Question

The faint light sometimes seen over Marshland at night, the "will-o'-the- wisp", is believed to come about as a result of the burning of a compound of phosphorus (P) and hydrogen (H). What is the formula of this compound?

Short answer

The compound formed by phosphorus (P) and hydrogen (H) has the chemical formula PH_3, also known as phosphine.

Step-by-step solution

Determine the valency of each element

Phosphorus (P) has a valency of 3 and hydrogen (H) has a valency of 1.

Write down the element symbols and their valencies

We will write the element symbols, followed by their valencies as superscripts. P^3 H^1

Cross multiply the valencies to determine the ratio

Now we will cross multiply the valencies to determine the ratio of the elements in the compound. P^3H^1 -> PH_3

Write the chemical formula

The chemical formula for the compound formed by phosphorus and hydrogen is PH_3. This compound is also known as phosphine.

Key concepts

Chemical Formula

A chemical formula is the shorthand representation of a compound using element symbols and numerical subscripts.
It shows the types and numbers of atoms present in the smallest unit of the compound. In the context of the given problem, the chemical formula represents the specific elements and their ratios found in phosphine.
For phosphine, the chemical formula is expressed as \( \text{PH}_3 \), where 'P' stands for phosphorus and 'H' stands for hydrogen.

Element Valency

Element valency refers to the ability of an atom to combine with other atoms.
It's determined by the number of electrons an atom can gain, lose, or share to achieve a full outer electron shell.
Phosphorus has a valency of 3, implying it can form three bonds with three hydrogen atoms, each having a valency of 1. This characteristic enables phosphorus and hydrogen to form a stable compound like phosphine.
Understanding valency is vital because it helps in predicting the chemical behavior and the type of bonds an element can form in a molecule.

Compound Formation

Compound formation involves the chemical combination of elements in specific ratios to form a new substance with distinct properties.
In forming a compound like phosphine, phosphorus, with a valency of 3, shares its three valency electrons with three hydrogen atoms, each contributing one electron.

• The chemical bond formed in phosphine is typically a covalent bond, where atoms share electrons to achieve stability.
• This new configuration gives rise to the stable compound with the formula \( \text{PH}_3 \).

Understanding compound formation is crucial to grasp how different elements chemically unite to form substances with new characteristics.

Phosphorus and Hydrogen

Phosphorus and hydrogen are the two elements that form the compound known as phosphine.

• Phosphorus (P) is a nonmetal element that usually forms trivalent compounds, meaning it can make three bonds with other atoms.
• Hydrogen (H), the simplest element, normally participates in forming single bonds due to its singular electron.

Together, phosphorus and hydrogen atoms combine to form \( \text{PH}_3 \), where the interaction is predominantly defined by their valencies.
This combination results in phosphine, which is a colorless, flammable gas under normal conditions and an essential part of the mystery of the 'will-o'-the-wisp' phenomenon.