Question

What is the basic procedure for writing a covalent Lewis structure?

Short answer

Count the total valence electrons, choose the central atom, distribute electrons to achieve octets on outer atoms, place remaining electrons on the central atom, and form multiple bonds if necessary to ensure all atoms satisfy the octet rule.

Step-by-step solution

Count the Total Number of Valence Electrons

Determine the total number of valence electrons in the molecule by adding up the valence electrons of each atom. Remember that hydrogen has 1, carbon has 4, nitrogen has 5, oxygen has 6, and halogens have 7 valence electrons.

Choose the Central Atom

Place the least electronegative atom in the center, except for hydrogen, which is almost always on the outside. The more electronegative atoms will surround this central atom.

Distribute Electrons to the Outer Atoms

Starting with the outer atoms, distribute the electrons until each one, except hydrogen, has 8 electrons to fulfill the octet rule. Hydrogen only needs 2 electrons.

Complete the Octets of Outer Atoms

If there are remaining electrons after the outer atoms have 8 electrons, place them on the central atom. Hydrogen can only hold 2 electrons.

Place Remaining Electrons on the Central Atom

Once the outer atoms have their octets, any leftover electrons should be placed on the central atom, even if it results in more than 8 electrons around it.

Rearrange Electrons to Form Multiple Bonds if Necessary

If there are not enough electrons to give the central atom an octet, form double or triple bonds by using lone pairs from the outer atoms.

Key concepts

Valence Electrons

Understanding valence electrons is the cornerstone of mastering the composition of covalent bonds and molecules. Simply put, valence electrons are the electrons that reside in the outermost shell of an atom and are responsible for the chemical properties of the element. When it comes to the creation of chemical bonds, these valence electrons are the ones that participate and interact with other atoms.

To clarify with an example, consider the element carbon (C) which has four valence electrons. This allows carbon to form four covalent bonds with other atoms. When writing a Lewis structure, the first step always involves counting the valence electrons of each atom to understand how many electrons are available for bonding.

• Hydrogen (H) has 1 valence electron.
• Carbon (C) has 4 valence electrons.
• Nitrogen (N) has 5 valence electrons.
• Oxygen (O) has 6 valence electrons.
• Halogens like Fluorine (F) have 7 valence electrons.

With these numbers, students can sketch out the potential links each atom can form.

Octet Rule

The octet rule is a chemical rule of thumb that states atoms 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. However, hydrogen is an exception as it seeks to emulate the nearest noble gas, helium, having only two electrons in its outer shell.

Thus, when distributing electrons in a Lewis structure, one aims to ensure that every atom (except hydrogen) achieves an octet. This may involve sharing electrons between atoms to obtain stable electronic configurations. For example, oxygen atoms will share two electrons each to create a double bond, reaching eight electrons in their shells. While working through textbook exercises, remember that atoms will either share, donate, or accept electrons to complete their valent shell to abide by the octet rule, which is a guide to potential bonding scenarios.

Electronegativity

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons towards itself. In the context of writing Lewis structures, knowing the relative electronegativity of different atoms can guide you in placing the atoms correctly within the molecule.

Typically, the least electronegative atom is chosen as the central atom because it is more likely to share electrons with its surrounding elements, not hogging the electrons as the more electronegative atoms might. An exception to this rule is hydrogen, which due to its small size and the fact it can only form one bond is almost never found in the center.

Imagine atoms in a tug-of-war for electrons; elements like fluorine or oxygen have a stronger 'grip' compared to elements like carbon or hydrogen, impacting where electrons are distributed and which atoms they are shared with. This knowledge is encapsulated in step two of the exercise solution, emphasizing the electronegativity considerations when choosing the central atom.

Multiple Bonds

Multiple bonds (double or triple bonds) are a type of chemical bond where two or three pairs of electrons are shared between atoms. They occur when adjacent atoms share more than one pair of valence electrons to fulfill the octet rule, especially when there aren't enough electrons to give each atom an octet by forming single bonds alone.

In the context of a Lewis structure, multiple bonds are often introduced in later steps, as one might initially place lone pairs to fulfill octets but then need to adjust by converting lone pairs into bonding pairs to ensure the central atom can also comply with the octet rule. This strategic redistribution of electrons is depicted in step six of the textbook solution.

• Double bonds involve the sharing of two pairs of electrons.
• Triple bonds involve the sharing of three pairs of electrons.

These bonds not only satisfy the octet rule but also impact the molecular geometry and rigidity of the molecule, influencing physical and chemical properties.