Answer:
Oxygen
Explanation:
The electrons present in an atom are arranged in shells or orbitals. The electrons are filled into these shells based on their energy levels with those shells closest to the nucleus filled first as they have the lowest energies.
The arrangement of electrons around the nucleus of an atom is known as electronic configuration. The outermost electron shell of an atom is known as a valence shell shell and the electrons found in this shell are known as valence electrons. The electrons usually exist in pairs. The first shell can only contain two electrons; the second shell can contain 8. the electrons are singly filled first before pairing occurs.
The electronic configuration of the atoms of the elements in the option are as follows:
Helium : 1s² ; valence electrons are 2 which are both paired
Hydrogen: 1s¹ ; valence electron is 1
Oxygen : 1s²2s²2p⁴ ; valence electrons are 6 wit two unpaired electrons
Lithium : 1s²2s¹ ; valence electron is 1
Helium has two valence electrons but they are paired as its 1s shell can only contain a maximum of two electrons.
Oxygen has 6 valence electrons. Two of these electrons are paired <em>while the other two are unpaired.</em>
