after 15 years?
How much of the isotope will you have left after 30 years? How many half-lives will
occur in 45 years?

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.

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