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Lewis Dot Structures: Polyatomic Ions

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  • 0:01 Lewis Dot Structure Definied
  • 1:14 Drawing Lewis Dot Structures
  • 4:06 Polyatomic Ions
  • 5:03 Lewis Dot Structures…
  • 8:45 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

This lesson defines Lewis dot structures and explains how to draw them for molecules in step-by-step detail. We'll also explore polyatomic ions and how to draw Lewis dot structures for them.

Lewis Dot Structures Defined

In science, we often use diagrams and shorthand notation to better understand a particular concept. One example of this is a Lewis dot structure. A Lewis dot structure is a simple diagram that shows the bonding between atoms and the arrangement of valence electrons around atoms within a molecule.

Atoms have electrons that can be divided into two categories: core and valence. Core electrons are not involved with bonding, so we get to ignore them completely when drawing or looking at Lewis dot structures. Our main concern is valence electrons, which are used in chemical bonding.

Let's take a look at an example of a Lewis dot structure:

Example Lewis dot structure
Lewis dot structure for O2

The structure incorporates three different things: an element symbol, dots, and lines. The nucleus of the atom is represented by the element symbol of the atom, and the valence electrons are represented by simple dots surrounding the atom. Lines are used to show the bonds. Two electrons can form a single bond, so we represent that with a single line. A double bond, which is made of four electrons, is represented by two lines. A triple bond is represented by three lines.

Drawing Lewis Dot Structures

Drawing Lewis dot structures can be challenging, but in a fun way! The main thing to keep in mind when drawing Lewis dot structures is the importance of the octet rule. The octet rule is a chemical rule that states all atoms want to have a stable, noble gas electron configuration consisting of eight valence electrons.

Atoms will share, lose, or gain electrons to achieve the octet rule. One of the major exceptions to this rule, though, is hydrogen, who is happy to have only two valence electrons. An easy way to remember the octet rule is to remember that the prefix 'oct-' refers to the number eight.

Let's draw a Lewis dot structure for a molecule to see it in action. I will use water (H2 O) as an example:

Step 1: Sum the number of valence electrons from each atom in the compound. You can find the number of valence electrons by looking at the column number that the atom lives in. Hydrogen is in column 1A (or column 1), so hydrogen has one valence electron. Since we have two hydrogen atoms in water, we need to multiply the number of valence electrons by two. Oxygen lives in column 6A (or column 16), so it has six valence electrons. We add oxygen's six electrons to the two valence electrons brought in by hydrogen for a total of eight electrons.

Step 2: Make a skeleton structure for your molecule by connecting atoms with single bonds. When doing this keep a couple things in mind: the central atom is usually the atom you have the least of. Hydrogens can never be bonded to more than one atom, so they will always be on the outside of the structure. In the case with water, I only have one oxygen, so that goes in the middle of the two hydrogens. I connect the hydrogens to the oxygen with single bonds.

Step 3: Determine how many electrons remain after making the skeleton structure. I used four electrons to make my skeleton structure, so four electrons remain.

Step 4: Place remaining electrons to satisfy the octet rule for atoms. Remember that hydrogen only needs two electrons to be satisfied. Because my hydrogens are satisfied, my only option is to place the remaining four electrons on oxygen. This works out perfectly because oxygen now has eight valence electrons: four involved in bonds and four not bonding.

Step 5: If there aren't enough electrons to satisfy the octet rule for all atoms, try doubling or tripling bonds. I have enough electrons for oxygen and the hydrogens, so I get to skip this step.

Step 6: Make sure all atoms are satisfied. If you want, draw a circle around each atom that has achieved the octet rule.

Polyatomic Ions

Now for some new information. Polyatomic ions may have a threatening name, but they are basically just a molecule with a charge. By definition, a polyatomic ion is an ion containing multiple atoms. If we dissect the term 'polyatomic ion,' we find that the term 'poly' means 'many,' 'atomic' means 'atom,' and 'ion' means 'charged particle.' Polyatomic ions form when a molecule needs to gain or lose electrons, so that all atoms inside the molecule can achieve the octet rule. In nature, polyatomic ions are incredibly common.

An example of a simple polyatomic ion is hydroxide (OH-). The Lewis dot structure follows the same principles we discussed before, but with a couple additions. The entire structure is bracketed, and the charge of the ion is included in the top right corner on the outside of the brackets.

Lewis Dot Structures for Polyatomic Ions

Drawing Lewis dot structures for polyatomic ions is essentially the same as Lewis dot structures for molecules. The only differences are that 1) you must add or subtract electrons from the valence electron total based on the charge of the ion, and 2) you must bracket your finished product and place the charge outside the brackets.

Let's draw the Lewis dot structure for hydroxide (OH-):

Step 1: Sum the number of valence electrons from each atom in the compound. Also, add in one extra electron because hydroxide has a one negative charge. We have six electrons from oxygen: one from hydrogen and one extra because this compound has gained an electron. I have a total of eight electrons.

Step 2: Make a skeleton structure for your molecule by connecting atoms with single bonds. I only have two atoms here, so I only have one bonding option! I connect oxygen to hydrogen with a single bond.

Step 3: Determine how many electrons remain after making the skeleton structure. I used two electrons to make my skeleton structure, so six electrons remain.

Step 4: Place remaining electrons to satisfy the octet rule for atoms. Hydrogen is satisfied, so I place the remaining electrons around oxygen.

Step 5: If there aren't enough electrons to satisfy the octet rule for all atoms, try doubling or tripling bonds. There are enough electrons, so I can skip this step.

Step 6: Make sure all atoms are satisfied.

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