Which part of an atom is involved in a chemical reaction

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Write the chemical symbols for three different atoms or atomic cations with 27 electrons. asap!

Indicate whether the specified alkyl halides will form primarily substitution products, only elimination products, both substitution and elimination products, or no products when they react with sodium methoxide. 1-bromobutane 1-bromo-2-methylpropane 2-bromobutane 2-bromo-2-methylpropane