(a) What functional group does the molecule ethanal contain? How can you tell? (b) Draw a dot diagram for the molecule. Answer: (a) The ending -al tells you this molecule is an aldehyde. It therefore contains the aldehyde functional group.

Understanding the aldehyde functional group is crucial for identifying compounds like ethanal. Aldehydes are characterized by the presence of a carbonyl group (a carbon-oxygen double bond) with at least one hydrogen atom attached directly to the carbon. This arrangement is noted as -CHO in molecular formulas and is what gives aldehydes their definitive properties.

In the case of ethanal, which is a two-carbon aldehyde, the molecule ends with the group -CH₂CHO. The hydrogen atom connected to the carbonyl carbon makes it an aldehyde rather than a ketone, where the carbonyl carbon would be bonded to two other carbon atoms instead of a hydrogen atom. Moreover, this simple aldehyde group is also responsible for the molecule’s reactivity, allowing it to participate in various chemical reactions, such as oxidation to form carboxylic acids or condensation reactions to form larger molecules.

Additionally, when naming aldehydes, the suffix '-al' is used, which helps in identifying the molecule's structure just by its name. Recognizing this is the key to mastering organic chemistry nomenclature and understanding molecular behavior.

The Lewis structure, also known as a dot diagram, is a representation that illustrates the valence electrons of atoms within a molecule and how these electrons are arranged in bonds between atoms. For ethanal, the Lewis structure helps visualize the electronic framework that underpins its physical and chemical properties.

To draw the Lewis structure of ethanal, we begin by arranging its atoms. There are two carbon atoms in ethanal. One carbon atom (the aldehyde carbon) will form a double bond with the oxygen atom and a single bond with a hydrogen atom and the second carbon atom. The final carbon atom has two hydrogen atoms bonded to it. This setup is a result of the carbons sharing electrons to ensure stability through full valence shells, following the octet rule.

Distribution of Electrons

After the construction of the basic skeletal structure, electrons are distributed around the atoms to complete the octets. The double bond between the carbon and oxygen uses four valence electrons, while single bonds with hydrogen use two electrons each. The remaining electrons form lone pairs around the oxygen, which are not shared with other atoms but complete the oxygen’s valence shell.

Valence electrons are the electrons located in an atom's outermost shell and play a pivotal role in chemical bonding and molecule formation. For atoms to achieve stability, their valence shells must be filled according to the octet rule, aiming to have eight electrons or to match the configuration of the nearest noble gas.

In the step-by-step solution to the exercise, calculating the total number of valence electrons for ethanal is a key step. Carbon atoms, being Group IV elements, have four valence electrons each. Hydrogen, found in Group I, has one valence electron, while oxygen, from Group VI, has six. Thus, the specific arrangement of these electrons in dot diagrams gives a clear view of potential bonds that can be formed: carbon with a typical valence of four forms four bonds, oxygen with a typical valence of six usually forms two bonds, and hydrogen with a valence of one forms a single bond.

Ethanal, with 22 valence electrons, uses these to create stable bonds represented in its Lewis structure. Recognition of valence electrons and their distribution is an essential skill in understanding and predicting molecular geometry and reactivity.