Question

Why can an atom in the second row of the periodic table not form expanded valence shell molecules?

Short answer

Atoms in the second row lack d orbitals, preventing expanded valence shells.

Step-by-step solution

Understanding the Concept of Valence Shell

The valence shell is the outermost shell of an atom, where the valence electrons are located. These electrons are responsible for chemical bonding.

Identify the Maximum Capacity of the Valence Shell

For atoms in the second row (or period) of the periodic table, such as Carbon (C), Nitrogen (N), and Oxygen (O), the valence shell is the second electron shell. The second shell can hold a maximum of 8 electrons total in s and p orbitals: 2 in the s orbital and 6 in the p orbital.

Explain Expanded Valence Shell Concept

Expanded valence shells are observed when an atom holds more than 8 electrons in its valence shell. This phenomenon is usually possible for atoms in the third period or below, which have d orbitals available.

Conclusion about Atoms in the Second Row

Atoms in the second row do not have d orbitals in their valence shell. Thus, they cannot accommodate more than 8 electrons and consequently cannot form expanded valence shell molecules.

Key concepts

Expanded Valence Shell

When we discuss atoms and their capabilities to form molecules, one intriguing concept is the "expanded valence shell". Most of us are familiar with the octet rule, where atoms strive to have eight electrons in their outermost valence shell. This is primarily because a full valence shell creates a stable electron configuration.

However, some atoms can exceed this eight-electron limit. This is what's known as an expanded valence shell. Atoms belonging to higher periods on the periodic table, especially those in the third period and beyond, have access to empty d orbitals. This additional space means they can accommodate more than eight electrons. For example, elements like phosphorus or sulfur often participate in covalent bonds that result in expanded valence shells. This is why compounds like \\[ \text{PCl}_5 \] (phosphorus pentachloride) exist. The ability to form expanded valence shells is crucial in creating complex and diverse molecules that deviate from those simply explained by the octet rule.

Second Row Elements

The second row of the periodic table consists of the elements from lithium to neon. These elements are unique in many ways. One of their defining characteristics is their electron configuration. Each element in the second row fills the second electron shell which includes the 2s and 2p orbitals.

Each orbital holds a specific number of electrons:

• The 2s orbital can hold up to 2 electrons.
• The 2p orbitals can collectively hold up to 6 electrons.
• In total, the second electron shell can hold up to 8 electrons.

This setup is crucial because it limits these elements to a maximum of 8 valence electrons. Therefore, the second row elements cannot expand their valence shell beyond this to hold more electrons, unlike the elements further down the periodic table. This electron limitation influences how these elements bond and what kinds of molecules they can form. Notable elements such as carbon, nitrogen, and oxygen all belong to this row, playing critical roles in organic chemistry and biological processes.

Periodic Table

The periodic table is a vital tool in chemistry, organizing elements based on their atomic number, electron configurations, and recurring chemical properties. It acts as a map for chemists, showing relationships between different elements and guiding predictions about how they will interact.

Understanding the layout helps clarify why certain elements behave the way they do. Each row tells us about the electrons' arrangement around the nucleus. Second row elements, for instance, are unable to form expanded valence shells because they lack d orbitals, which other rows might have. As you move down the table, additional electron shells become available.

• The first row (hydrogen and helium) fills the 1s orbital, supporting only 2 electrons.
• The second fills up to the 2p orbital, accommodating up to 8 electrons.
• Subsequent rows introduce s, p, and then d orbitals.

These details help scientists and students predict behavior in chemical reactions, such as why silicon can form bonds that carbon cannot. The periodic table thus is more than just an organizational chart – it is foundational to understanding chemistry and the properties of matter.