Why does atomic radius decrease across a period, but increase down a group?

Across a period, from left to right, the number of protons in the nucleus increases, leading to a higher effective nuclear charge ($Z_{\text{eff}}$). Although electrons are added to the same principal shell, the increased nuclear pull draws the electron cloud closer to the nucleus, resulting in a decrease in atomic radius. Down a group, new electron shells are added with each successive element. These new shells are further from the nucleus, and the inner electrons provide a significant shielding effect, causing the atomic radius to increase despite the increasing nuclear charge.

What is the difference between ionization enthalpy and electron gain enthalpy?

Ionization enthalpy (IE) is the energy required to *remove* an electron from an isolated gaseous atom. It's always an endothermic process (energy is absorbed). Electron gain enthalpy (EGE) is the energy change when an electron is *added* to an isolated gaseous atom. It can be exothermic (energy released, negative value) or endothermic (energy absorbed, positive value), depending on the element's stability upon gaining an electron.

Why is the electron gain enthalpy of chlorine more negative than that of fluorine?

This is a key exception. While fluorine is smaller and more electronegative, its very compact 2p subshell experiences significant electron-electron repulsion when an incoming electron is added. Chlorine, being larger, has a more diffuse 3p subshell, which can accommodate the incoming electron with less inter-electronic repulsion. This makes the electron addition process more favorable (more exothermic, hence more negative EGE) for chlorine than for fluorine.

Explain the concept of effective nuclear charge ($Z_{\text{eff}}$).

Effective nuclear charge ($Z_{\text{eff}}$) is the net positive charge experienced by an electron in a multi-electron atom. It's not the full nuclear charge (Z) because inner electrons shield the outer electrons from the nucleus's full attractive force. $Z_{\text{eff}}$ is calculated as $Z - S$, where S is the shielding constant. A higher $Z_{\text{eff}}$ means the outer electrons are more strongly attracted to the nucleus, influencing properties like atomic size and ionization enthalpy.

How does metallic character change across a period and down a group?

Metallic character refers to an element's tendency to lose electrons and form positive ions. Across a period, metallic character decreases because the effective nuclear charge increases, making it harder for atoms to lose electrons. Down a group, metallic character increases because atomic size increases, and the outermost electrons are further from the nucleus and experience less attraction, making them easier to remove.

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Periodic Trends in Properties

Chemistry

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Version 1Updated 21 Mar 2026

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Periodic trends refer to the systematic and predictable variations in the physical and chemical properties of elements as one moves across a period or down a group in the modern periodic table. These trends are a direct consequence of the periodic recurrence of similar electronic configurations in the outermost shells of elements, which in turn dictates their chemical behavior and physical charact…

Quick Summary

Periodic trends are systematic variations in elemental properties across periods and down groups in the periodic table, driven by changes in electronic configuration, effective nuclear charge ( $Z_{\text{eff}}$ ), and shielding effect.

Key trends include atomic and ionic radii, which generally decrease across a period (due to increasing $Z_{\text{eff}}$ ) and increase down a group (due to added shells). Ionization enthalpy, the energy to remove an electron, generally increases across a period and decreases down a group.

Electron gain enthalpy, the energy change upon adding an electron, typically becomes more negative across a period and less negative down a group, with notable exceptions like chlorine having a more negative value than fluorine.

Electronegativity, an atom's ability to attract shared electrons, increases across a period and decreases down a group, with fluorine being the most electronegative. Metallic character decreases across a period and increases down a group, while non-metallic character shows the opposite trend.

These patterns are crucial for predicting chemical behavior.

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Key Concepts

Effective Nuclear Charge (

Z_{\text{eff}}

) and its Impact

The effective nuclear charge ( $Z_{\text{eff}}$ ) is the actual positive charge felt by an electron from the…

Ionization Enthalpy (IE) Trends and Exceptions

Ionization enthalpy (IE) is the energy required to remove the most loosely bound electron from a gaseous…

Ionic Radii of Isoelectronic Species

Isoelectronic species are atoms or ions that have the same number of electrons. When comparing their ionic…

Atomic Radius: —

\downarrow

Group (increases),

\rightarrow

Period (decreases). \n- Ionic Radius: Cations < Parent atom; Anions > Parent atom. Isoelectronic: Size

\propto 1/Z

. \n- Ionization Enthalpy (IE):

\downarrow

Group (decreases),

\rightarrow

Period (increases). Exceptions: G13 < G2, G16 < G15. \n- Electron Gain Enthalpy (EGE):

\downarrow

Group (less negative),

\rightarrow

Period (more negative). Exceptions: Noble gases (positive), G2/G15 (positive/near zero), Cl > F (more negative). \n- Electronegativity:

\downarrow

Group (decreases),

\rightarrow

Period (increases). F is highest. \n- Metallic Character:

\downarrow

Group (increases),

\rightarrow

Period (decreases). \n- Non-metallic Character:

\downarrow

Group (decreases),

\rightarrow

Period (increases). \n- Oxide Nature: Basic (metals)

\rightarrow

Amphoteric (metalloids)

\rightarrow

Acidic (non-metals) across period.

All Ions Except Noble Gases Exhibit Regular Trends: \n\nAtomic Radius: Increases Down, Decreases Across. \nIonization Energy: Decreases Down, Increases Across.

\nElectron Gain Enthalpy: Less Negative Down, More Negative Across. \nNegativity (Electronegativity): Decreases Down, Increases Across. \n\n*Remember F is the most electronegative, and Cl has more negative EGE than F!

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