You can simply subtract the atomic number from the mass number in order to find the number of neutrons.
Answer:
4
Explanation:
Relationship between wavenumber and Rydberg constant (R) is as follows:

Here, Z is atomic number.
R=109677 cm^-1
Wavenumber is related with wavelength as follows:
wavenumber = 1/wavelength
wavelength = 253.4 nm

Z fro Be = 4

Therefore, the principal quantum number corresponding to the given emission is 4.
Opposite pairs form ionic bonds, due to this the answer is D - Li and Br; They have unlike charges.
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