A Se ion has a mass number of 78 and a charge of -2. Determine the number of neutrons, protons, and electrons in this ion.

Answer 1

Hello Marline,

There are a few important concepts to remember that are important to solving this problem. These concepts include atomic number and mass number. The atomic number is the number of protons an element always has. An element must always have the same number of protons, however atoms of the same element can have different numbers of electrons (ions) or neutrons (isotopes). You can get the atomic number from the periodic table. Selenium has an atomic number of 34, so it has 34 protons. The mass number is the number of protons plus neutrons. So we would subtract the atomic number from the mass number here to get your number of neutrons. 78-34=44 neutrons.

To find the number of electrons in the ion you have to remember that the element would have the same number of electrons as it has protons in its original state. This is a -2 ion so it would have two extra electrons.

34 electrons + 2 electrons = 36 electrons. This ion has 36 electrons.

Answer 2

Hi, Marlene,

Although it might seem like you don't have enough information here, you actually do!! The mass number for an element is defined as follows: "The mass number is a count of the total number of protons and neutrons in an atom's nucleus." If you consult the Periodic Table, you'd find that selenium's (Se) atomic number is 34.

With this information, you can calculate the number of neutrons because the number of neutrons is equal to the atomic mass number minus the atomic number:

Se Atomic Mass Number (sum of protons and neutrons) = 78

Se Atomic Number (number of protons) = 34

Se Number of neutrons = 78-34 or 44

Now, if this was an uncharged Se atom, we would HAVE to have an equal number of protons (positively charged) and electrons (negatively charged). So, for uncharged Se it would be 44 neutrons, 34 protons, and 34 electrons ...

HOWEVER, the question is to determine the number of neutrons, protons, and electrons for a Se ion with a -2 charge!! That means this ion has 2 more electrons than it would normally as a neutral ion, so the answer would be:

Se^-2: 44 neutrons, 34 protons, and 36 electrons.

'Hope that helps,

Chris