Why does resistance of a metallic conductor increase with increase in temperature?

When a metallic conductor is heated, the atoms in the metal vibrate with greater amplitude and frequency. Due to increase in temperature, the thermal velocities of free electrons also increases. Therefore, the number of collisions between free electrons and atoms increases. This increases the opposition to the movement of electrons and hence the resistance of the conductor.

Valence shell where the valence electrons are present which are loosely held and are easily removed from the metal atom.

The rest part of the atom which consists of the nucleus and all the shells except the valence shell is called the kernel. According to electron gas theory, a metal is a sea of valence electrons where the positively charged kernels are dipped or immersed. The valence electrons can flow throughout the body of the metal.

With the increase in temperature the kernels vibrate with greater amplitude and creates greater obstacles to the path of free electrons .i. e. the metal’s resistance to the movement of electrons increase.