0.54 mol of Hy is contained in a 2.00 L container at 20.0°C. What is the pressure in this container in atm?

Yes it would be a physical change.

If a hydrogen atom is neutral, it must also have  one electron. an oxygen atom (o) has an atomic number of eight (8). this means a neutral oxygen atom has eight protons and eight electrons. the element actium (ac) has an atomic number of 89, so it has 89 protons and 89 electrons in a neutral atom