Day7

Day 7 note

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Introduction

About 50% of the mass of the earth's crust is oxygen.

Large part of plants and animals contain oxygen in combination with carbon, hydrogen and nitrogen.

Roughly 96% of(by mass) of human body is made up of just four elements: oxygen, carbon, hydrogen and nitrogen.

Astatine is a radioactive halogen. Halogens are the most reactive non-metals. They react with most metals to form salt.

Halogen means salt former in Greek.