Problem 143 - Chemistry Matter and Change

Question

Determine the molecular formula for luminol and draw its Lewis structure.

Step-by-Step Solution

Answer

The molecular formula for luminol is C8H7N3O2, and its Lewis structure can be drawn as: O \ || N C----C \ || / \ C==N NH2 // \\ N-C==C \\ O==C This structure follows the octet rule with every atom having a complete octet (except for hydrogen atoms which have 2 electrons).

Step 1: Identify the molecular formula of luminol from its IUPAC name

Luminol has an IUPAC name of 5-amino-2,3-dihydro-1,4-phthalazinedione. From this name, we can deduce the molecular formula as follows: - 5-amino means there is an NH2 group on carbon 5. - 2,3-dihydro means that there are two H atoms on carbons 2 and 3. - 1,4-phthalazinedione means that there are two carbonyl (C=O) groups on carbons 1 and 4. Considering these functional groups, we can deduce the molecular formula of luminol to be C8H7N3O2.

Step 2: Count the number of valence electrons in the molecule

We need to count the valence electrons of each atom in the molecule, which will be used in creating the Lewis structure: - Carbon (C) has 4 valence electrons. - Hydrogen (H) has 1 valence electron. - Nitrogen (N) has 5 valence electrons. - Oxygen (O) has 6 valence electrons. Using the molecular formula (C8H7N3O2), we can calculate the total number of valence electrons: - 8 Carbon atoms: 8 × 4 = 32 valence electrons - 7 Hydrogen atoms: 7 × 1 = 7 valence electrons - 3 Nitrogen atoms: 3 × 5 = 15 valence electrons - 2 Oxygen atoms: 2 × 6 = 12 valence electrons Total valence electrons = 32 + 7 + 15 + 12 = 66 valence electrons

Step 3: Create the Lewis structure for luminol

To draw the Lewis structure of luminol, follow these steps: 1. Arrange the atoms in the structure of the molecule. Place nitrogen and oxygen atoms around the carbons and hydrogens according to the IUPAC name. 2. Connect the atoms with single bonds, using 2 electrons for each bond. 3. Distribute the remaining electrons around the atoms to complete the octet rule for each atom, except for hydrogen, which only requires 2 electrons. 4. Check for resonance structures and adjust the structure if needed. Arranging the atoms and distributing the valence electrons, we get the following Lewis structure: O \ || N C----C \ || / \ C==N NH2 // \\ N-C==C \\ O==C This Lewis structure is based on the octet rule with every atom having a complete octet, except for the hydrogen atoms which only have 2 electrons. In conclusion, the molecular formula for luminol is C8H7N3O2, and its Lewis structure is determined by arranging the atoms and distributing the valence electrons accordingly.

Key Concepts

Molecular Formula

The molecular formula of a compound is a representation of the types and numbers of atoms present in a molecule. It provides essential information about the proportions of the elements that compose the substance, and can offer insights into the chemical properties and reactivity of the molecule.

For example, the molecular formula for luminol, derived from its IUPAC name, is C8H7N3O2. This formula tells us there are 8 carbon atoms, 7 hydrogen atoms, 3 nitrogen atoms, and 2 oxygen atoms in each molecule of luminol. Knowing the molecular formula is the starting point for numerous chemical analyses, including the determination of the molecular structure and the creation of a Lewis structure.

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a key role in chemical bonding and reactions. In the Lewis structure exercise for luminol, counting valence electrons is crucial since they participate directly in bond formation.

In simple terms, different elements have different numbers of valence electrons: Carbon has 4, Hydrogen has 1, Nitrogen has 5, and Oxygen has 6. By multiplying these numbers with the amount of each atom in the molecular formula (C8H7N3O2), we can determine that luminol has 66 valence electrons in total. These valence electrons are subsequently arranged in the Lewis structure to illustrate potential bonding scenarios and electron distribution among the atoms within the molecule.

Octet Rule

The octet rule is a fundamental concept in chemistry which postulates that atoms tend to form bonds in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. There are some exceptions, but for many elements, this rule guides the distribution of electrons in a Lewis structure.

When applying this rule to luminol's Lewis structure, each carbon, nitrogen, and oxygen atom needs to be surrounded by eight electrons to satisfy the octet rule. Hydrogen, an exception to this principle, is satisfied with two electrons. The Lewis structure of luminol portrays this electron arrangement, demonstrating all the atoms connected with bonds and lone pairs in a way that fulfills the octet rule for each atom accordingly, creating a stable structure.