Isotopes= a group of atoms with the same no. of protons , but different no. of neutrons
Hydrogen isotopes

The name of the second isotope is deuterium and the name of the third isotope is tritium(radioactive).The most abundant isotope is
The writers of the Articles of Confederation wanted to preserve the states' sovereignty.
To fill the other element's shell.
Their is 2 valence electrons
Answer: The correct option is (c). The total pressure doubles.
Solution:
Initially, only 4 moles of oxygen gas were present in the flask.
(
) ( according to Dalton's law of partial pressure)
....(1)
= Total pressure when only oxygen gas was present.
Final total pressure when 4 moles of helium gas were added:

partial pressure of oxygen in the mixture :
Since, the number of moles of oxygen remains the same, the partial pressure of oxygen will also remain the same in the mixture.

= Total pressure of the mixture.
from (1)

On rearranging, we get:

The new total pressure will be twice of initial total pressure.