The flow of electrical current is possible because electrons are

able to move from one atom to another; the atom that loses an electron becomes positively charged and the atom that gains an electron becomes negatively charged
charged particles that move freely between all atoms; when an atom loses an electron it becomes negatively charged
negatively charged and can move between atoms; the atom that loses an electron becomes negatively charged and the atom that gains an electron becomes positively charged
positively charged particles that when transferred between different atoms create ions that are positively charged