Answer:
Covalent Bonds are formed when two non-metals share electrons
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Answer:
Ammonia is the richest source of nitrogen on a mass percentage basis because it has 82.35% of nitrogen by mass.
Explanation:
Percentage of element in compound :

(a) Urea, 
Molar mass of urea = 60 g/mol
Atomic mass of nitrogen = 14 g/mol
Number of nitrogen atoms = 2

(b) Ammonium nitrate, 
Molar mass of ammonium nitrate = 80 g/mol
Atomic mass of nitrogen = 14 g/mol
Number of nitrogen atoms = 2

(c) Nitric oxide, NO
Molar mass of nitric oxide = 30 g/mol
Atomic mass of nitrogen = 14 g/mol
Number of nitrogen atoms = 1

(d) Ammonia, 
Molar mass of ammona = 17 g/mol
Atomic mass of nitrogen = 14 g/mol
Number of nitrogen atoms = 1
Ammonia is the richest source of nitrogen on a mass percentage basis because it has 82.35% of nitrogen by mass.
Answer : The mass of sample is, 267.5 grams.
Explanation :
Density : It is defined as the mass of a substance contained per unit volume.
Formula used :

Given:
Volume of Pb = 
Density of Pb = 
Now put all the given values in the above formula, we get the mass of Pb.


Therefore, the mass of sample is, 267.5 grams.
A mixture is a material system made up of two or more different substances, which are mixed but not combined chemically. A mixture refers to the physical combination of two or more substances in which the identities of the individual substances are retained.
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Answer:
Many years ago, people living on the Earth, desired to go to the outer space.
Explanation:
When man initiate to go outer space:
Many times ago, people used to think about the world out of Earth, the space and the sky. These thoughts generated the idea of thinking and imagining the space and the moon.
The idea turned into a movie by Georges Méliès, which was inspired by a book written by Jules Verne. The movie gave the idea to Konstantin Tsiolkovsky, a Russian man to develop many ideas which were implemented by Russia and America to build airplanes and rockets that we see launching at present.
A small though became an idea which gave birth to a new invention through which people live their dreams of going in the outer space. Incredible!