Question 2 of 48intermediate💡 UnderstandShort Answer2 marks

How would you justify the presence of 18 elements in the 5th period of the Periodic Table?

Correct Answer

For the 5th period, the highest principal quantum number is n=5n=5. For this value of nn, the azimuthal quantum number ll can be 0,1,2,30, 1, 2, 3.

The available orbitals for filling in the 5th period are 5s5s, 4d4d, and 5p5p, and the order of their increasing energy is 5s<4d<5p5s < 4d < 5p.

The total number of orbitals available in these subshells is 9 (one from 5s5s, five from 4d4d, and three from 5p5p). The maximum number of electrons that can be accommodated in these 9 orbitals is 9×2=189 \times 2 = 18. Since each element corresponds to the filling of one electron, there are 18 elements in the 5th period.

Exercise: IN-TEXT QUESTIONS (PROBLEMS) | Q: Problem 3.2 | (Page 9)
For More Understanding

Explanation

The number of elements in a period is determined by the maximum number of electrons that can be accommodated in the orbitals available for that period. For the 5th period, the filling occurs in the 5s, 4d, and 5p orbitals, which provide a total of 9 orbitals, holding up to 18 electrons.

Solution Steps

  1. Step 1: Identify the principal quantum number for the 5th period, which is n=5n=5, giving l=0,1,2,3l=0, 1, 2, 3.

  2. Step 2: Determine the available orbitals and their energy order: 5s<4d<5p5s < 4d < 5p.

  3. Step 3: Calculate the total number of orbitals: 1 (from 5s) + 5 (from 4d) + 3 (from 5p) = 9 orbitals.

  4. Step 4: Calculate the maximum electrons: 9 orbitals × 2 electrons/orbital = 18 electrons, justifying 18 elements in the 5th period.

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