# Atomic structure - 2 | The Atomic Theory

Determine the number of protons, neutrons and electrons of the following elements:

${}_{34}{}^{78}\mathrm{Se}$ , ${}_{8}{}^{17}\mathrm{O}^{+}$ , ${}_{26}{}^{55}\mathrm{Fe}^{2+}$ , ${}_{17}{}^{35}\mathrm{Cl}^{-}$

${}_{34}{}^{78}\mathrm{Se}$ ⇒ 34 protons, 44 neutrons, 34 electrons

${}_{8}{}^{17}\mathrm{O}^{+}$ ⇒ 8 protons, 9 neutrons, 7 electrons

${}_{26}{}^{55}\mathrm{Fe}^{2+}$ ⇒ 26 protons, 55 neutrons, 24 electrons

${}_{17}{}^{35}\mathrm{Cl}^{-}$ ⇒ 17 protons, 18 neutrons, 18 electrons