Read the text and complete the sentencesAll substances, solids , liquids or gases, are composed of one or more of the chemical element s. Each element is composed of identical atoms. Each atom is composed of a small central nucleus consisting of protons and neutrons around which orbit shells of electrons. These electrons are very much smaller than protons and neutrons.The electrons in the outermost shell are called valence electrons and the electrical properties of the substance depend on the number of these electrons. Neutrons have no electric charge, but protons have a positive charge while electrons have a negative charge. In some substances, usually metals, the valence electrons are free to move from one atom to another and this is what constitutes an electric current.