A certain element has only two naturally occurring isotopes: one with 18 neutrons and the other with 20 neutrons. The element forms \(1-\) charged ions when in ionic compounds. Predict the identity of the element. What number of electrons does the \(1-\) charged ion have?

Isotopes are atoms of the same element with the same number of protons, but different numbers of neutrons. They can be represented using isotopic notation, where the symbol X is used for the element, A for the mass number (number of protons + neutrons), and Z for the atomic number (number of protons): \[ _{Z}^{A}\textrm{X} \]

We know that the element has two naturally occurring isotopes with 18 and 20 neutrons. Since the element forms a \(1-\) charged ion in ionic compounds, this means it gains 1 electron when forming an ion. We will use the information about isotopes to predict the identity of the element. Elements in the same group on the periodic table tend to have similar chemical properties, including the formation of ions with the same charges. For isotopes, the atomic number stays the same, which means the number of protons is constant. To find the atomic number (number of protons), we will consider periodic trends. We know that elements in Group 17 (Halogens) tend to form ions with a charge of \(-1\) by gaining one electron. Based on this periodic trend, assume that our unknown element is likely a halogen and therefore has 7 valence electrons. When it gains 1 electron to form an ion, this would mean it has 8 valence electrons.

Using the information that our unknown element likely has 7 valence electrons and given the number of neutrons for both isotopes as 18 and 20, we can now calculate the atomic number (number of protons) and mass numbers. The number of protons equals the atomic number. Since we've assumed our element is a halogen, we'll use Z = 17. For the isotope with 18 neutrons: Atomic number (Z) = 17 (number of protons) Mass number (A) = number of protons (Z) + number of neutrons = 17 + 18 = 35 For the isotope with 20 neutrons: Atomic number (Z) = 17 (number of protons) Mass number (A) = number of protons (Z) + number of neutrons = 17 + 20 = 37 These isotopes would be represented as: \[ _{17}^{35}\textrm{X} \quad \textrm{and} \quad _{17}^{37}\textrm{X} \]

Using the atomic number of 17 (number of protons) and referencing the periodic table, we can identify the element as Chlorine (Cl). The two isotopes are: \[ _{17}^{35}\textrm{Cl} \quad \textrm{and} \quad _{17}^{37}\textrm{Cl} \]

When a neutral Chlorine atom gains 1 electron, it forms a \(1-\) charged ion. Since a neutral Chlorine atom has 17 electrons (equal to the atomic number), the \(1-\) charged ion will have 18 electrons (1 extra electron gained): Number of electrons in \(1-\) charged Chlorine ion = 17 + 1 = 18 In conclusion, the identity of the element is Chlorine (Cl) with atomic number 17. The \(1-\) charged ion formed has 18 electrons.