Answer: (3) silicon
Explanation:
Element is a pure substance which is composed of atoms of similar elements.It can not be decomposed into simpler constituents using chemical reactions.Example: Silicon (Si)
Compound is a pure substance which is made from atoms of different elements combined together in a fixed ratio by mass.It can be decomposed into simpler constituents using chemical reactions. Example: ethane
, propanone
and water 
Answer:
6.2 g
Explanation:
In a first-order decay, the formula for the amount remaining after <em>n</em> half-lives is
where
<em>N</em>₀ and <em>N</em> are the initial and final amounts of the substance
1. Calculate the <em>number of half-lives</em>.
If
2. Calculate the <em>final mass</em> of the substance.
O2- stands for superoxide anion, S2- stands for sulfide anion, H- stands for hydrogen anion, and Na+ stands for sodium this all ions possess the electron configuration of noble gas because each of them (O2-, S2-, H-, and Na+) has a full outer shell.