Question

Two elements \(P\) and \(Q\) combine to form a cormpound. If \(P\) has 2 and \(Q\) has 6 electrons in their outermost shell, what will be formula of the compound formed? (a) \(\mathrm{PQ}\) (b) \(P_{2} Q\) (c) \(P_{2} Q_{3}\) (d) \(\mathrm{PQ}_{2}\)

Short answer

The formula of the compound formed is \textbf{(a) PQ}.

Step-by-step solution

Identify Valency

Valency is determined by the number of electrons an element needs to either gain or lose to achieve a noble gas electronic configuration, which is typically 8 electrons in the outermost shell. Element P, with 2 electrons in its outermost shell, will want to lose 2 electrons to reach this stable configuration, giving it a valency of +2. Element Q, with 6 electrons, needs 2 more to reach stability, thus its valency is -2.

Determine the Formula

The compound is formed when elements combine in a ratio that balances their valencies. P, with a valency of +2, will combine with Q, with a valency of -2, in a 1:1 ratio to balance the charges, so that the total positive charge equals the total negative charge, resulting in a neutral compound.

Write the Chemical Formula

Considering the 1:1 ratio, the formula of the compound will be PQ as each will contribute their respective valencies to form a stable compound.

Key concepts

Valency

Valency is a fundamental concept in chemistry that refers to the combining power of an element. Specifically, it is the number of hydrogen atoms that can combine with or replace (in part or whole) one atom of the element, forming a compound. Elements show their valency through their tendency to either lose, gain, or share electrons to achieve a full outer shell, much like that of the closest noble gas.

In simple terms, valency tells us how 'eager' an atom is to bond with others. For example, in the exercise we reviewed, element P has a valency of +2 because it needs to lose 2 electrons to reach a stable configuration. Conversely, element Q has a valency of -2 since it requires 2 additional electrons to be stable. Understanding valency is essential for predicting how elements interact and ultimately for determining the chemical formula of compounds they form.

Chemical Bonding

Chemical bonding is all about how atoms connect to each other to form molecules or compounds. Atoms bond in order to achieve a more stable electron configuration, usually aiming for a full outer shell, which is typically associated with the lowest energy or the most stable state of the atom.

There are several types of chemical bonds, including ionic, covalent, and metallic bonds, each with unique properties. Ionic bonds occur when one atom donates electrons to another, leading to the formation of positive and negative ions that attract each other. Covalent bonds happen when two atoms share electrons. In our exercise, the bond formed between element P and Q would be ionic because P is losing electrons while Q is gaining them, thereby creating an electrostatic attraction that holds the atoms together to form a neutral compound.

Electron Configuration

Electron configuration describes the distribution of electrons in an atom's shells and subshells. It is significant because it determines an atom's chemical properties and its behavior in bonding. The most stable electron configuration is that of a noble gas, where the outermost shell is either completely full or has eight electrons (an octet), making it highly unreactive or inert.

In our problem, the electron configuration of the elements directly informs their valency and thus their behavior in forming compounds. Element P wants to reach the stable electron configuration by losing 2 electrons, while Q wants to gain 2 to achieve stability. Thus, understanding electron configurations can help us predict how an atom will react chemically.

Outermost Shell Electrons

The electrons found in the outermost shell of an atom, also known as valence electrons, play a crucial role in determining an element's chemical properties. These electrons are the ones involved in forming chemical bonds with other atoms. An atom's reactivity is tied to how many electrons are in its outermost shell and how many it needs to achieve a stable octet.

For elements P and Q in the exercise, P has 2 electrons and wants to lose them, while Q has 6 and wants to gain more to fill its outer shell to 8 electrons. This desire to fill or empty the outer shell drives the chemical bonding process, and is why element P bonds with element Q, each providing what the other needs, to form a compound with the formula PQ, given their respective valencies and the need to achieve balance and stability.