Question

Draw Lewis structures for the following: (a) \(\mathrm{SiH}_{4}\), (b) \(\mathrm{CO},(\mathrm{c}) \mathrm{SF}_{2}\) (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 short answer with the Lewis structures for the given molecules is: (a) SiH4: \( H \) \( | H--Si--H | H \) (b) CO: \( O \equiv C: \) (The two lone pairs on O are not shown) (c) SF2: \( F | (LP)--> S -- F (LP) | (LP) \) (LP represents lone pairs) (d) H2SO4: \( O || H -- O -- S -- O | H \) (e) ClO2-: \( - O -- Cl -- O \) (Lone pair on Cl is not shown) (f) NH2OH: \( O | N -- H (LP)| H \)

Step-by-step solution

(a) SiH4

1. Count the total number of valence electrons: Silicon has 4 valence electrons and hydrogen has 1. There are 4 hydrogen atoms, so the total number of valence electrons is 4 + (4 × 1) = 8. 2. Place the Si atom in the center and draw single bonds with each hydrogen atom, which fulfills the octet rule for Si and the duet rule for each H atom. Lewis structure for SiH4 is: H | H--Si--H | H

(b) CO

1. Count the total number of valence electrons: Carbon has 4 valence electrons and oxygen has 6. The total number of valence electrons is 4 + 6 = 10. 2. Place the C atom beside the O atom. Start with a single bond and complete the octet rule for O using the remaining valence electrons. 3. Apply the octet rule for C by forming a triple bond with O. The Lewis structure for CO is: O ≡ C: (The two lone pairs on O are not shown)

(c) SF2

1. Count the total number of valence electrons: Sulfur has 6 valence electrons and fluorine has 7. There are 2 fluorine atoms, so the total number of valence electrons is 6 + (2 × 7) = 20. 2. Place the S atom in the center and make single bonds with each fluorine atom. 3. Complete the octet rule for each F and S using the remaining valence electrons. The Lewis structure for SF2 is: F | (LP)--> S -- F (LP) | (LP) (LP represents lone pairs)

(d) H2SO4

1. Count the total number of valence electrons: S has 6 valence electrons, oxygen has 6, and hydrogen has 1. There are 4 oxygen atoms and 2 hydrogen atoms, so the total number of valence electrons is 6 + (6 × 4) + (2 × 1) = 32. 2. Place S in the center. Add single bonds with two oxygen atoms, and double bonds with the other two oxygen atoms. 3. Add hydrogen atoms to the oxygen atoms with single bonds and complete the octet rule for all atoms. The Lewis structure for H2SO4 is: O || H -- O -- S -- O | H

(e) ClO2-

1. Count the total number of valence electrons: Cl has 7 valence electrons, oxygen has 6, and the overall ion has a charge of -1. There are 2 oxygen atoms, so the total number of valence electrons is 7 + (2 × 6) + 1 = 20. 2. Place Cl in the center and form single bonds with the 2 oxygen atoms. 3. Complete the octet rule for each O atom. Add the remaining valence electron as a lone pair to Cl, which will have 8 electrons total (1 lone pair, 2 single bonds). The Lewis structure for ClO2- is: - O -- Cl -- O (Lone pair on Cl is not shown)

(f) NH2OH

1. Count the total number of valence electrons: N has 5 valence electrons, oxygen has 6, and hydrogen has 1. There are 3 hydrogen atoms, so the total number of valence electrons is 5 + 6 + (3 × 1) = 14. 2. Place N in the center and form single bonds with both H atoms and the O atom. 3. Complete the octet rule for O and add a hydrogen atom to it. The N atom will not follow the octet rule, only having 7 electrons (2 single bonds and 1 lone pair). The Lewis structure for NH2OH is: O | N -- H (LP)| H

Key concepts

Valence Electrons

Valence electrons are the electrons in the outermost shell of an atom that are available for bonding with other atoms. These electrons play a crucial role in the formation of chemical bonds, as they are the ones involved in interactions that lead to the formation of molecules. For instance, in the Lewis structure exercise, we calculated the total valence electrons for different molecules before drawing the structures. Silicon in SiH₄, for example, has 4 valence electrons, which is evident from its group number in the periodic table.

To determine the valence electrons for a given atom, one can look up its position in the periodic table. Elements in the same group (vertical column) typically have the same number of valence electrons. For example, all Group 1 elements have 1 valence electron, Group 2 elements have 2, and so on, with the transition metals being more complex.

Knowledge of valence electrons is essential when predicting how atoms will bond together. When drawing Lewis structures, the first step is always to count the total number of valence electrons available to participate in bonding. This count guides the arrangement of electrons in the Lewis structure to ensure that each atom achieves a stable electronic configuration.

Octet Rule

The octet rule is a fundamental concept in chemistry that explains the stability of atoms when they have eight electrons in their valence shell. Atoms strive to reach an octet of electrons to attain the same electronic configuration as the nearest noble gas, leading to chemical stability.

In the Lewis structure drawings, we used the octet rule to distribute electrons among the atoms. For example, carbon in CO needs four more electrons to fulfill its octet, which it achieves by forming a triple bond with oxygen, which also needs two more electrons. Likewise, other structures like SF₂ and ClO₂^- are drawn ensuring that each atom (except hydrogen) follows the octet rule where possible.

It should be noted that there are exceptions to the octet rule. Some elements can have fewer than eight electrons, like hydrogen, which follows the 'duet rule', and others can expand their octet if they have d-orbitals, such as sulfur. In NH₂OH, nitrogen has only seven valence electrons and does not follow the octet rule, which can happen in some compounds.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It influences the physical and chemical properties of compounds, such as boiling and melting points, reactivity, and polarity. In Lewis structures, after placing the atoms and distributing the electrons, we can often predict the molecular geometry using VSEPR (Valence Shell Electron Pair Repulsion) theory.

For instance, SiH₄ has a tetrahedral geometry as the silicon atom forms four equivalent bonds with hydrogen atoms spread out to minimize repulsion. In CO, due to the triple bond, the molecule is linear with oxygen and carbon atoms at opposite ends. SF₂ has a bent geometry because of the lone pairs on the sulfur that push the fluorine atoms closer together.

While Lewis structures provide a two-dimensional representation, understanding the three-dimensional shape is crucial for comprehending concepts like bond angles and molecular polarity. For example, the bent shape of water (H₂O) makes it a polar molecule, which is crucial for many of its properties, including its solvent capabilities. The knowledge of molecular geometry is vital in fields such as drug design, where the shape of molecules directly affects their interaction with biological targets.