Chapter 7: Problem 27
How does atomic radius change (a) from left to right across a period and (b) from top to bottom in a group?
Short Answer
Expert verified
Atomic radius decreases from left to right across a period and increases from top to bottom within a group.
Step by step solution
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Periodic Table
Understanding the periodic table is crucial for mastering concepts like atomic radius and other properties of elements. The periodic table is a systematic arrangement of elements, organized by atomic number, electron configuration, and recurring chemical properties. The table is structured into rows called periods and columns known as groups or families.
- **Periods (Rows):** As you move across a period from left to right, properties like atomic radius change due to increasing nuclear charge. - **Groups (Columns):** Elements within a group share similar chemical behaviors and properties, mainly because they have the same number of electrons in their outermost shell, which affects the overall atomic radius as you move down a group.
The placement of an element in the periodic table can provide significant insights into its atomic radius and other chemical properties.
- **Periods (Rows):** As you move across a period from left to right, properties like atomic radius change due to increasing nuclear charge. - **Groups (Columns):** Elements within a group share similar chemical behaviors and properties, mainly because they have the same number of electrons in their outermost shell, which affects the overall atomic radius as you move down a group.
The placement of an element in the periodic table can provide significant insights into its atomic radius and other chemical properties.
Effective Nuclear Charge
Effective nuclear charge (
Z_{ ext{eff}}
) plays a vital role in determining atomic radius. It is the net positive charge experienced by an electron in a multi-electron atom. This charge affects how tightly electrons are held within an atom.
- **Increase Across a Period:** As you move from left to right across a period, the effective nuclear charge increases. This is because additional protons in the nucleus increase the overall positive charge, attracting electrons more strongly and pulling them closer to the nucleus. As a result, the atomic radius decreases. - **Shielding Effect:** Even though the number of electrons increases across a period, they don't contribute to a greater pull on outer electrons because they are being added to the same energy level, and their repulsion is minimized by inner electrons.
Understanding Z_{ ext{eff}} helps in explaining why some elements have smaller atomic radii despite having higher atomic numbers.
- **Increase Across a Period:** As you move from left to right across a period, the effective nuclear charge increases. This is because additional protons in the nucleus increase the overall positive charge, attracting electrons more strongly and pulling them closer to the nucleus. As a result, the atomic radius decreases. - **Shielding Effect:** Even though the number of electrons increases across a period, they don't contribute to a greater pull on outer electrons because they are being added to the same energy level, and their repulsion is minimized by inner electrons.
Understanding Z_{ ext{eff}} helps in explaining why some elements have smaller atomic radii despite having higher atomic numbers.
Energy Levels
Energy levels are essentially regions around an atom's nucleus where electrons are most likely to be found. Electrons within an atom occupy different energy levels, or shells, which can be thought of as layers surrounding the nucleus.
- **Increase Down a Group:** When moving down a group in the periodic table, each successive element has more energy levels. This is why the atomic radius increases as you descend through the group. More layers mean a larger overall size, despite the nuclear charge increase. - **Electron Configuration:** Additional energy levels cause outer electrons to be further removed from the nucleus, which affects the radius.
Energy levels are fundamental in determining the size and various chemical characteristics of an atom.
- **Increase Down a Group:** When moving down a group in the periodic table, each successive element has more energy levels. This is why the atomic radius increases as you descend through the group. More layers mean a larger overall size, despite the nuclear charge increase. - **Electron Configuration:** Additional energy levels cause outer electrons to be further removed from the nucleus, which affects the radius.
Energy levels are fundamental in determining the size and various chemical characteristics of an atom.
Proton-Electron Interactions
The interaction between protons (positively charged) and electrons (negatively charged) within an atom is critical for understanding atomic structure.
- **Attraction Force:** Electrons are attracted to the nucleus due to the positive charge of protons. This force is a key factor in determining atomic radius, with greater attractions leading to smaller radii. - **Movement Across a Period:** As you move across a period, the increase in protons enhances this attractive force, pulling electrons closer to the nucleus and decreasing the atomic radius. - **Influence of Added Energy Levels:** In contrast, moving down a group adds new energy levels, lessening this attraction due to the shielding effect, thus resulting in a larger atomic radius.
Proton-electron interactions highlight the balance between attractive forces and the spatial demand of additional energy levels, significantly impacting atomic size and behavior.
- **Attraction Force:** Electrons are attracted to the nucleus due to the positive charge of protons. This force is a key factor in determining atomic radius, with greater attractions leading to smaller radii. - **Movement Across a Period:** As you move across a period, the increase in protons enhances this attractive force, pulling electrons closer to the nucleus and decreasing the atomic radius. - **Influence of Added Energy Levels:** In contrast, moving down a group adds new energy levels, lessening this attraction due to the shielding effect, thus resulting in a larger atomic radius.
Proton-electron interactions highlight the balance between attractive forces and the spatial demand of additional energy levels, significantly impacting atomic size and behavior.
