CBSE Class 10 Answered

The electrons in the outermost shell are called valence electrons. When enough external energy, such as heat, light, or electrical energy, is applied to the atoms of certain materials,

some of the valence electrons will leave the atom.   These electrons are called  free  electrons.  

  It  is  the  movement  of  free  electrons  that  provides  electric  current  in  a metal conductor.  

An atom that has lost or gained one or more electrons is said to be ionized or to have an ion change.   If the atom loses one or more electrons, it becomes positively charged and  is  referred  to  as  a  positive  ion.    If  an  atom  gains  one  or  more  electrons,  it  becomes negatively charged and is referred to as a negative ion.