The atomic numbers of four elements P, Q, R and S are 6, 8, 10 and 12 respectively. Which two elements combine to form a covalent compound?

The atomic numbers of four elements P, Q, R, and S are 6, 8, 10, and 12 respectively. P & S are the two elements combined to form a covalent compound.

The covalent bond is formed by the mutual sharing of electrons between two atoms. 

The electronic configuration of elements of P is 2, 4. The element P will make a covalent bond with another element and needs four electrons to complete its octet. 

Q has an electronic configuration of 2, 8 means it has completed eight electrons in its outermost shell and does not form any bond. 

R is a metal that has 2 electrons in its last shell as its electronic configuration is 2, 8, 2, and can lose 2 of its electrons. 

S needs one electron to complete its octet and can either gain or share the electron with another element as its electronic configuration is 2, 8, 7. 

Therefore, P and two Q will combine with each other to form a covalent compound. 

P: Atomic number 6 → Element symbol: C (Carbon)
Q: Atomic number 8 → Element symbol: O (Oxygen)

The compound formed is ${\mathrm{CO}}_2$.