Question

How do I find electron configuration using only the periodic table and what are some strategies?

Answer 1

Using an ADOMAH Periodic Table :-

1.Understand the ADOMAH Periodic Table. This method of writing electron configurations doesn't require memorization. However, it does require a rearranged periodic table, because in a traditional periodic table, beginning with 4th row, period numbers do not correspond to the electron shells. Find an ADOMAH Periodic Table, a special type of periodic table

1. • In the ADOMAH Periodic Table, horizontal rows represent groups of elements, such as halogens, inert gases, alkali metals, alkaline earths, etc. Vertical columns correspond to electron shells and so called “cascades” (diagonal lines connecting s,p,d and f blocks) correspond to periods.
   • Helium is moved next to Hydrogen, since both of them are characterized by the 1s orbital. Blocks of periods (s,p,d and f) are shown on the right side and shell numbers are shown at the base. Elements are presented in rectangular boxes that are numbered from 1 to 120. These numbers are normal atomic numbers that represent total number of electrons in a neutral atom.

2. Find your atom in the ADOMAH table. To write electron configuration of an element, locate its symbol in ADOMAH Periodic Table and cross out all elements that have higher atomic numbers. For example, if you need to write electron configuration of Erbium (68), cross out elements 69 through 120.

   • Notice numbers 1 through 8 at the base of the table. These are electron shell numbers, or column numbers. Ignore columns which contain only crossed out elements. For Erbium, remaining columns are 1,2,3,4,5 and 6.
3. Count orbital sets up to your atom. Looking at the block symbols shown on the right side of the table (s, p, d, and f) and at the column numbers shown at the base and ignoring diagonal lines between the blocks, break up columns into column-blocks and list them in order from the bottom up. Again, ignore column blocks where all elements are crossed out. Write down the column-blocks beginning with the column number followed by the block symbol, like this: 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 6s (in case of Erbium).
   • Note: The above electron configuration of Er is written in the order of ascending shell numbers. It could also be written in the order of orbital filling. Just follow cascades from top to bottom instead of columns when you write down the column-blocks: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹².

4. Count electrons for each orbital set. Count elements that were not crossed out in each block-column, assigning 1 electron per element, and write down their quantity next to the block symbols for each block-column, like this: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹² 5s² 5p⁶ 6s². In our example, this is the electron configuration of Erbium.

5. Know irregular electron configurations. There are eighteen common exceptions to electron configurations for atoms in the lowest energy state, also called the ground state. They deviate from the general rule only by last 2 to 3 electron positions. In these cases, the actual electron configuration keeps the electrons in a lower-energy state than in a standard configuration for the atom. The irregular atoms are:
   • Cr (..., 3d5, 4s1); Cu (..., 3d10, 4s1); Nb (..., 4d4, 5s1); Mo (..., 4d5, 5s1); Ru (..., 4d7, 5s1); Rh (..., 4d8, 5s1); Pd (..., 4d10, 5s0); Ag (..., 4d10, 5s1); La (..., 5d1, 6s2); Ce (..., 4f1, 5d1, 6s2); Gd (..., 4f7, 5d1, 6s2); Au (..., 5d10, 6s1); Ac (..., 6d1, 7s2); Th (..., 6d2, 7s2); Pa (..., 5f2, 6d1, 7s2); U (..., 5f3, 6d1, 7s2); Np (..., 5f4, 6d1, 7s2) and Cm (..., 5f7, 6d1, 7s2).