Answer: Elements in Group 2
Explanation: The periodic table was arranged by Dmitri Mendeleev specifically around similarites in their chemical behaviors. He found that as atomic number increases, at some point an element starts to react in a manner similar to a previous one. When that happened, he would place the larger element under the smaller one, and eventually noticed a periodicity in the table. Elements in a column (Groups) had similiar chemical properties. We know today that these similarities are due to the electron configuration, and that these configurations repeat themselves. He left gaps in the table when he could find an existing element with properties similar to others in that group. I big leap of faith, but it worked. Elements for those missing boxes were eventually discovered.
Answer:
The volume of hydrogen gas produced at STP is 4.90 liters.
Explanation:

Moles of aluminium =
According to reaction , 2 moles of aluminium gives 3 moles of hydrogen gas.
Then 0.1333 moles of aluminium will give:
of hydrogen gas
Volume of 0.2 moles of hydrogen gas at STP = V
Temperature at STP = T = 298.15 K
Pressure at STP = P = 1 atm
n = 0.2 mol
PV = nRT (Ideal gas equation)

The volume of hydrogen gas produced at STP is 4.90 liters.
Explanation:
At each reflecting surface (benzene and glass) there will be 180 degree phase change.
Now, for constructive interference the optical path in benzene is
.
Formula to calculate thickness of a benzene thin film is as follows.
Optical path length through benzene (
) = 
Hence, substituting the given values into the above formula as follows.
Optical path length through benzene = 
d = 
=
=
(as 1 nm = 
= 204.9 m
Thus, we can conclude that minimum thickness of benzene is 204.9 m.