Isotopes of an element differ in the number of ______ in their nuclei.

When there are two electrons in the same orbital, they have _____ spins.

The outer electronic configuration of the ground state chromium atom is 3d⁴ 4s²

Calculate the wavelength in Angstrom of the photon that is emitted when an electron in the Bohr orbit, n = 2 returns to the orbit, n = 1 in the hydrogen atom. The ionization potential of the ground state hydrogen atom is 2.17 $$\times$$ 10^-11 erg per atom.