Answer is: D.
Atomic number is the number of protons, which is characteristic of a chemical element: Boron is an element with atomic number 5, which means it has 5 protons and 5 electrons, bcause atom has neutral charge.
Electron configuration of boron: ₅B: 1s² 2s² 2p¹, it has three valence electrons in 2s and 2p orbitals.
