Question

Draw Lewis structures for the following: (a) \(\mathrm{NCl}_{3}\) (b) \(\mathrm{H}_{2} \mathrm{CO}_{3}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{6}\) (d) \(\mathrm{NaNO}_{3}\)

Short answer

The Lewis structures for the given compounds are: (a) NCl3 with a nitrogen atom surrounded by three chlorine atoms, each with three lone pairs of electrons; (b) H2CO3 with a central carbon atom double-bonded to two oxygens and single-bonded to an oxygen, which is single-bonded to two hydrogens; (c) C2H6 with two carbon atoms single-bonded to each other, each surrounded by three hydrogen atoms; (d) NaNO3 with sodium as a cation and the NO3- anion having a nitrogen atom single-bonded to two oxygen atoms, and double-bonded to one oxygen atom with a -1 charge.

Step-by-step solution

Determine the Total Number of Valence Electrons for NCl3

For NCl3, nitrogen (N) has 5 valence electrons and each chlorine (Cl) has 7 valence electrons. Since there are three Cl atoms, the total number of valence electrons is 5 + (3 * 7) = 26 valence electrons.

Draw the Skeletal Structure for NCl3

Nitrogen is less electronegative and will be the central atom. Arrange the three chlorine atoms around the nitrogen atom. Connect each Cl to the N with a single bond.

Place Remaining Electrons Around Outer Atoms for NCl3

Distribute the remaining valence electrons as lone pairs to satisfy the octet rule for the chlorine atoms.

Determine the Total Number of Valence Electrons for H2CO3

For H2CO3, carbon (C) has 4 valence electrons, each hydrogen (H) has 1, and each oxygen (O) has 6. The total is 4 + (2 * 1) + (3 * 6) = 24 valence electrons.

Draw the Skeletal Structure for H2CO3

Carbon will be the central atom, with two oxygens double-bonded to it and one oxygen single-bonded. The hydrogens will be single-bonded to the single-bonded oxygen.

Place Remaining Electrons Around Outer Atoms for H2CO3

Distribute the remaining valence electrons as lone pairs around the oxygen atoms to satisfy their octets.

Determine the Total Number of Valence Electrons for C2H6

Each carbon (C) has 4 valence electrons and each hydrogen (H) has 1. The total is (2 * 4) + (6 * 1) = 14 valence electrons.

Draw the Bond Structure for C2H6

Connect the two carbon atoms with a single bond and arrange the six hydrogen atoms, three around each carbon, all with single bonds.

Place Remaining Electrons Around Atoms for C2H6

All valence electrons are used in the bonds, and each atom already satisfies the octet (for C) or duet (for H) rule.

Determine the Total Number of Valence Electrons for NaNO3

Sodium (Na) has 1 valence electron, nitrogen (N) has 5, and each oxygen (O) has 6. The total is 1 + 5 + (3 * 6) = 24 valence electrons. However, since Na is a metal, it donates its electron and is not part of the Lewis structure for the polyatomic ion NO3-.

Draw the Skeletal Structure for NO3-

Nitrogen is the central atom. Connect it to three oxygen atoms, one with a double bond and the others with single bonds. Add a single negative charge to account for the extra electron from the Na atom.

Place Remaining Electrons Around Outer Atoms for NO3-

Arrange the remaining electrons to satisfy the octet rule for the oxygen atoms. The nitrogen atom in NO3- will have an exception to the octet rule and carry a formal charge.

Key concepts

Valence Electrons

Valence electrons are the electrons that reside in the outermost shell of an atom, and these are instrumental in the process of chemical bonding. For example, in the case of nitrogen (\textsc{N}), which has five valence electrons, or chlorine (\textsc{Cl}) with seven, understanding their number is the first step in predicting how these atoms will bond with others.

During chemical reactions, valence electrons are either shared with or transferred to other atoms, leading to the formation of chemical bonds. When drawing Lewis structures, the count of valence electrons determines how atoms connect and arrange themselves in a molecule. For instance, in the exercise given for \textsc{NCl}\(_{3}\), counting valence electrons (26 in total) helps illustrate how the nitrogen and chlorine atoms will share electrons to form stable bonds.

Octet Rule

The octet rule is a chemical rule of thumb 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. The rule applies to most elements, although there are some exceptions. For example, hydrogen (\textsc{H}) and helium (\textsc{He}) follow the 'duet rule', aiming for two electrons in their outer shell.

In the step-by-step solutions for \textsc{H}\(_2\)\textsc{CO}\(_3\) and \textsc{C}\(_2\)\textsc{H}\(_6\), arranging the remaining electrons to satisfy the octet rule for the oxygen and carbon atoms is a crucial step, ensuring stability and a lower energy state for the molecules. This is why, after creating bonds, the additional electrons are placed as lone pairs around the more electronegative atoms to complete their octets.

Chemical Bonding

Chemical bonding refers to the force that holds atoms together in chemical compounds. This occurs when atoms either share or transfer valence electrons, resulting in the formation of ionic, covalent, or metallic bonds. For instance, in \textsc{NaNO}\(_3\), sodium (\textsc{Na}) transfers its one valence electron to the polyatomic nitrate ion (\textsc{NO}\(_3^-\)), forming an ionic bond. The nitrate ion itself is held together by covalent bonds, where nitrogen shares its electrons with the oxygen atoms.

Understanding Lewis structures, like those drawn for \textsc{NaNO}\(_3\), helps in visualizing how atoms are bonded in molecules. It's a significant step in comprehending not only the structure of a molecule but also how the individual atoms within it interact through chemical bonding.