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Atoms are composed of electrons. These electrons are arranged in shells. The number of valence electrons (outermost shell electrons) determine the kind of bonds that can be formed in a molecule.
In a molecule, charges are distributed in a unique way and the charge distribution in a molecule determines how the molecule interacts with other molecules. Different molecules can be made by arranging the same basic atoms in different ways.
Harmful substances can be formed when atoms of elements combine in new ways in the environment.
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