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4th Period has 18 Elements from K to Kr (atomic nos. 19 to 36) and is the first long Period. The first two and last size elements resemble the elements above them in the 2nd and 3rd Periods. The Ten Elements Candium (21) to Zinc (30) called transition elements have no Counterparts in the 2nd and 3rd Periods. This is the first transition series.
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