To determine which element can exhibit both covalency and electrovalency, we need to understand the concepts of covalent and electrovalent bonds and how they are formed.



A covalent bond is formed between two atoms when they share electrons. This type of bond usually occurs between nonmetals. In a covalent bond, atoms share electrons to achieve a stable electron configuration.



An electrovalent bond, also known as an ionic bond, is formed when there is a complete transfer of electrons from one atom to another. This type of bond usually occurs between a metal and a nonmetal. In an electrovalent bond, one atom loses electrons to become a positively charged cation, while the other atom gains electrons to become a negatively charged anion.



Given that the atomic numbers of the four elements are as follows:

- P: 4
- Q: 18
- R: 10
- S: 16



To determine which element can exhibit both covalency and electrovalency, we need to consider their positions in the periodic table.

- Element P has an atomic number of 4, which corresponds to beryllium (Be). Beryllium is a metal and is more likely to form electrovalent bonds rather than covalent bonds.

- Element Q has an atomic number of 18, which corresponds to argon (Ar). Argon is a noble gas and does not readily form compounds through either covalent or electrovalent bonds.

- Element R has an atomic number of 10, which corresponds to neon (Ne). Neon is also a noble gas and does not readily form compounds through either covalent or electrovalent bonds.

- Element S has an atomic number of 16, which corresponds to sulfur (S). Sulfur is a nonmetal and can form both covalent and electrovalent bonds depending on the elements it interacts with.



Among the given elements, only sulfur (S) with the atomic number 16 can exhibit both covalency and electrovalency. Sulfur can form covalent bonds with other nonmetals and electrovalent bonds with metals. Therefore, the correct answer is option 'A' - S.