Why Do Elements Show Different Trends in Groups and Periods?
Understanding how various properties of elements change as you move down a group or across a period is a key part of mastering the periodic table for exams like JEE. These trends help us predict the physical and chemical behavior of elements and their compounds. On this page, you’ll find a thorough guide to the "Variation Down a Group and Along a Period," featuring explanations, examples, exceptions, and tables—all designed for clarity and exam readiness.
What is Periodicity in the Periodic Table?
Periodicity refers to the repetition of similar properties at regular intervals when elements are arranged by increasing atomic number. This is due to the recurring electronic configurations in their outermost shells. For example, all alkali metals in Group 1 share a tendency to lose one electron and form +1 ions.
Core Periodic Properties and Their Variations
- Atomic Size (Atomic Radius)
- Ionisation Enthalpy (First Ionisation Energy)
- Electron Affinity (Electron Gain Enthalpy)
- Electronegativity
- Nature of Oxides
Variation of Atomic Size
Atomic size increases as you move down a group. This is because each period adds a new energy shell, making the atom larger. Across a period (left to right), atomic size decreases. The nuclear charge increases, drawing electrons closer without adding a new shell.
| Direction | Trend in Atomic Size | Reason |
|---|---|---|
| Across a Period (Left to Right) | Decreases | Increased nuclear charge pulls electrons closer |
| Down a Group (Top to Bottom) | Increases | Additional energy levels (shells) |
Variation of Ionisation Enthalpy
Ionisation enthalpy is the energy needed to remove an electron from a gaseous atom. It increases across a period because atomic size decreases and nuclear charge increases, holding the electrons more tightly. Down a group, it decreases due to increased atomic size and greater shielding effect, so electrons are held less tightly.
- Exception: Elements with half-filled or fully filled subshells (e.g., Be and N) have higher than expected ionisation enthalpy.
Variation of Electron Affinity
Electron affinity is the energy released when an atom gains an electron. It generally increases across a period due to increasing nuclear charge and decreasing atomic size, which means a greater tendency to gain electrons. Down a group, electron affinity usually decreases because atoms become larger and the added electron is less attracted to the nucleus.
- Exception: Electron affinity of O and F is less negative than S and Cl, due to the high electron-electron repulsion in their small n=2 quantum level.
Variation of Electronegativity
Electronegativity measures an atom’s ability to attract electrons in a bond. It follows the same trend as ionisation enthalpy: increases across a period and decreases down a group.
Nature of Oxides: Acidic, Basic, and Amphoteric Character
The type of oxide formed by an element reflects its metallic or non-metallic character. As you move from left to right across a period, the oxides become less basic and more acidic. Down a group, basic character increases.
| Period (Left to Right) | Group (Top to Bottom) |
|---|---|
| Basic → Amphoteric → Acidic | Basic character increases |
| e.g., Na2O (strongly basic) to Cl2O7 (strongly acidic) | e.g., NaOH is less basic than RbOH or CsOH |
Key Points for JEE
- Trends are due to electron configuration, nuclear charge, shielding effect, and sublevel filling.
- Group 17 Elements (halogens) show high electron affinity and form acidic oxides.
- Group 13 Elements like boron form amphoteric oxides.
- Group 15 Elements show unique ionisation energy exceptions due to half-filled stability.
- Periodicity also affects melting/boiling points, metallic/non-metallic behavior, and chemical reactivity.
Example: Third Period Oxides
| Element | Oxide | Nature |
|---|---|---|
| Na | Na2O | Strongly Basic |
| Mg | MgO | Basic |
| Al | Al2O3 | Amphoteric |
| Si | SiO2 | Weakly Acidic |
| P | P2O5 | Acidic |
| S | SO3 | Strongly Acidic |
| Cl | Cl2O7 | Extremely Acidic |
Common Questions & Answers
-
How does melting point of alkali metal chlorides vary down the group?
It decreases as lattice energy falls due to increasing ionic size. The correct order is NaCl > KCl > CsCl > LiCl.
