Question

Draw acceptable Lewis structures for the molecular ion (radical cation) formed from the following molecules when each is bombarded by high-energy electrons in a mass spectrometer. a. b. c. d.

Short answer

Answer: The Lewis structure of the radical cation formed from Methane (CH4) when it loses an electron in mass spectrometry is: H | H-C⋅ | H The process includes drawing the Lewis structure of Methane (CH4), removing one electron from a C-H bond to form a radical, and checking the formal charge and octet rule. In the radical cation, carbon has an unpaired electron (indicated by a dot), and the octet rule is not satisfied due to the loss of an electron.

Step-by-step solution

Identify the molecule

Let's first consider the case of Methane (CH4). Methane has the molecular formula CH4 and contains one carbon atom bonded to four hydrogen atoms.

Draw the Lewis structure of the molecule

The Lewis structure of Methane (CH4) can be drawn as follows: H | H-C-H | H In this structure, carbon has 4 covalent bonds to hydrogen atoms, and each hydrogen atom has a single covalent bond to carbon.

Remove one electron to form the radical cation

When Methane is bombarded by high-energy electrons in a mass spectrometer, it loses an electron, forming a radical cation. To represent this, we need to remove one electron from one of the covalent bonds in Methane's Lewis structure.

Draw the Lewis structure of the radical cation

We will remove an electron from one of the C-H bonds. The new C-H bond will now have only one shared electron, forming a radical. The Lewis structure for the radical cation of Methane (CH4+) is: H | H-C⋅ | H Here, the unpaired electron on carbon is represented by a dot (⋅), indicating that it is a radical.

Check the formal charge and octet rule

The formal charge on the carbon atom in the CH4+ radical cation is +1, while each hydrogen atom has a formal charge of 0. This indicates that the radical cation has a net charge of +1. In the CH4+ radical cation, the octet rule is not satisfied for the carbon atom, as it has only 7 electrons around it. However, this is expected since it is a radical cation created by the loss of an electron.

Key concepts

Chemical Bonding

Chemical bonding is the key to understanding molecular structure and stability. Bonds are formed when atoms share or transfer electrons to achieve a more stable electron configuration, often reaching a noble gas electron arrangement. In organic compounds like methane (\( \text{CH}_4 \)), carbon forms four covalent bonds with hydrogen atoms, sharing electrons to fill its outer shell. Covalent bonding is critical in forming the wide variety of organic molecules fundamental to life and many materials.

When dealing with radical cations, the usual bonding patterns can be disrupted. Radical cations, such as the one formed from methane in mass spectrometry (discussed later), occur when a molecule loses an electron. This often leads to an unpaired electron, creating a radical. It's important to remember that even though these species are less stable and don't follow the octet rule strictly, they are pivotal in understanding reaction mechanisms and the behavior of molecules under certain conditions, like those in a mass spectrometer.

Mass Spectrometry

Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions. It's invaluable in structural determination and the analysis of organic compounds. In mass spectrometry, a molecule is bombarded with high-energy electrons, causing it to lose an electron and form a radical cation. The instrument then separates these ions based on their mass-to-charge ratio. For example, methane (\( \text{CH}_4 \)) in a mass spectrometer will form a radical cation (\( \text{CH}_4^+ \)), losing an electron and becoming a positively charged ion.

Understanding the formation and structure of molecular ions in mass spectrometry is vital for interpreting spectra and identifying substances. Each peak in a mass spectrum can give clues about the molecular weight and structure of the compound being analyzed, a crucial step in many fields like drug discovery, environmental monitoring, and forensic analysis.

Molecular Ions

Molecular ions are charged molecules created when atoms within a molecule gain or lose electrons. In the context of mass spectrometry, they often form when a molecule is ionized by the loss of an electron to produce a cation. They are typically represented by adding a plus sign to indicate a positive charge (\( \text{CH}_4 \rightarrow \text{CH}_4^+ \)). These ions may be radicals if they have an unpaired electron left behind, as in the case of radical cations.

It's crucial to handle the Lewis structure for molecular ions carefully, taking into account the change in electron count and how it impacts bonding and molecular geometry. The creation of a molecular ion can drastically affect a molecule's reactivity, often making it more reactive due to the presence of an unpaired electron or a formal charge.

Octet Rule

The octet rule is a fundamental concept in chemistry that refers to the tendency of atoms to prefer having eight electrons in their valence shell. This desire for a complete set of eight valence electrons drives atoms to bond with each other. For example, carbon, with four valence electrons, will commonly form four covalent bonds, as evidenced in methane (\( \text{CH}_4 \)), to complete its octet.

However, in the case of radical cations such as \( \text{CH}_4^+ \), not all atoms can satisfy the octet rule due to the loss of an electron. Typically, this results in a less stable, more reactive species. The presence of an unpaired electron in such ions does not negate the importance of the octet rule but illustrates its limitations and the existence of exceptions, particularly in the realm of transient species in reactions or under the high-energy conditions of mass spectrometry.