Problem 63 - General Chemistry: Principles and Modern Applications

Question

Draw plausible Lewis structures for (a) dimethylhydrazine, \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NNH}_{2}\) (b) nitryl chloride, \(\mathrm{ClNO}_{2}\) (Central atom is \(\mathrm{N}\) ) (c) phosphorous acid, a diprotic acid with the empirical formula \(\mathrm{H}_{3} \mathrm{PO}_{3}\).

Step-by-Step Solution

Answer

The Lewis structures for dimethylhydrazine, nitryl chloride, and phosphorous acid are obtained by placing the corresponding central atom surrounded by other atoms and distributing valence electrons among the atoms to fulfill the octet rule.

Step 1: Identify the Central Atom and Determine the Number of Valence Electrons

In \((\mathrm{CH}_{3})_{2} \mathrm{NNH}_{2}\), nitrogen is the central atom. Each Carbon atom has 4 valence electrons, each Hydrogen atom has 1 and each Nitrogen atom has 5. Hence, total valence electrons are \(2*4 + (4+4+3)*1 + 2*5 = 18\).\n\n In \(\mathrm{ClNO}_{2}\), Nitrogen is the central atom. Each Oxygen atom has 6, Chlorine atom has 7 and Nitrogen atom has 5. Hence, total valence electrons are \(2*6 + 7 + 5 = 24\).\n\n In \(\mathrm{H}_{3} \mathrm{PO}_{3}\), Phosphorus is the central atom. Each Hydrogen atom has 1, each Oxygen atom has 6 and the Phosphorus atom has 5. Hence, total valence electrons are \(3*1 + 3*6 + 5 = 24\).

Step 2: Draw the Lewis Structure

For \((\mathrm{CH}_{3})_{2} \mathrm{NNH}_{2}\), Place the Nitrogen atoms in the center surrounded by Carbon atoms and Hydrogen atoms. Distribute electrons so that each Carbon and Nitrogen atom gets 8 electrons and each Hydrogen atom gets 2 electrons.\n\n For \(\mathrm{ClNO}_{2}\), Place the Nitrogen atom at the center surrounded by one Chlorine atom and two Oxygen atoms. Allocate the electrons so each Oxygen and Chlorine atom gets 8 electrons and Nitrogen atom gets 8 electrons.\n\n For \(\mathrm{H}_{3} \mathrm{PO}_{3}\), place the Phosphorus atom in the center surrounded by three Hydrogen atoms and three Oxygen atoms. Distribute electrons so each Oxygen atom gets 8 electrons, Phosphorus atom gets 8 electrons and each Hydrogen atom gets 2 electrons.

Step 3: Check for stability

Check that all atoms in each molecule follow the octet rule (2 for Hydrogen atoms), except where exceptions occur. If all atoms in a molecule satisfy the octet rule, the Lewis structure is correctly drawn.

Key Concepts

Valence Electrons

Valence electrons are the electrons found in the outer shell of an atom. These are crucial in determining how an atom interacts with other atoms to form chemical bonds.

The importance of valence electrons lies in their ability to form bonds, as they are the ones involved in chemical reactions. In simpler terms, valence electrons can be thought of as the outermost 'layer' of an atom that can be "shared" or "exchanged" with other atoms to create molecules. When drawing Lewis structures, the first step is determining the number of valence electrons each atom contributes to the molecule:

• Carbon has 4 valence electrons
• Hydrogen has 1 valence electron
• Nitrogen has 5 valence electrons
• Oxygen has 6 valence electrons
• Chlorine has 7 valence electrons
• Phosphorus has 5 valence electrons

Knowing these numbers helps us count the total number of valence electrons, which assists in correctly arranging the electrons in a Lewis structure. For example, in dimethylhydrazine \((CH_{3})_{2}NNH_{2}\), and determining the valence electrons for each atom is essential to drawing the structure correctly.

Octet Rule

The octet rule is a fundamental concept in chemistry that describes how atoms strive to have eight electrons in their valence shell, mimicking the electron configuration of a noble gas. This rule is often used when drawing Lewis structures to ensure that the atoms have a stable configuration.

Atoms not naturally with a full octet often react with other atoms to achieve this. For instance, oxygen needs two more electrons to complete its octet, so it forms bonds where it can gain or share these electrons, such as in \(ClNO_2\). Each atom forms enough bonds to fill its valence shell to reach this configuration.The octet rule is the guiding principle during Step 2 in the solution, where electrons are distributed around the atoms:

• Carbon aims for 8 electrons in its valence shell.
• Nitrogen similarly aims for 8 due to its need for stability.
• Oxygen and chlorine strive to complete their respective octets as well.

Hydrogen is the exception since it only requires two electrons in its valence shell. It's important to note that some atoms can have fewer or more electrons than an octet, resulting in exceptions to the rule such as in molecules with odd numbers of total valence electrons.

Chemical Bonding

Chemical bonding refers to the force that holds atoms together in molecules and compounds. Understanding these bonds is essential for drawing accurate Lewis structures and predicting molecular behavior.

There are major types of chemical bonds: ionic, where electrons are transferred, and covalent, where electrons are shared. In the context of Lewis structures, we focus primarily on covalent bonding, where nonmetals share valence electrons to fulfill the octet rule. For example, in nitryl chloride \(ClNO_2\), nitrogen forms covalent bonds with oxygen and chlorine by "sharing" its valence electrons.Covalent bonds can be single, double, or even triple bonds:

• Single bonds share one electron pair between two atoms.
• Double bonds share two pairs of electrons, as seen commonly with oxygen molecules.
• Triple bonds share three pairs, though less common, especially in simple diatomic molecules.

The concept of chemical bonding helps to explain how molecules like dimethylhydrazine and phosphorous acid are structured and how their Lewis structures map onto their chemical and physical properties.