Electron Configurations from the Periodic Table (a) Based on its position in the periodic table, write the condensed electron configuration for bismuth, element 83. (b) How many unpaired electrons does a bismuth atom have? A certain atom has an [noble gas]5s24cf105p4 electron configuration. Which element is it? (a) Cd (b) Te (c) Sm (d) Hg (e) More information is needed Solution: Here, we are going to identify the element having electronic configuration [noble 2? 10? 4 gas]5s? 4d? 5p?
Solution 1PE Step1: The distribution of electrons in different orbitals is known as electronic configuration of the atom. The filling of orbitals are governed by the following rules: a) Aufbau principle: The Aufbau principle states that in the ground state(lowest energy state) of an atom, an electron enters the orbital of lowest energy first and subsequent electrons are fed in the order of increasing energies. The orbital should be filled in the following sequence: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, ………. b) Pauli’s exclusion principle: According to this principle an orbital can accommodate maximum of two electrons and these must have opposite spins. c) Hund’s rule of maximum multiplicity: According to this rule, electron pairing will not take place in orbitals of same energy(subshell) until each orbital is singly filled. For example, there are three p- orbitals(p , p and px) y the p zsubshell in a principal energy level. According to Hund’s rule, each of the three p-orbitals must get one electron of parallel spin before any one of them receives the second electron of opposite spin. The three rules are applied simultaneously to get the electronic configuration. In writing the electronic configuration, an orbital is shown by using its proper symbol.The number of electrons present in an orbital is shown as the right-hand superscript of the symbol of the orbital. 1 For example, the electronic configuration of hydrogen is 1s . This means that the single electron in hydrogen occupies the s orbital present in the principal energy level n = 1. Similarly the electronic configuration of other elements are written. Furthermore, for simplicity a common convention is used. In this, the detailed electronic configuration of the noble gas core preceding the valence shell is represented by the symbol of the noble gas in square brackets. Then, the configuration of the valence shell is written after the symbol. For example, the electronic configuration of sodium may be written as Na(Z = 11) : [Ne]3s . 1 Step2: Given, the electronic configuration of the element is [noble gas]5s 4d 5p 2 10 4 Looking at the sequence of the orbitals to be filled, as given by Aufbau, the complete electronic configuration of the element is 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 2 10 4 Thus, total number of electrons in the element = 2+2+6+2+6+2+10+6+2+10+4 = 52 The element having atomic number 52 is T ellurium(Te). Hence, the correct option is (b). ----------------- 2 10 4 o identify the element having electronic configuration [noble gas]5s 4d 5p