SF 6 gas has excellent insulating property and it has a high electronegativity. Due to high electronegativity, it has a high affinity for absorbing free electrons. It absorbes electrons to form a negative ions. The attachment of electron with SF 6 molecules may occur in two different reactions as shown below,
Ions are obviousely much heavier than a free electron and therefore over all mobility of charged particles in SF 6 gas is much less as compared to the other common gases.