A laboratory sample contains ions of an element represented by X\mathrm{X}X. Each ion was formed when a neutral atom of the element lost one outer electron. Each ion contains 18 electrons, and its nucleus contains 20 neutrons.
Which nuclear symbol represents the neutral atom before it lost the electron?
1920X{}^{20}_{19}\mathrm{X}1920X
1939X{}^{39}_{19}\mathrm{X}1939X
1820X{}^{20}_{18}\mathrm{X}1820X
1839X{}^{39}_{18}\mathrm{X}1839X