-
Which s-block elements are liquids at or near room temperature?
Caesium (Cs) and Francium (Fr) are liquids between 25°C and 40°C because of their large atomic sizes and weak metallic bonds.
Summary
The periodic trends in atomic size, ionisation enthalpy, electron affinity, and oxide nature determine much of an element’s physical and chemical behavior. Grasping these variations helps you understand group-wise and period-wise similarities and differences, making the periodic table logical rather than just for memorization. For more specific trends in main group chemistry, review the pages on Group 14 Elements and Group 16 Elements. Vedantu’s resources are built to make these concepts clear and exam-ready for Indian school students.
FAQs on Trends of Elements: Variation Down a Group and Across a Period
1. What is the variation of atomic radius down a group and across a period?
Atomic radius increases down a group and decreases across a period.
• Down a group, each element has an extra energy shell, so size increases.
• Across a period, more protons pull electrons closer, decreasing atomic size.
This trend is important for understanding periodic properties in the modern periodic table.
2. Why does ionization energy decrease down a group?
The ionization energy decreases down a group due to increased atomic size and shielding effect.
• Outer electrons are farther from the nucleus.
• Shielding by inner electrons makes it easier to remove electrons.
This explains why alkali metals at the bottom of the group lose electrons more easily.
3. How does electronegativity change along a period and down a group?
Electronegativity increases across a period and decreases down a group.
• Across a period, atoms have a greater pull for electrons due to increased nuclear charge.
• Down a group, increased distance and shielding reduce the nucleus’s pull.
This affects how elements form chemical bonds in compounds.
4. What is the trend of metallic character in the periodic table?
The metallic character increases down a group and decreases along a period.
• Down a group, elements lose electrons more easily.
• Along a period, elements gain electrons more easily and become nonmetallic.
Understanding this helps to identify metals, nonmetals, and metalloids.
5. Explain the variation of valency in a group and period.
The valency of elements remains the same down a group but changes progressively across a period.
• In a group, elements have the same number of valence electrons.
• In a period, the number of valence electrons increases from 1 to 8.
This trend determines chemical reactivity and bonding characteristics.
6. What causes the decrease in atomic size across a period?
The atomic size decreases across a period because nuclear charge increases while electrons fill the same shell.
• More protons increase the pull on electrons.
• Electrons are drawn closer to the nucleus.
This results in smaller atomic radii for elements toward the right.
7. Why do elements in the same group have similar chemical properties?
Elements in the same group have the same valence electron configuration, which leads to similar chemical properties.
• All elements in a group react in comparable ways.
• Their outer shell electrons are alike, influencing reactivity and bond formation.
8. State the reason for the increase in non-metallic character across a period.
The non-metallic character increases across a period due to higher electronegativity and decreasing atomic size.
• Atoms attract electrons more strongly.
• They tend to gain rather than lose electrons during reactions.
This is clearly observed from left to right in any period.
9. How does the reactivity of alkali metals vary down the group?
The reactivity of alkali metals increases down the group.
• Ease of losing the single valence electron rises.
• Larger atomic size and increased shielding make reactions faster.
Thus, cesium is more reactive than sodium or lithium.
10. What is the variation of electron affinity in the periodic table?
Electron affinity generally becomes more negative (increases) across a period and less negative (decreases) down a group.
• Left to right: Higher nuclear charge, easier to add electrons.
• Top to bottom: More shielding, harder to gain electrons.
This influences element behavior in chemical reactions.
11. Why does the atomic size increase down the group?
Atomic size increases down the group because every step adds a new electron shell.
• New shells make atoms bigger.
• Increased shielding reduces nuclear pull on outer electrons.
This is seen in groups such as the alkali metals and halogens.
12. How does the trend of periodic properties help in predicting the properties of elements?
Periodic trends like atomic size, ionization energy, and electronegativity allow scientists to predict element properties.
• Similar trends indicate similar behaviors.
• New element properties can be forecasted based on position in the periodic table.
These trends make the study of periodic classification vital in chemistry.
