## Chemistry: The Science in Context (4th Edition)

(a) To find out the number of electrons in this ion of oxygen (O), we first look for oxygen on the periodic table. Oxygen has an atomic number of 8, so it has 8 protons. In a neutral atom, the number of protons and electrons are equal. Since this ion of oxygen has two extra negative charges (or two extra electrons), it actually has 10 electrons instead of the normal 8. (b) To find out the number of electrons in this ion of sulfur (S), we first look for sulfur on the periodic table. Sulfur has an atomic number of 16, so it has 16 protons. In a neutral atom, the number of protons and electrons are equal. Since this ion of sulfur has two extra negative charges (or two extra electrons), it actually has 18 electrons instead of the normal 16. (c) To find out the number of electrons in this ion of selenium (Se), we first look for selenium on the periodic table. Selenium has an atomic number of 34, so it has 34 protons. In a neutral atom, the number of protons and electrons are equal. Since this ion of selenium has two extra negative charges (or two extra electrons), it actually has 36 electrons instead of the normal 34. (d) To find out the number of electrons in this ion of tellurium (Te), we first look for tellurium on the periodic table. Tellurium has an atomic number of 52, so it has 52 protons. In a neutral atom, the number of protons and electrons are equal. Since this ion of tellurium has two extra negative charges (or two extra electrons), it actually has 54 electrons instead of the normal 52. Option (b) is the correct answer because S$^{2-}$ has 18 electrons.