How many valence electrons does an atom of Cr possess?
9
3
1
2
11
The correct answer and explanation is:
The correct answer is 6.
Chromium (Cr) is an element with an atomic number of 24. To understand how many valence electrons it has, we first need to look at its electron configuration. The electron configuration for chromium is:
Cr: [Ar]3d54s1\text{Cr:} \, [Ar] 3d^5 4s^1
This shows that chromium has an electron arrangement that ends in the 3d and 4s orbitals. The valence electrons are the electrons in the outermost shell of an atom, which are involved in chemical bonding. In chromium’s case, the outermost shell is the 4th energy level, and it contains one electron in the 4s orbital and five electrons in the 3d orbital, totaling six valence electrons.
It’s important to note that while chromium has two 4s electrons in its neutral state, it has a unique electron configuration. Chromium is one of the elements that does not follow the typical electron configuration expected from the Aufbau principle (which suggests filling lower energy orbitals first). Instead, chromium promotes one of its 4s electrons into the 3d orbital to achieve a more stable half-filled 3d subshell configuration. As a result, it only has one 4s electron and five 3d electrons in its valence shell, adding up to six valence electrons.
These six electrons are available for bonding, which is why chromium typically forms a variety of oxidation states, including +2, +3, and +6, depending on the number of valence electrons it loses or shares in chemical reactions. The flexibility in chromium’s bonding behavior is part of what makes it so versatile in various chemical compounds.