Q1-1-2P - Organic Chemistry (Mcmurry)

Question

How many electrons does each of the following elements have in its outermost electron shell?

a) Magnesium (b) Cobalt (c) Selenium

Step-by-Step Solution

Answer

a) There are two electrons in the outermost electron shell of magnesium.

             Energy diagram with filled electrons

b) There are two electrons in the outermost electron shell of cobalt.

               Energy diagram with filled electrons

c) There are six electrons in the outermost electron shell of selenium.

                  Energy diagram with filled electrons

Arrangement of electrons

To know the number of electrons in the outermost shell, we need to put the number of electrons a magnesium atom has into the appropriate shells. The highest filled energy shell will be the outermost electron shell and then we need to count the number of electrons in that shell. Magnesium is in period three, so we can only consider shells up to 3p. Therefore, the shell energy diagram will be represented as follows:

                           Energy diagram for magnesium

Energy diagram for selenium

a) Magnesium

Magnesium’s atomic number is 12, so for a neutral atom, there are 12 protons and 12 electrons. Using the Aufbau, Hund, and Pauli principles we should fill in the electrons. As per the energy level diagram the 3s shell is the highest filled shell and that it contains two electrons.

Therefore, there are two electrons in the outermost shell of magnesium.

                         Energy diagram with filled electrons

b) Cobalt

Cobalt’s atomic number is 27, so for a neutral atom there are 27 protons and 27 electrons. Using the Aufbau, Hund, and Pauli principles we should fill the electrons in a subshell. The only minor exception is that the 4s orbital is filled first before the 3d orbital.

As per the energy level diagram 4s shell is the highest filled shell and that it contains two electrons. Thus, there are two electrons in the outermost electron shell of cobalt.

                           Energy diagram with filled electrons

c) Selenium

Selenium’s atomic number is 34, so for a neutral atom there are 34 protons and 34 electrons. Using the Aufbau, Hund, and Pauli principles, we should fill in the electrons in the subshell.

As per the energy level diagram, the orbitals 4s and 4p shells is the highest filled shell, and they contain six electrons. Therefore, there are six electrons in the outermost electron shell of selenium.

                       Energy diagram with filled electrons