A student was asked to give the electron configuration for the element Iodine that has 53 electrons. The answer was given as [Ar] 5s2 4d10 5p5 His answer is not correct. Explain

the electron configuration:

1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁶, 5s², 4d¹⁰, 5p⁶, etc
Ar = [Ne] 3s² 3p⁶ ( 18)
Iodine = 53 = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁶, 5s², 4d¹⁰, 5p⁵

if use Ar (18) ⇒ [Ar] 4s², 3d¹⁰, 4p⁶, 5s², 4d¹⁰, 5p⁵

if use Kr (36) ⇒ [Kr] 5s², 4d¹⁰, 5p⁵

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.

un recipient conține 6,4 moli de gaz. gazul hidrogen reprezintă 25% din totalul kilometrilor din container. în cazul în care presiunea totală este de 1.24 atm ceea ce este