All atoms have two parts - a positively charged nucleus , and negatively charged electrons held together by the positive charge of the nucleus.  In some atoms the electrons are tightly held together and in some the electrons in the valence shell (which is the outer shell of an atom) are loosely bound , hence can be easily knocked off when an electric charge passes. Electrons are responsible  in the electrical conductivity in solids. 
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atoms consist of a heavy central portion called the nucleus containing neutrons and protons ..lighter particles are called electrons they revolve around the nucleus .....neutrons are neutral...protons are positively charged and electrons are negatively charged .. 
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