The atomic number of this element is 15, so its electronic configuration will be `underset("2, 8, 5")("K L M")`. It has 5 electrons in its outermost shell, so it needs 3 more electrons to complete the eight electron, inert gas configuration. Since one atom of this element needs 3 electrons to achieve the inert gas electron configuration, therefore, its valency is 3. (The element having atomic number 15 is actually ? phosphorus).
