Problem 94 - Introductory Chemistry Atoms First

Question

How many covalent bonds do you think would form between the phosphorus atoms in the molecule \(\mathrm{P}_{2}\) ? Explain your answer and draw a dot diagram.

Step-by-Step Solution

Answer

There is one covalent bond between the phosphorus atoms in the molecule P2. This is because each phosphorus atom has 5 valence electrons, and they need to share 3 electrons to achieve a full outer shell based on the octet rule. However, when sharing electrons via covalent bonding, each electron being shared gets counted by both atoms in the bond, leading them to share two electrons, which results in one covalent bond. The Lewis dot structure for P2 can be represented as: :P:::P:

Step 1: Determine the number of valence electrons

Phosphorus is in Group 15 of the periodic table, which means it has 5 valence electrons.

Step 2: Apply the octet rule

The octet rule states that atoms tend to form covalent bonds in such a way that they achieve an electron configuration similar to that of a noble gas, with a full outer shell containing eight electrons. Since each phosphorus atom has 5 valence electrons, both atoms will need 3 more electrons to achieve a full outer shell. However, when sharing electrons via covalent bonding, each electron being shared gets counted by both atoms in the bond. So if one shared electron pair accounts for two electrons being counted for each atom, then each phosphorus needs to share 1.5 pairs of electrons, or 3. Since bonds have to be whole (no half-bonds), two of the electrons in each atom form a covalent bond. Both atoms will share these electrons so each can achieve 6 valence electrons. If the P-P bond held 3 electron pairs, one phosphorus would be sharing away all 6 valence electrons, which no element does. P may form multiple bonds, but no more than 3 individual bonds and no more than 5 total bond electrons.

Step 3: Draw a dot diagram (Lewis structure)

To represent this covalent bonding, we will draw a Lewis dot structure. Place a single bond between the two phosphorus atoms and add the remaining three valence electrons for each phosphorus atom as dots around the symbol. The Lewis dot structure for P2 is: :P:::P: In summary, there is one covalent bond between the phosphorus atoms in the molecule P2.

Key Concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom. These are the electrons that participate in chemical bonding.
Atoms in the same group of the periodic table generally have the same number of valence electrons. This is why they exhibit similar chemical properties.
Phosphorus, for example, is in Group 15 and has 5 valence electrons.

• Understanding the number of valence electrons helps predict how many covalent bonds an atom can form.
• More valence electrons mean more possible bonds.
• Phosphorus needs 3 more electrons to fill its outer shell, resembling a noble gas configuration, hence, it can form up to three covalent bonds.

Knowing the valence electrons is crucial when drawing Lewis dot structures, as it depicts the atom's ability to share or gain electrons in bonding.

Octet Rule

The octet rule is a fundamental concept in chemistry, stating that atoms tend to form chemical bonds in such a way that each atom has eight electrons in its valence shell.
This configuration is similar to that of a noble gas, which is a very stable arrangement.

• To achieve this, atoms can share, gain, or lose electrons, forming covalent or ionic bonds.
• Phosphorus, for instance, needs to form three covalent bonds to achieve an octet configuration, as it has five valence electrons and needs three more to make eight.
• This drive for stability explains why phosphorus in \(\mathrm{P}_{2}\) forms the bonds it does, each atom sharing electrons to try and reach this stable state.

The octet rule guides the formation of molecules, ensuring they occupy a low-energy, stable state, mirroring noble gases.

Lewis Dot Structure

The Lewis dot structure is a simple diagrammatic method to represent the valence electrons in an atom and how they participate in bonding.
These visual frameworks help predict the number and type of bonds in a molecule.

• Each dot represents a valence electron.
• A single line between atoms represents a pair of shared electrons, or a covalent bond.
• For \(\mathrm{P}_{2}\), we start by placing each phosphorus atom with its five valence electrons.

To draw the Lewis structure for \(\mathrm{P}_{2}\), we account for the shared electrons (indicated by a line: :P:::P:), showing each phosphorus atom with a total of eight electrons around it, to follow the octet rule.
Phosphorus forms a single covalent bond in \(\mathrm{P}_{2}\), illustrating the sharing of electrons and achieving the desired electronic configuration.