Answer:
Calcium
Explanation:
Clacium has atomic number 20. The atomic number. This means that it has 20 protons and 20 electrons. it has 21 neutrons.
Answer : The value of 'R' is 
Solution : Given,
At STP conditions,
Pressure = 1 atm
Temperature = 273 K
Number of moles = 1 mole
Volume = 22.4 L
Formula used : 
where,
R = Gas constant
P = pressure of gas
T = temperature of gas
V = volume of gas
n = number of moles of gas
Now put all the given values in this formula, we get the values of 'R'.


Therefore, the value of 'R' is
.
Answer:-
Ca-Cl
Explanation:-
The more the difference in electronegativity more the polar bond.
In case of Ca-Cl the difference = 3.16 - 1= 2.16
It is more than all the other combinations given.
Hence Ca-Cl is the most polar covalent bond.