20. Which of the following statements regarding subshell filling order for a neutral atom is/are CORRECT? 1. Electrons are assigned to the 4f subshell before they are assigned to the 6s subshell. 2. Electrons are assigned to the 5d subshell before they are assigned to the 6p subshell. 3. Electrons are assigned to the 4p subshell before they are assigned to the 4s subshell. a. 1 only b. 2 only c. 3 only d. 1 and 3 e. 1, 2, and 3

2 only

Explanation:

Electrons are filled in atoms according to the Aufbau principle. Electrons are filled into lower energy orbital before the filling of higher energy orbitals and this sequence must be followed in filling electron orbitals.

The order of arrangement of energy levels may be shown as follows; 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p. This implies that 4f>6s, hence 6s is filled before 4f.

Also, the 6p level > 5d level hence this is the correct option. You must fill the 5d level before you feel the 6p level.

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