• 当然とうぜん — naturally / as a matter of course
• 自然しぜん — nature
• 偶然ぐうぜん — coincidence / chance
• 必然ひつぜん — necessity / inevitability
• 全然ぜんぜん — completely / totally
• 突然とつぜん — suddenly

The character 然 likely depicts something “as it is,” historically analyzed as a piece of meat over fire (⺼ + 灬), and now functions widely in compounds to express state, naturalness, or “thus/so.”

彼かれは 自然しぜんが好すきです。
He loves nature.

偶然ぐうぜんに彼女かのじょと会あいました。
I ran into her by chance.

当然とうぜんの結果けっかです。
It’s the natural result.