Helium has a structure 1s2. The electron is being removed from the same orbital as in hydrogen's case. It is close to the nucleus and unscreened. The value of the first ionization energy (24.6 eV) of helium is much higher than hydrogen (13.6 eV). It is well known that the energy of 13.6 eV is given by applying the Bohr formula, while for the first ionization energy of 24.6 eV of helium so far no one was able to formulate any successful formula. 

Effective Nuclear Charge:
The effective nuclear charge experienced by an electron in an atom is the net positive charge experienced by the electron due to the nucleus. It takes into account the shielding effect of other electrons in the atom.

First Ionization Energy:
The first ionization energy is the energy required to remove the outermost electron from an atom in its gaseous state. It is a measure of the atom's ability to hold onto its electrons.

Calculation of Effective Nuclear Charge:
The effective nuclear charge (Zeff) can be calculated using the formula:

Zeff = Z - S

where Z is the atomic number (number of protons) and S is the shielding constant.

For helium (He), the atomic number is 2. Since helium has two electrons, we need to calculate the shielding constant.

Shielding Constant:
The shielding constant is a measure of the electron-electron repulsions within an atom. It takes into account the shielding effect of electrons between the nucleus and the electron in question.

For helium, the two electrons are in the 1s orbital. The electron-electron repulsions between them reduce the effective nuclear charge experienced by each electron.

The shielding constant (S) for helium can be calculated using the formula:

S = 0.35 * (n - 1)

where n is the principal quantum number of the electron. For helium, n = 1.

Therefore, S = 0.35 * (1 - 1) = 0

Calculation of Effective Nuclear Charge for Helium:
Using the formula for Zeff:

Zeff = Z - S

Zeff = 2 - 0 = 2

Therefore, the effective nuclear charge for helium is 2.

Calculation of First Ionization Energy for Helium:
The first ionization energy (IE) for helium can be calculated using the formula:

IE = -13.6 * (Zeff)^2/n^2

where Zeff is the effective nuclear charge and n is the principal quantum number of the electron.

IE = -13.6 * (2)^2/1^2 = -13.6 * 4 = -54.4 eV

Since ionization energy is a positive quantity, we take the magnitude of the value.

The first ionization energy of helium atom is 54.4 eV.

Conversion to Joules:
To convert the ionization energy from electron volts (eV) to joules (J), we use the conversion factor:

1 eV = 1.602 x 10^-19 J

IE (in J) = -54.4 * 1.602 x 10^-19 = -8.72 x 10^-18 J

Since ionization energy is a positive quantity, we take the magnitude of the value.

The first ionization energy of helium atom is 8.72 x 10^-18 J.

Answer:
The correct answer is option C) 39.3 eV.