Write the number of periods the modern periodic table has. How do the valency and metallic character of elements vary on moving from left to right in a period? How do the valency and atomic size of elements vary down a group?

Q: Write the number of periods the modern periodic table has. How do the valency and metallic character of elements vary on moving from left to right in a period? How do the valency and atomic size of elements vary down a group?

1. **Number of Periods:** The Modern Periodic Table has **7 periods** (horizontal rows). 2. **Trends Across a Period (Left to Right):** * **Valency:** On moving from left to right in a period, the valency of elements first increases from $1$ to $4$ and then decreases to $0$. * **Metallic Character:** On moving from left to right in a period, the metallic character of elements **decreases**. This is because the effective nuclear charge acting on the valence electrons increases, making it harder for atoms to lose electrons and form positive ions. 3. **Trends Down a Group:** * **Valency:** Down a group, the valency **remains constant** (the same) because all elements in a group have the same number of valence electrons. * **Atomic Size:** Down a group, the atomic size **increases**. This is because new electronic shells are added at each step down the group, which increases the distance between the outermost electrons and the nucleus, overcoming the increase in nuclear charge.

Metals and Non-metals

Periodic Trends in Modern Periodic Table

Official Answer

Number of Periods:

The Modern Periodic Table has 7 periods (horizontal rows).

Trends Across a Period (Left to Right):

Valency: On moving from left to right in a period, the valency of elements first increases from $1$ to $4$ and then decreases to $0$ .
Metallic Character: On moving from left to right in a period, the metallic character of elements decreases. This is because the effective nuclear charge acting on the valence electrons increases, making it harder for atoms to lose electrons and form positive ions.

Trends Down a Group:

Valency: Down a group, the valency remains constant (the same) because all elements in a group have the same number of valence electrons.
Atomic Size: Down a group, the atomic size increases. This is because new electronic shells are added at each step down the group, which increases the distance between the outermost electrons and the nucleus, overcoming the increase in nuclear charge.

7 periodsvalency first increases then decreasesmetallic character decreasesvalency remains constantatomic size increasesaddition of new shells

Marking Scheme

1Stating that there are 7 periods (0.5 marks)
2Explaining the variation of valency across a period (increases 1 to 4, then decreases to 0) (0.5 marks)
3Explaining the variation of metallic character across a period (decreases) (0.5 marks)
4Explaining the variation of valency down a group (remains same) (0.5 marks)
5Explaining the variation of atomic size down a group (increases with reason) (1 mark)

Hint

Recall that metallic character is the ability to lose electrons easily, which is highest for elements on the far left (like alkali metals) and decreases as you move towards non-metals on the right.

Quick Oral Answer

The modern periodic table has 7 periods. Across a period, valency first increases then decreases, while metallic character decreases. Down a group, valency remains the same, and atomic size increases.

Analysis & Explanation

Let's break down the trends:

Periods: There are 7 horizontal rows in the periodic table.
Across a Period (Left to Right):
Valency depends on the number of valence electrons. For Groups 1 to 14, it increases from 1 to 4. For Groups 15 to 18, it decreases from 3 to 0 (Valency = $8 - \text{valence electrons}$ ).
Metallic character is the tendency to lose electrons. As we move right, the nuclear charge increases and atomic size decreases, so electrons are held more tightly, decreasing metallic character. This is why metals are on the left and non-metals are on the right of the periodic table.
Down a Group:
Valency remains the same because the number of valence electrons is constant (e.g., all alkali metals like $\text{Na}$ and $\text{K}$ have a valency of 1).
Atomic size increases because each successive period adds a new energy shell, pushing the valence shell further from the nucleus.

Common Mistakes

1Stating that metallic character increases across a period.
2Confusing atomic size trend down a group, thinking it decreases due to higher nuclear charge.

Interesting Facts

The decrease in metallic character across a period explains why we transition from highly reactive metals (like sodium) on the left, to metalloids (like silicon), and finally to non-metals (like chlorine) and noble gases on the far right.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2013?

This question carries 3 marks in the CBSE Class 10 Science 2013 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Metals and Non-metals" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Periodic Trends in Modern Periodic Table" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2013 paper?

This is a Short Answer question from Section A in the CBSE Class 10 Science 2013 paper.