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Period 4 element with highest energy level filled - krypton
Elements in a group are chemically similar to each other.
The periodic table is an arrangement of elements in groups and periods. The elements in the same group share a lot of chemical similarity with each other. The elements that are in the same period only have the same number of valence shells.
An atom is most stable if its outermost energy level contains as many electrons as it can hold. For example, helium has two electrons, both in the first energy level. This energy level can hold only two electrons, so helium's only energy level is full. This makes helium a very stable element.
Valence electrons are the highest energy electrons in an atom and are therefore the most reactive.
To learn more about elements in periodic table from given link
https://brainly.com/question/14658497
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