Question

An element with atomic number 20 will be placed in which period of the periodic table :

• A. 4
• B. 3
• C. 2
• D. 1

Hint:

To find the period of an element with a given atomic number (Z): 1. Write down the electronic configuration by filling shells (K, L, M, N...). Shell capacities (maxima, but Bohr-Bury rules apply for filling order): K=2, L=8, M=8 (initially, then up to 18), N=2 (then up to 8, then 18, then 32). 2. For Z=20: K-shell: 2 electrons L-shell: 8 electrons (Total 10) M-shell: 8 electrons (Total 18) N-shell: 2 electrons (Total 20) Configuration: 2, 8, 8, 2. 3. The number of shells used is the period number. Here, 4 shells (K,L,M,N) are used. So, the element is in the 4th period.

Answer 1

The Correct Option is A

Concept: The period number in the periodic table corresponds to the highest principal energy level (electron shell) occupied by electrons in an atom of that element. The atomic number (Z) equals the number of electrons in a neutral atom. Step 1: Determine the number of electrons The atomic number of the element is 20. For a neutral atom, the number of electrons = atomic number = 20. Step 2: Write the electronic configuration (distribution of electrons in shells) Electrons fill shells in the order K, L, M, N... Maximum electrons per shell:
K-shell (n=1): Can hold up to 2 electrons.
L-shell (n=2): Can hold up to 8 electrons.
M-shell (n=3): Can hold up to 18 electrons (but often fills to 8 before the N-shell starts, as per Bohr-Bury rules for simpler configurations).
N-shell (n=4): Can hold up to 32 electrons. Let's fill the 20 electrons:
K-shell: 2 electrons (Remaining electrons: \(20-2=18\))
L-shell: 8 electrons (Remaining electrons: \(18-8=10\))
M-shell: According to the Bohr-Bury scheme, the M-shell will take up to 8 electrons before the N-shell begins to fill if the M-shell is the outermost or penultimate shell being filled for these lighter elements. So, 8 electrons go into the M-shell. (Remaining electrons: \(10-8=2\))
N-shell: The remaining 2 electrons will go into the N-shell. So, the electronic configuration is 2 (K), 8 (L), 8 (M), 2 (N). The handwritten note "2+8+8+2" on the image confirms this configuration. Step 3: Identify the period number The period number is determined by the highest principal energy level (shell) that contains electrons. In this configuration (2, 8, 8, 2):
K-shell is the 1st shell.
L-shell is the 2nd shell.
M-shell is the 3rd shell.
N-shell is the 4th shell. Since electrons are present in the N-shell (the 4th shell), the element belongs to the 4th period. (The element with atomic number 20 is Calcium, Ca).