Problem 8 - Chemistry: The Central Science

Question

(a) What is the octet rule? (b) How many electrons must a sulfur atom gain to achieve an octet in its valence shell? (c) If an atom has the electron configuration \(1 s^{2} 2 s^{2} 2 p^{3}\), how many electrons must it gain to achieve an octet?

Step-by-Step Solution

Answer

(a) The octet rule states that atoms tend to combine in a way that they have eight electrons in their valence shell, which corresponds to a stable electronic configuration like that of noble gases. (b) A sulfur atom must gain 2 electrons to achieve an octet in its valence shell. (c) The atom with the given electron configuration \(1s^{2} 2s^{2} 2p^{3}\) must gain 3 electrons to achieve an octet.

Step 1: (a) Definition of Octet Rule

The octet rule is a chemical rule of thumb that states that atoms of low atomic number (mainly belonging to the main-group elements) tend to combine such that they have eight electrons in their valence shell, which corresponds to a stable electronic configuration like that of noble gases. In other words, atoms will gain, lose, or share electrons to have a full set of eight electrons in their outermost electron shell.

Step 2: (b) Electrons needed by sulfur to achieve an octet

A sulfur atom has an atomic number of 16, which means it has 16 electrons. Its electron configuration is \(1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{4}\). The valence shell of sulfur is the third shell, which currently holds 6 electrons (2 in the 3s subshell and 4 in the 3p subshell). In order to achieve an octet, it needs 8 electrons in its valence shell. Hence, a sulfur atom must gain 2 electrons to achieve an octet in its valence shell.

Step 3: (c) Electrons needed by given atom to achieve an octet

The given electron configuration is \(1s^{2} 2s^{2} 2p^{3}\). The valence shell is the second shell, which currently holds 5 electrons (2 in the 2s subshell and 3 in the 2p subshell). In order to achieve an octet, it needs 8 electrons in its valence shell. Therefore, the atom must gain 3 electrons to achieve an octet.

Key Concepts

Valence Electrons

Valence electrons are the electrons located in the outermost shell of an atom and are involved in chemical bonding. Understanding valence electrons is crucial because they determine how atoms will interact with each other. Atoms strive to have a full outer shell of electrons, typically resembling the nearest noble gas configuration. For the majority of atoms, this means an ideal of eight valence electrons, which is where the octet rule originates from.

For instance, the sulfur atom, with atomic number 16, has six electrons in its valence shell (3s² 3p⁴). To comply with the octet rule and become more stable, it aims to fill its outermost shell up to eight electrons. Consequently, sulfur needs to acquire two additional electrons to fulfill this stable electronic arrangement.

In another example from the textbook, an atom with electron configuration 1s² 2s² 2p³ contains five valence electrons. This atom would need to gain three more electrons to reach the octet and achieve desired stability. A closer look at the valence electrons can reveal much about an element's reactivity and the types of bonds it will form in compounds.

Electron Configuration

Electron configuration refers to the distribution of electrons among the different orbits (shells) and subshells (s, p, d, f) around an atomic nucleus. This configuration shapes how atoms interact and ultimately influences the chemical behavior of different elements.

Considering the sulfur example again, its electron configuration ends in 3s² 3p⁴, which implies there are six electrons in the valence shell (third shell). The configuration shows that sulfur is two electrons short of the stable octet we talk about in the octet rule. For the atom with the configuration 1s² 2s² 2p³, it is in need of three more electrons to fill the 2p subshell completely (which can hold up to six electrons) and satisfy the octet rule.

Electron configuration is a powerful tool in predicting the chemical formula of compounds that atoms will form, as atoms will typically bond in patterns that allow them to share or transfer electrons in such a way that all participating atoms reach a stable electron configuration.

Chemical Stability

Chemical stability is a term that describes the likelihood of an atom or compound to maintain its chemical structure without undergoing a chemical reaction or transformation. An atom achieves chemical stability when it has a full set of valence electrons, conforming to the octet rule. This rule helps us to predict and understand the reactivity of elements. Atoms with a full octet are less likely to participate in chemical reactions because they are already in a low-energy, stable state.

In contrast, atoms that do not have a full valence shell are more likely to engage in chemical reactions to reach the stable octet. These reactions can involve gaining, losing, or sharing electrons through ionic or covalent bonding. For example, a sulfur atom will often form sulfide ions (S^2-) by gaining two electrons to achieve stability. In a molecule, sulfur may share its valence electrons to create covalent bonds and attain a full octet.

Understanding how elements achieve chemical stability via the octet rule is foundational in the study of chemistry, as it informs us about the potential reactions and compounds that can be formed. It also sheds light on the inherent stability of noble gases, which naturally possess a full octet and are typically unreactive.