Question

Select the molecule in which the central atom has an incomplete octet.
(a) Beryllium
(b) Phosphorous
(c) Silicon
(d) Chlorine

Answer 1

In order to answer this question, to know the exact option or the molecule in which the central atom has an incomplete octet, first we should know the condition of being an octet and then we will check whether which one is not satisfying the condition.

Complete answer:
In the above options, Beryllium $(Be)$ is the molecule in which the central atom has incomplete octet.
Beryllium doesn't fulfill the octet rules because of its valence electrons and the orbitals they occupy. Just like $H,{\text{ }}He,{\text{ }}Li,{\text{ }}and{\text{ }}Be$ , they are all exceptions to the octet rules mainly because they don't need 8 to feel complete. Their $s - orbital$ do not need 8 electrons to complete the shell to feel full, so they don't follow the octet rule.
Beryllium is a chemical element with atomic number 4 and the symbol $Be$ . It's a relatively uncommon feature in the universe, typically resulting from the spallation of larger atomic nuclei colliding with cosmic rays. Beryllium is depleted in the cores of stars when it is mixed into heavier elements. It's a divalent element that only appears in nature in conjunction with other elements.
Because of the toxicity of inhaled beryllium-containing dusts, which can induce a chronic life-threatening allergic disease called berylliosis in certain individuals, commercial use of beryllium necessitates the use of adequate dust control equipment and industrial controls at all times.
Hence, the correct option is (A).

Note:
Beryllium is a chemical element that belongs to the alkaline-earth metals of Group 2 of the periodic table. It's used as a hardening agent in metallurgy and in a variety of outer space and nuclear applications. At room temperature, it is a steel-gray metal that is very brittle.