Short Answer

Periodic trends like atomic size, ionization energy, electronegativity, and electron affinity directly influence chemical reactivity.

• Across a period, reactivity of metals decreases while reactivity of non-metals increases due to higher nuclear charge and smaller atomic radius.
• Down a group, metals become more reactive as atomic size increases, and non-metals become less reactive as electron attraction decreases.

Detailed Explanation :

Definition of Periodic Trends

Periodic trends are patterns observed in properties of elements across periods and down groups in the periodic table. These include atomic size, ionization energy, electronegativity, electron affinity, and metallic/non-metallic character. These trends determine how easily an element can gain, lose, or share electrons, which directly affects chemical reactivity.

Effect of Periodic Trends on Metals

1. Atomic Size (Radius):
   • Metals are more reactive when the atomic radius is large, as outer electrons are far from the nucleus and easier to remove.
   • Example: In Group 1, cesium is more reactive than lithium because cesium’s valence electron is farther from the nucleus.
2. Ionization Energy:
   • Metals with low ionization energy lose electrons easily → high reactivity.
   • Across a period, ionization energy increases → metal reactivity decreases.
   • Down a group, ionization energy decreases → metal reactivity increases.
3. Electronegativity and Electron Affinity:
   • Metals have low electronegativity → less tendency to attract electrons.
   • Elements with very low electronegativity tend to react vigorously with non-metals.

Effect of Periodic Trends on Non-Metals

1. Atomic Size (Radius):
   • Smaller non-metal atoms have electrons closer to the nucleus, so they can attract additional electrons easily → higher reactivity.
   • Example: Fluorine is more reactive than iodine due to smaller atomic radius.
2. Electronegativity:
   • Non-metals with high electronegativity strongly attract electrons during chemical reactions → high reactivity.
   • Across a period, electronegativity increases → reactivity of non-metals increases.
3. Electron Affinity:
   • High electron affinity in non-metals favors formation of anions, enhancing chemical reactivity.
4. Group Trend:
   • Down a group, atomic size increases → electron attraction decreases → non-metal reactivity decreases.

Examples of Reactivity Trends

1. Alkali Metals (Group 1):
   • Reactivity increases down the group: Li < Na < K < Rb < Cs
   • Larger atoms → valence electron is loosely held → easily lost.
2. Halogens (Group 17):
   • Reactivity decreases down the group: F > Cl > Br > I
   • Smaller atoms → stronger pull on electrons → highly reactive.
3. Across a Period:
   • Sodium (metal) is more reactive than magnesium → easier to lose one electron.
   • Chlorine (non-metal) is more reactive than sulfur → easier to gain an electron.

Role of Metallic and Non-Metallic Character

1. Metallic Character:
   • Metals tend to lose electrons → reactivity increases with larger atomic size and lower ionization energy.
2. Non-Metallic Character:
   • Non-metals tend to gain electrons → reactivity increases with smaller atomic size, higher electronegativity, and higher electron affinity.
3. Transition Across Periods:
   • Metal reactivity decreases while non-metal reactivity increases due to changes in atomic size, nuclear charge, and electron configuration.

Conclusion

Periodic trends such as atomic radius, ionization energy, electronegativity, and electron affinity play a crucial role in determining chemical reactivity. Metals are more reactive when atomic size is large and ionization energy is low, whereas non-metals are more reactive when atomic size is small and electronegativity is high. Understanding these trends helps predict behavior of elements in chemical reactions, explaining patterns in reactivity across periods and down groups in the periodic table.