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1. The reason electrons are not included in the mass of an atom is that the mass of an electron is so small that it wouldn't do much to the atomic mass of an atom anyway.

2. The chemical symbol for tin is: Sn

3. Tellurium is a metalloid

Explanation:

the number of protons in the nucleus of a given element. has slight luster and conducts electricity at high temperatures like metal.but like a nonmetal,boron is brittle and does not conduct electricity well at low temperatures. in graphite, carbon is a soft,brittle solid.it is reflective,ductile,or malleable.