Question

Each compound contains both ions and covalent bonds. Draw the Lewis structure for each compound. Show with dashes which are covalent bonds and show with charges which are ions. (a) Sodium methoxide, \(\mathrm{CH}_{3} \mathrm{ONa}\) (b) Ammonium chloride, \(\mathrm{NH}_{4} \mathrm{C}\) (c) Sodium bicarbonate, \(\mathrm{NaHCO}_{3}\) (d) Sodium borohydride, \(\mathrm{NaBH}_{4}\) (e) Lithium aluminum hydride, \(\mathrm{LiAlH}_{4}\)

Short answer

Question: Draw the Lewis structures for the following compounds, showing covalent bonds as dashes and ions with charges: (a) Sodium methoxide, (b) Ammonium chloride, (c) Sodium bicarbonate, (d) Sodium borohydride, and (e) Lithium aluminum hydride. Answer: (a) Sodium methoxide: H | H - C - O (-) - Na (+) | H (b) Ammonium chloride: (NH4)+ (+) - Cl(-) (c) Sodium bicarbonate: HO - (-) - Na (+) \ C / \\ O O (d) Sodium borohydride: (–) - B - H | H - (–) - Na (+) H (e) Lithium aluminum hydride: (–) - Al - H | H - (–) - Li (+) H

Step-by-step solution

(a) Sodium Methoxide: CH3ONa

To draw the Lewis structure for sodium methoxide, first identify the atoms in the compound: Carbon (C), Hydrogen (H), Oxygen (O), and Sodium (Na). Carbon has 4 valence electrons, Hydrogen has 1 valence electron, Oxygen has 6 valence electrons, and Sodium has 1 valence electron. We first draw the Lewis structure for methoxide (CH3O-), where Carbon is connected to three Hydrogens and one Oxygen. Carbon forms three single covalent bonds with three Hydrogens. Between Carbon and Oxygen, there is a single covalent bond. Oxygen, being more electronegative, has a negative charge. Sodium with its one valence electron donates it to the methoxide ion, and thus form an ionic bond. The final structure will be: H | H - C - O (-) - Na (+) | H

(b) Ammonium Chloride: NH4Cl

To draw the Lewis structure for ammonium chloride, first identify the atoms in the compound: Nitrogen (N), Hydrogen (H), and Chlorine (Cl). Nitrogen has 5 valence electrons, Hydrogen has 1 valence electron, and Chlorine has 7 valence electrons. We first draw the Lewis structure for ammonium ion (NH4+), where Nitrogen is connected to four Hydrogens. Nitrogen forms four single covalent bonds with four Hydrogens and has a positive charge. Chlorine needs one electron to complete its octet and thus gain a negative charge. The ionic bond forms between the ammonium ion (NH4+) and chloride ion (Cl-). (NH4)+ (+) - Cl(-)

(c) Sodium Bicarbonate: NaHCO3

To draw the Lewis structure for sodium bicarbonate, first identify the atoms in the compound: Sodium (Na), Hydrogen (H), Carbon (C), and Oxygen (O). Sodium donates its one valence electron to the bicarbonate ion (HCO3-). Bicarbonate ion consists of a central carbon atom connected to three oxygen atoms and one hydrogen atom. One of the oxygen atoms forms a single covalent bond with hydrogen, and the other two oxygen atoms are double bonded to carbon. HO - (-) - Na (+) \ C / \\ O O

(d) Sodium Borohydride: NaBH4

To draw the Lewis structure for sodium borohydride, first identify the atoms in the compound: Sodium (Na), Boron (B), and Hydrogen (H). Sodium donates its one valence electron to the borohydride ion (BH4-). Boron is central and has 3 valence electrons, which are used to form single covalent bonds with three Hydrogens. An additional hydrogen is also bonded to boron forming a total of 4 single covalent bonds. The boron atom has a negative charge. (-) - B - H | H - (-) - Na (+) H

(e) Lithium Aluminum Hydride: LiAlH4

To draw the Lewis structure for lithium aluminum hydride, first identify the atoms in the compound: Lithium (Li), Aluminum (Al), and Hydrogen (H). In this compound, lithium donates its one valence electron to the aluminum hydride ion (AlH4-). Aluminum is central and has 3 valence electrons, which are used to form single covalent bonds with three Hydrogens. An additional hydrogen is also bonded to aluminum forming a total of 4 single covalent bonds. The aluminum atom has a negative charge. (-) - Al - H | H - (-) - Li (+) H

Key concepts

Ionic and Covalent Bonds

Understanding the distinction between ionic and covalent bonds is crucial for grasping how elements combine to form compounds. Ionic bonds occur when electrons are transferred from one atom to another, resulting in the formation of ions. These ions, carrying opposite charges, attract each other to maintain electrical neutrality. A classic example is table salt (NaCl), where sodium (Na) donates an electron to chlorine (Cl), creating positively charged sodium ions (Na⁺) and negatively charged chloride ions (Cl^-).

In contrast, covalent bonds are formed when two atoms share one or more pairs of valence electrons, creating a stable balance of attractive and repulsive forces between atoms. For instance, in a water molecule (H₂O), each hydrogen atom shares an electron with oxygen, resulting in covalent bonds that hold the atoms together.

Many compounds, like sodium methoxide (CH₃ONa), exhibit both ionic and covalent bonding characteristics. The carbon (C) bonds covalently with oxygen (O) and hydrogen (H), while sodium (Na) forms an ionic bond with the negatively charged oxygen.

Valence Electrons

The outermost electrons of an atom are known as valence electrons, and they play a pivotal role in chemical bonding. Valence electrons are responsible for the reactivity of an element and determine the type of bond that will form during a chemical reaction. For example, in sodium methoxide, the sodium atom has only one valence electron, which it readily donates, becoming a positively charged ion.

On the other hand, carbon, with four valence electrons, seeks to achieve a stable configuration by sharing its electrons with hydrogen and oxygen atoms, exhibiting covalent bonding. In drawing Lewis structures, as seen in the exercises for compounds like ammonium chloride (NH₄Cl) and sodium bicarbonate (NaHCO₃), the valence electrons are represented as dots around the chemical symbols, with dashes depicting the shared electron pairs in covalent bonds.

Molecular Geometry

The spatial arrangement of atoms within a molecule, known as molecular geometry, is influenced by the number of electron pairs, both bonding and non-bonding, that surround its central atom. This distribution of electron pairs determines the molecule's shape, which can affect properties like polarity, reactivity, and biological activity.

For example, in the Lewis structure of sodium bicarbonate, the carbon's electron pairs are tetrahedrally arranged, leading to a planar configuration around the carbon atom. In sodium borohydride (NaBH₄), boron's electron pairs form a tetrahedral shape, which is a common molecular geometry for compounds with four single covalent bonds. Understanding molecular geometry helps in predicting how molecules interact with each other, which is an important aspect of both organic and inorganic chemistry.