# Why does nitrogen gas' pressure not change after an addition of helium

a 9 L vessel contains 3 moles of helium and 3 moles of nitrogen at a pressure of 10 atm. Maintaining constant temperature, an additional 2 moles of helium are added. Assuming gases behave ideally, what are the partial pressures of nitrogen and helium at the end?

Initially there's: 3mol He/(3mol + 3mol) * 10 atm = 5 atm pressure of He
And: 3/6 * 10atm = 5 atm pressure of N2 as well

now that 2 moles of He are added to the vessel, why wouldn't N2's partial pressure increase? The way I see it, there are now more atoms, so more of them would knock on the walls, therefore pressure would increase overall, and since there are now more atoms, space between atoms is tightened, so ...
Anyway, overall pressure is P1/n1 = P2/n2
(10 atm/9 mol) = (P2/11 mol); P2 = 12.2 atm new total pressure
Partial PHe * Ptotal = 5/11 * 12.2 atm = 5.545 atm He
That's for He gas, but why shouldn't partial pressure change for N2 gas?