Answer:
C) to show that atoms are conserved in chemical reactions
Explanation:
When writing a chemical reaction, we should always consider the Mass Conservation Law, which basically states that; in an isolated system; the total mass should remain constant, this is, the total mass of the reactives should be equal to the total mass of the products
For this case, we should add the apporpiate coefficients in order to be in compliance with this law:
2H₂ + O₂ → 2H₂O
So, we can check the above statement:
For reactives (left side):
4H
2O
For product (right side):
4H
2O
Based on the periodic table, the element mendeleev called eka-manganese is now called technetium. Technetium<span> is a silvery-gray metal that tarnishes slowly in moist air. Hope this answers the question. Have a nice day. Feel free to ask more questions.</span>
ANSWER:
4 a) Specific elements have more than one oxidation state, demonstrating variable valency.
For example, the following transition metals demonstrate varied valence states:
,
,
, etc.
Normal metals such as
also show variable valencies. Certain non-metals are also found to show more than one valence state 
4 b) Isotopes are members of a family of an element that all have the same number of protons but different numbers of neutrons.
For example, Carbon-14 is a naturally occurring radioactive isotope of carbon, having six protons and eight neutrons in the nucleus. However, C-14 does not last forever and there will come a time when it loses its extra neutrons and becomes Carbon-12.
5 a)
→
5 b)
→ 
5 c)
→
(already balanced so don't need to change)
5 d)
→
5 e)
→ 
EXPLANATION (IF NEEDED):
1. Write out how many atoms of each element is on the left (reactant side) and right (product side) of the arrow.
2. Start multiplying each side accordingly to try to get atoms of the elements on both sides equal.
EXAMPLE OF BALANCING:
Alpha particle is equivalent to B. Helium atom (2 protons, 2 neutrons)