Explanation:
(a) The given data is as follows.
B = 
Z = 4 for Be
Now, for the first excited state 
= 2; and 
if it is ionized.
Therefore, ionization energy will be calculated as follows.
I.E = 
= 
Converting this energy into kJ/mol as follows.
= 5249 kJ/mol
Therefore, the ionization energy of the 
ion in its first excited state in kilojoules per mole is 5249 kJ/mol.
(b) Change in ionization energy is as follows.
= 
or, = 
Therefore, wavelength of light given off from the ion by electrons dropping from the fourth (n = 4) to the second (n = 2) energy levels .
Carbon has four valence electrons. Meaning, it has 4 electrons in it's last shell. These 4 electrons will form a covalent bond with other elements to get the full set of 8 electrons needed for it to be stable. That is why it's considered as tetravalent.
The family on the periodic table that has a filled outer
energy level is VIIIA. The answer is letter D. They are also called the noble
gases or inert gases. They are virtually unreactive towards other elements or
compounds. They are found in trace amounts in the atmosphere. Their elemental form
at room temperature is colorless, odorless and monatomic gases. They also have
full octet of eight valence electrons in their highest orbitals so they have a
very little tendency to gain or lose electrons to form ions or share electrons with
other elements in covalent bonds.
