Question

Draw Lewis structures for (a) selenium trioxide, (b) the carbonate ion, (c) hydrogen carbonate ion, and (d) selenium dioxide.

Short answer

Lewis structures: (a) SeO3 has a triangular planar structure with double bonds to each O. (b) CO3^2- has a trigonal planar structure with one double bond and two single bonds to the O atoms. (c) HCO3^- has one H bonded to an O, two single-bonded O atoms and one double-bonded O atom to C. (d) SeO2 has a bent structure with double bonds between Se and each O atom.

Step-by-step solution

Selenium Trioxide (SeO3)

First, determine the total number of valence electrons available for the SeO3 molecule. Selenium (Se) has 6 valence electrons and each oxygen (O) has 6 valence electrons, making a total of 24 electrons (6+3*6). Arrange the Se atom as the central atom and distribute the O atoms around it. Form a single bond between Se and each O atom, using 6 electrons. Now, add lone pairs around the O atoms to complete their octets; you should use 18 electrons for this. The remaining 0 electrons are used to form double bonds between Se and the O atoms to satisfy the octet rule for Se.

Carbonate Ion (CO3^2-)

For the CO3^2- ion, start by counting valence electrons, which is 4 for carbon (C), 6 for each oxygen (O), and 2 additional electrons for the charge, totalling 24 electrons. Place the C atom in the center with the O atoms surrounding it. Use single bonds to connect C with each O atom, and place the remaining electrons as lone pairs on the O atoms. With remaining electrons, form multiple bonds to one or two oxygen atoms to complete the octet for carbon and satisfy the octet rule for all atoms. Finally, add brackets with the charge of -2 to denote the ion.

Hydrogen Carbonate Ion (HCO3^-)

For HCO3^-, we have 4 valence electrons from C, 6 from each of the 3 O, and 1 from the H, plus 1 additional electron for the charge, giving us 24 electrons. C will be central, with O atoms and the H atom surrounding it. Form single bonds between C and each O and C and H, using 8 electrons. Complete O octets with lone pairs using the next 18 electrons. If there are remaining electrons, which there are not, form double bonds as needed. Enclose the structure in brackets and indicate the charge of -1.

Selenium Dioxide (SeO2)

Selenium dioxide will have a total of 18 valence electrons from Se (6 valence electrons) and two O atoms. Se is the central atom, and form a single bond with each O atom using 4 electrons. Add lone pairs to the O atoms to complete their octets with the next 12 electrons. The remaining 2 electrons will go to the Se atom, which will share them in double bonds with the O atoms to satisfy the octet rule for selenium, as Se can have an expanded octet.

Key concepts

Valence electrons

Valence electrons are the electrons present in the outermost shell of an atom that participate in chemical bonding. Knowing the number of valence electrons an element possesses is crucial for predicting how it will interact with other atoms. For example, in the carbon dioxide molecule, carbon has 4 valence electrons and the oxygen atoms each have 6 valence electrons. To draw its Lewis structure, we must account for these valence electrons in the formation of bonds.

Elements in the same group of the periodic table have the same number of valence electrons, which defines their chemical behavior. For instance, all alkaline earth metals have two valence electrons, which typically lead them to lose these electrons and form cations with a +2 charge. When creating Lewis structures, we distribute valence electrons to form bonds and fulfill the octet rule where applicable. This foundational concept of valence electrons is essential for grasping chemical bonding and molecule formation.

Octet rule

The octet rule is a chemical rule of thumb that states atoms tend to bond in such a way that they each have eight electrons in their valence shell, giving them the same electronic configuration as a noble gas. The rule applies to most elements, with the notable exceptions being hydrogen (which seeks to fill its valence with two electrons) and elements in periods 3 and below of the periodic table, which can have more than eight valence electrons due to the availability of d-orbitals.

When drawing Lewis structures, like those of selenium trioxide (SeO3) or the carbonate ion (CO3^2-), we aim to satisfy the octet rule for each atom. There can be exceptions, such as in the hydrogen carbonate ion (HCO3^-) where certain elements will not have a complete octet and can exhibit an expanded octet as selenium does in selenium dioxide (SeO2). The octet rule is a simplification and does not predict all aspects of molecular structure, but it is a helpful principle for understanding the basics of chemical bonding.

Chemical bonding

Chemical bonding is the process by which atoms combine to form new substances. Bonds are formed by the interaction of valence electrons between atoms. The primary types of chemical bonds include ionic, covalent, and metallic bonds, but in the context of Lewis structures, we are primarily concerned with covalent bonds, where electrons are shared between atoms.

When we draw Lewis structures, such as for the selenium dioxide (SeO2), we illustrate these covalent bonds as lines between atoms. Sometimes, to complete an atom's octet, we must form double or triple bonds, which involve the sharing of two or three pairs of electrons, respectively. In the carbonate ion (CO3^2-), resonance structures are used to show the equivalence of different possible Lewis structures, reflecting the delocalization of electrons within the molecule. The arrangement of these bonds and electrons plays a pivotal role in determining the shape and stability of the resulting molecule.