## Chemistry (4th Edition)

$^{137}_{55}$Cs
We know that the mass number is the sum of the number of protons and neutrons in an atom. If we are given that the element we are working with has a mass number of 137 and that the number of neutrons in the atom's nucleus is 82, then we can subtract the number of neutrons from the mass number to get the number of protons, which, in this case, is 55. The number of protons in an atom is also its atomic number, which identifies that atom as a member of a certain element. In this case, the element with atomic number 55 is cesium (Cs). In a neutral atom, the number of protons and the number of electrons equal one another, but since this atom contains one fewer electron than a neutral atom, the atom is a cation with a single positive charge. We write the mass number as a superscript to the left of the chemical symbol and the atomic number as a subscript to the left of the chemical symbol. Therefore, the chemical symbol for this atom of cesium is $^{137}_{55}$Cs.