Video Transcript
Which electron dot diagram shows the correct structure for sodium oxide?
The question tells us we are working with electron dot diagrams, which can also be referred to as Lewis dot diagrams, which are simple schematic illustrations that show how valence shell electrons are shared or exchanged between atoms in covalently or ionically bonded compounds. The compound in question is sodium oxide. Using its name, we can locate the elements of this compound on the periodic table and we can determine the type of bonding and ratio of each type of atom present.
Sodium is atomic number 11, and it is a metal, as it lies to the left of the dotted zigzag line. Additionally, sodium is found in group one, which means it has one valence electron. To illustrate this using an electron dot diagram, we would write the chemical symbol for sodium Na. And using dots surrounding the chemical symbol, we would draw the number of valence electrons. This is the electron dot diagram for a neutral sodium atom.
We can determine the second element in this compound using the second part of the compound name, oxide. The ending has been changed to “ide.” However, we can identify the element using the root “ox.” This tells us that oxygen, atomic number eight, is the second element in the compound, which is a nonmetal, as it lies to the right of the dotted zigzag line, and can be found in group 16. Therefore, it has six valence electrons. We can draw the electron dot diagram using the chemical symbol for oxygen, O, and by drawing six dots representing its six valence electrons. This is the electron dot diagram for a neutral oxygen atom.
To determine the correct electron dot diagram for the compound sodium oxide, we must determine the type of bonding taking place between sodium and oxygen. Since sodium is a metal and oxygen is a nonmetal, we can expect that they will exchange electrons, forming ions and, thus, forming ionic bonds. We can next determine the ratio of sodium to oxygen in the compound by examining the number of electrons exchanged when sodium and oxygen form ionic bonds. We can do this by using a rule of thumb called the octet rule, which tells us that generally an atom will tend to react to achieve eight electrons in its outer shell.
Let’s first look at sodium. The octet rule of thumb allows us to predict that sodium when reacting will likely lose its single valence electron, becoming a positively charged ion, which we can express using square brackets, with the charge indicated on the outside of the brackets. Don’t worry! We will come back to this lost electron in a moment. We can also show this on the electron dot diagram by removing the dot representing the valence electron and indicating that sodium has become a positively charged ion by using square brackets, with the charge indicated on the outside of the brackets, showing that sodium has become an ion with a one plus charge.
With this information, we can eliminate answer choice (B), which shows sodium taking on a negative charge. This would not occur for sodium when becoming an ion.
Let’s now look at oxygen. With only six valence electrons, oxygen will likely gain two electrons when becoming an ion. In sodium oxide, oxygen will gain the valence electrons lost from sodium. However, one sodium atom can only donate one valence electron. So another sodium atom in an effort to become a one plus ion will lose its single valence electron, which will fill the valence shell of the oxygen atom, forming a second sodium one plus ion and an oxide ion with a charge of two minus. This means that in the compound sodium oxide, there are two positive sodium ions formed for every one oxide ion. We can represent this ratio in our electron dot diagram. We can show oxygen with eight electrons in its outer shell having gained two electrons from two sodium atoms, forming two sodium ions with one plus charges and one oxide ion with a two minus charge.
Let’s look at our remaining answer choices. We can eliminate answer choices (C) and (D), which do not show sodium and oxygen as charged ions. We can also eliminate answer choice (E), which does show sodium and oxygen with their correct charges when forming ions but not in the correct ratio we would expect for the compound sodium oxide. Answer choice (A) is the only electron dot diagram given which shows the correct structure for sodium oxide. Therefore, the correct answer to this question is (A).