Question

Draw Lewis structures for the following: (a) \(\mathrm{SiH}_{4}\), (b) \(\mathrm{CO}\), (c) \(\mathrm{SF}_{2}\), (d) \(\mathrm{H}_{2} \mathrm{SO}_{4}\) ( \(\mathrm{H}\) is bonded to \(\mathrm{O}\) ), (e) \(\mathrm{ClO}_{2}^{-}\), (f) \(\mathrm{NH}_{2} \mathrm{OH}\).

Short answer

The Lewis structures for the given compounds are as follows: (a) SiH4: Si surrounded by 4 H atoms, each connected to Si with a single bond. (b) CO: C and O connected with triple bond, and O has one lone pair. (c) SF2: S surrounded by 2 F atoms, each connected to S with a single bond, and each F has 3 lone pairs, S with 2 lone pairs. (d) H2SO4: Central S connected to two O with double bonds, and to two other O with single bonds, each of these two O connected to H with single bonds, and each O has two lone pairs. (e) ClO2-: Cl surrounded by 2 O atoms, each connected to Cl with a single bond, each O has 3 lone pairs, and Cl has 2 lone pairs and one extra electron from the -1 charge. (f) NH2OH: N connected to O with single bond, two H atoms connected to N and one H atom connected to O with single bonds, N with one lone pair and O with two lone pairs.

Step-by-step solution

(a) Drawing the Lewis structure for SiH4

First, we will identify the number of valence electrons for each atom in the compound. Silicon (Si) has 4 valence electrons, and hydrogen (H) has 1 valence electron. Since there are 4 hydrogen atoms, the total number of valence electrons is 4 + (4 × 1) = 8. 1. Place the Silicon (Si) atom at the center and distribute the 4 Hydrogen atoms around it. 2. Connect the Hydrogen atoms to the Silicon atom with single bonds. This uses up 4 valence electrons from Si and 1 valence electron from each H atom. 3. Check the octet rule: Si has a full octet of 8 electrons, and each H atom has 2 electrons. The Lewis structure for SiH4 is complete.

(b) Drawing the Lewis structure for CO

Carbon (C) has 4 valence electrons, and oxygen (O) has 6 valence electrons. The total number of valence electrons is 4 + 6 = 10. 1. Place the Carbon (C) atom adjacent to the Oxygen (O) atom. 2. Connect the C and O atoms with a single bond. This uses up 2 valence electrons. 3. Distribute the remaining electrons as lone pairs: 2 lone pairs (4 electrons) to O and 2 lone pairs (4 electrons) to C. 4. Check the octet rule: C has only 6 electrons and O has 8 electrons. To complete the octet for C, change the single bond between C and O to a triple bond. The Lewis structure for CO is complete.

(c) Drawing the Lewis structure for SF2

Sulfur (S) has 6 valence electrons, and fluorine (F) has 7 valence electrons. Since there are 2 fluorine atoms, the total number of valence electrons is 6 + (2 × 7) = 20. 1. Place the Sulfur (S) atom at the center and distribute the 2 Fluorine atoms around it. 2. Connect each F atom to the S atom with a single bond. This uses up 2 valence electrons from S and 1 valence electron from each F atom. 3. Distribute the remaining electrons as lone pairs: 3 lone pairs (6 electrons) to each F atom and 2 lone pairs (4 electrons) to S. 4. Check the octet rule: S and both F atoms have full octets with 8 electrons each. The Lewis structure for SF2 is complete.

(d) Drawing the Lewis structure for H2SO4

Sulfur (S) has 6 valence electrons, oxygen (O) has 6 valence electrons, and hydrogen (H) has 1 valence electron. The total number of valence electrons is 6 + (4 × 6) + (2 × 1) = 32. 1. Place the Sulfur (S) atom at the center, distribute the 4 Oxygen atoms around it, and add 2 Hydrogen atoms to 2 of the Oxygen atoms. 2. Connect each O atom to the S atom and each H atom to the O atom with single bonds. This uses up 6 valence electrons from S, 1 valence electron from each H atom, and 1 valence electron from each O atom. 3. Distribute the remaining electrons as lone pairs: 3 lone pairs (6 electrons) to each O atom connected to a H atom, and 2 lone pairs (4 electrons) to the other two O atoms. 4. Finally, add a double bond between the S and the O atoms that are not connected to H atoms to satisfy the octet rule for S. The Lewis structure for H2SO4 is complete.

