The valence electronic configuration of an atom is 6s2. It d orbital of the penultimate shell contains two unpaired electrons, calculate the atomic number.

To calculate the atomic number, we need to understand the electron configuration of the atom.

1. The given valence electronic configuration is 6s2. This means that the atom has 6 energy levels and the outermost energy level (6th level) has 2 electrons in the s orbital.

2. The d orbital of the penultimate shell (5th level, because penultimate means second to last) contains two unpaired electrons. The d orbital can hold up to 10 electrons, so this means there are 8 more slots that could be filled.

3. Now, let's calculate the total number of electrons, which is also the atomic number of the atom.

• The first energy level (1s) can hold 2 electrons.
• The second energy level (2s and 2p) can hold 8 electrons.
• The third energy level (3s, 3p, and 3d) can hold 18 electrons.
• The fourth energy level (4s, 4p, 4d, and 4f) can hold 32 electrons.
• The fifth energy level (5s, 5p, 5d, and 5f) can hold 32 electrons, but we know that the 5d subshell has only 2 electrons, so this level holds 18 electrons (2 in 5s, 6 in 5p, and 2 in 5d).
• The sixth energy level (6s) has 2 electrons.

4. Adding these up, the total number of electrons is 2 + 8 + 18 + 32 + 18 + 2 = 80.

So, the atomic number of the atom is 80.