Problem 98 - Introductory Chemistry Atoms First

Question

Which atom has a smaller valence shell, lithium (Li) or sodium (Na)? Explain your answer.

Step-by-Step Solution

Answer

Lithium (Li) has a smaller valence shell than sodium (Na). This is because Li is located in period 2 and Na in period 3 of the periodic table, and atomic radii increase moving down a group. Thus, Li's valence shell is closer to its nucleus than Na's valence shell.

Step 1: Identify the electron configurations

Determine the electron configurations for lithium (Li) and sodium (Na). Li - atomic number: 3 Li electron configuration: 1s² 2s¹ Na - atomic number: 11 Na electron configuration: 1s² 2s² 2p⁶ 3s¹

Step 2: Understand the periodic table trends

Going across a period in the periodic table, atomic radii generally decrease as the elements' effective nuclear charge increases. This is due to the greater attraction between the nucleus and the valence electrons. On the other hand, moving down a group in the periodic table, atomic radii increase as more electron shells are added, causing the outermost electrons to be further away from the nucleus.

Step 3: Compare the positions of Li and Na in the periodic table

Locate lithium (Li) and sodium (Na) in the periodic table. We find that both Li and Na are in Group 1 (alkali metals group), with Li in period 2 and Na in period 3. Since Na is below Li in the same group, it indicates that Na has a larger atomic radius compared to Li, meaning it has a larger valence shell.

Step 4: Conclusion

Based on the analysis of their electron configurations and their positions in the periodic table, we conclude that lithium (Li) has a smaller valence shell than sodium (Na).

Key Concepts

Valence Shell

The valence shell of an atom is the outermost shell that contains the electrons involved in chemical reactions. These electrons, known as valence electrons, play a crucial role in chemical bonding. The size of the valence shell, therefore, determines many of an atom's chemical properties. For lithium (Li), the valence shell is the second shell, whereas sodium (Na) has its valence shell in the third energy level. The further away the valence shell is from the nucleus, the larger it is.

• Valence electrons determine reactivity.
• Lithium's valence shell is smaller than sodium's.

Understanding the size of the valence shell helps explain why sodium is generally more reactive than lithium; its valence shell can be influenced more readily by interacting atoms.

Electron Configuration

Electron configuration is a method of representing the arrangement of electrons around an atom's nucleus. This configuration follows established principles such as the Aufbau principle, Pauli exclusion principle, and Hund's rule. For lithium, the electron configuration is written as 1s² 2s¹, which means it has two electrons in the first energy level and one electron in the second.
For sodium, the electron configuration is 1s² 2s² 2p⁶ 3s¹, indicating that it fills three energy levels.

• Electron configurations describe electron positions.
• They help determine the atom's chemical behavior.

For students, understanding electron configuration is essential as it directly influences how atoms interact chemically, ensuring the correct prediction of bonding patterns.

Periodic Table Trends

The periodic table is arranged in a way that exhibits recurring trends or patterns in element properties. Two significant trends include the decrease in atomic radii across a period (row) and the increase in atomic radii down a group (column).
As you move from left to right across a period, elements gain more protons, which increases the effective nuclear charge, pulling electrons closer and reducing the atomic radii.

• Atomic size decreases across periods.
• Size increases down groups.

When comparing lithium and sodium, which are both alkali metals in Group 1, sodium displays a larger atomic radius because it exists in a lower period, contributing to its larger size compared to lithium.

Atomic Radii

Atomic radius is a term used to describe the size of an atom, typically measured from the center of the nucleus to the outermost edge of the electron cloud. It can be impacted by several factors, such as the number of electron shells and the nuclear charge.
Given lithium and sodium, sodium's larger atomic radius results from having an additional electron shell and being located in a lower period in the periodic table.

• Atomic radii provide insights into element size.
• Greater radii suggest more electron shells.

Thus, knowing the atomic radii helps predict how atoms will interact in terms of bonding, with larger atoms generally participating in bonds more readily due to their more distended electron clouds.