(e) Drawing the Lewis structure for ClO2-

Chlorine (Cl) has 7 valence electrons, oxygen (O) has 6 valence electrons, and the -1 charge adds 1 electron. The total number of valence electrons is 7 + (2 × 6) + 1 = 20. 1. Place the Chlorine (Cl) atom at the center and distribute the 2 Oxygen atoms around it. 2. Connect each O atom to the Cl atom with single bonds. This uses up 2 valence electrons from Cl and 1 valence electron from each O atom. 3. Distribute the remaining electrons as lone pairs: 3 lone pairs (6 electrons) to each O atom and 2 lone pairs (4 electrons) to the Cl atom. 4. Check the octet rule: O atoms have full octets with 8 electrons each, but Cl has 10 electrons due to the extra electron from the -1 charge. The Lewis structure for ClO2- is complete.

(f) Drawing the Lewis structure for NH2OH

Nitrogen (N) has 5 valence electrons, oxygen (O) has 6 valence electrons, and hydrogen (H) has 1 valence electron. The total number of valence electrons is 5 + 6 + (3 × 1) = 14. 1. Place the Nitrogen (N) atom adjacent to the Oxygen (O) atom, and add 2 Hydrogen atoms to N and 1 Hydrogen atom to O. 2. Connect each H atom to N or O with single bonds, and connect N to O with a single bond. This uses up 1 valence electron from each H atom, 3 valence electrons from N, and 2 valence electrons from O. 3. Distribute the remaining electrons as lone pairs: 1 lone pair (2 electrons) on N and 2 lone pairs (4 electrons) on O. 4. Check the octet rule: N has 8 electrons, O has 8 electrons, and each H atom has 2 electrons. The Lewis structure for NH2OH is complete.

Key concepts

Valence Electrons

Valence electrons are the electrons located in the outermost shell of an atom. These electrons play a crucial role in bond formation and the chemical reactivity of an atom. In the process of drawing Lewis structures, the number of valence electrons for each atom must be identified to determine how these atoms will bond together.

For instance, silicon (Si) in the SiH₄ molecule has four valence electrons, and hydrogen (H) has one. By knowing these numbers, we can predict the types of bonds that will form. Each H atom shares one electron with Si, satisfying the H's need for a full shell, in this case, a duet, and contributing towards Si's full octet. It's important to remember that elements in the same group of the periodic table have the same number of valence electrons, which is essential in predicting molecular structures.

When improving exercise explanations, it's beneficial to reinforce the idea that valence electrons are the foundation for understanding Lewis structures. A handy tip for students is to recall that the group number for main-group elements (excluding transition metals) often indicates the number of valence electrons, making it easier to start any Lewis structure problem.

Octet Rule

The octet rule is a fundamental concept that states atoms tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. This is a guiding principle for determining the structure of molecules. There are exceptions to the rule, particularly for elements that can have more or fewer than eight electrons.

In the context of Lewis structures, the octet rule helps us decide how atoms within a molecule share electrons. Hydrogen is an exception, as it is content with two electrons (a duet), which is the case in compounds like SiH₄. However, in a molecule like CO, the carbon atom initially lacks a full octet, prompting the formation of a triple bond with oxygen to satisfy the rule for both atoms. When explaining this concept, emphasize that while the octet rule is a helpful guideline, it's also important to acknowledge its limitations, such as with molecules exhibiting expanded octets like SF₂ or those with odd-electron molecules.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. The shape of a molecule is influenced by the number of electron pairs, both bonding and lone pairs, surrounding its central atom. This arrangement is crucial in understanding the molecule's reactivity, polarity, and physical properties.

VSEPR theory (Valence Shell Electron Pair Repulsion) is employed to predict molecular geometry by assuming that electron pairs repel each other and will, therefore, arrange themselves as far apart as possible. For example, the methane molecule (CH₄) has a tetrahedral shape as the four hydrogen atoms spread out to minimize repulsion. In contrast, the bent shape of water (H₂O) arises because the oxygen atom has two lone pairs of electrons, in addition to the two bonding pairs.

When clarifying the concept of molecular geometry, visualize common geometries like linear, bent, tetrahedral, trigonal planar, and trigonal bipyramidal. Offering real-life examples, such as water's bent shape leading to its unique properties, can make the concept more relatable. Also, remind them that lone pairs occupy more space than bonding pairs and that molecular geometry is determined not just by the number of bonding pairs but by the total number of electron pairs (bonding and lone pairs) around the central atom.