Question

Draw Lewis structures for (a) \(\mathrm{NO}^{+},\) (b) \(\mathrm{HClO}_{2}\), (c) \(\mathrm{H}_{2} \mathrm{SeO}_{3}\), and (d) \(\mathrm{H}_{3} \mathrm{AsO}_{4}\).

Short answer

The Lewis structures show that \(\mathrm{NO}^{+}\) has a double bond between N and O with N having a lone electron; \(\mathrm{HClO}_{2}\) has Cl at the center, with one O bound to H and a lone pair on Cl; \(\mathrm{H}_{2} \mathrm{SeO}_{3}\) has Se at the center, each O is single-bonded to H and there are two lone pairs on Se; \(\mathrm{H}_{3} \mathrm{AsO}_{4}\) has As at the center with O atoms around and three of them are single-bonded to H.

Step-by-step solution

Drawing Lewis Structure for \(\mathrm{NO}^{+}\)

Count the total number of valence electrons. Nitrogen has 5 and oxygen has 6, giving a total of 5 + 6 - 1 (because of the positive charge) = 10 electrons. Arrange the electrons to satisfy the octet rule for oxygen, which means oxygen will have 8 electrons around it. Place the remaining electrons around nitrogen, and remember there will be a shortage due to the positive charge, indicating a bond exists between N and O.

Drawing Lewis Structure for \(\mathrm{HClO}_{2}\)

Firstly, count the valence electrons. Chlorine has 7, oxygen has 6 (for each of the two oxygens), and hydrogen has 1, totaling 7 + 6*2 + 1 = 20 electrons. Hydrogen will be bonded to one of the oxygens. Chlorine will be the central atom because it is the least electronegative. Distribute the electrons to form single bonds first and then complete the octets by adding pairs of electrons. Additionally, place one pair of electrons on chlorine to indicate a lone pair.

Drawing Lewis Structure for \(\mathrm{H}_{2} \mathrm{SeO}_{3}\)

Count the valence electrons available. Selenium has 6, each oxygen also has 6, and hydrogen has 1, for a total of 6 + 6*3 + 1*2 = 26 electrons. Place selenium as the central atom surrounded by three oxygen atoms. Place the remaining electrons to complete the octets by adding two lone pairs to selenium and attaching each hydrogen to an oxygen through single bonds.

Drawing Lewis Structure for \(\mathrm{H}_{3} \mathrm{AsO}_{4}\)

Calculate the total number of valence electrons. Arsenic has 5, each of the four oxygens has 6, and hydrogen has 1, for a total of 5 + 6*4 + 1*3 = 32 electrons. Arsine will be in the center with four oxygens around it. Three hydrogens will each be attached to three different oxygens. Distribute the remaining electrons to complete the octets and place any leftover pairs on the central arsenic atom.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom, which are primarily responsible for the chemical properties of the element. They are the electrons that are involved in forming bonds with other atoms. For example, in the exercise where you were asked to draw the Lewis structure for \(\mathrm{NO}^{+}\), you start by counting the valence electrons. Nitrogen, being in group 15 of the periodic table, has 5 valence electrons while oxygen, from group 16, has 6. This counting is crucial as the number of valence electrons determines how atoms will bond and form molecules. The positive charge on \(\mathrm{NO}^{+}\) indicates that one valence electron is missing, thus, we subtract one. It's important to follow this kind of electron accounting to accurately represent the molecule.

Remembering the number of valence electrons in each element can be simplified if you're familiar with the periodic table's group numbers. For most elements, the group number equates to the number of valence electrons (this doesn't apply to transition metals). For the exercises involving \(\mathrm{HClO}_{2}\), \(\mathrm{H}_{2} \mathrm{SeO}_{3}\), and \(\mathrm{H}_{3} \mathrm{AsO}_{4}\), you deal with more complex molecules, yet the initial step always involves counting these significant electrons to determine the total needed for bonding.

Octet Rule

The octet rule is a chemical rule of thumb that reflects observation that atoms of main-group elements 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, excluding hydrogen (which seeks a duet of electrons) and the transition metals (which have complex subshell filling).

In the Lewis structure exercises, it's shown that satisfying the octet rule is a key consideration in placing electrons around atoms. When we distribute the valence electrons for \(\mathrm{HClO}_{2}\), for instance, we first create single bonds, then add pairs of electrons to either fulfill the octet or represent lone pairs. However, there can be exceptions to the octet rule; some elements can have fewer or more than eight electrons. These are often referred to as hypovalent or hypervalent, but such cases are not mentioned in the given exercise.

The concepts of the octet rule not only help in deducing the structure of the molecule but also guide the understanding of the stability and reactivity of different molecules.

Chemical Bonding

Chemical bonding is the physical process that allows for the formation of molecules and compounds from atoms. It occurs when atoms share or transfer valence electrons to achieve a more stable electron configuration. There are three primary types of chemical bonds: ionic, covalent, and metallic bonds. In the context of the Lewis structures you're working with, covalent bonding is most relevant.

Covalent bonds form when two atoms share a pair of valence electrons. For example, in the \(\mathrm{H}_{2} \mathrm{SeO}_{3}\) molecule from the exercise, selenium shares electrons with oxygen to form covalent bonds. It's important to note that the number of shared electron pairs can vary, resulting in single, double, or triple covalent bonds. In Lewis structures, these are depicted as single lines (for single bonds), double lines (for double bonds), or triple lines (for triple bonds) between the atoms involved.

Through exercises such as drawing Lewis structures, students gain insight into the arrangements of atoms within molecules and the types of bonds that hold these atoms together. Understanding chemical bonding is fundamental in chemistry as it explains the formation of different substances and guides predictions about their behavior and interactions.