When current flows through a conductor, it is said that the flowing electrons collide with the molecules or atoms of that conductor which causes resistance. The collision of electrons with molecules seems confusing and unclear to me. What is actually happening there, and how does the electron collide with molecules or atoms?
When electric current in a material is proportional to the voltage across it, the material is said to be "ohmic", or to obey Ohm's law. A microscopic view suggests that this proportionality comes from the fact that an applied electric field superimposes a small drift velocity on the free electrons in a metal. For ordinary currents, this drift velocity is on the order of millimeters per second in contrast to the speeds of the electrons themselves which are on the order of a million meters per second. Even the electron speeds are themselves small compared to the speed of transmission of an electrical signal down a wire, which is on the order of the speed of light, 300 million meters per second.
The scatters are random, dependent on the electric and magnetic fields in the lattice where the electron is essentially free in metals.
When one goes to the particle level, in this case electrons in the lattice of a conductor, one has to realize that it is quantum mechanics that describes the actual scatters.
It is a many body problem, and the model used is the "band theory of solids" which takes into account the energy levels in a solid:
A useful way to visualize the difference between conductors, insulators and semiconductors is to plot the available energies for electrons in the materials. Instead of having discrete energies as in the case of free atoms, the available energy states form bands. Crucial to the conduction process is whether or not there are electrons in the conduction band.
In terms of the band theory of solids, metals are unique as good conductors of electricity. This can be seen to be a result of their valence electrons being essentially free. In the band theory, this is depicted as an overlap of the valence band and the conduction band so that at least a fraction of the valence electrons can move through the material.
Thus in conductors resistivity is very small in insulators very high.
So the "collision" of individual electrons is a change in the energy level while absorbing or giving up energy to the lattice. The larger the gap between the conduction band and the valence band the more energy has to be supplied and the resistance is higher.
Electron collision with atoms and molecules causing resistance to current can be visualised as the river flowing.
River flow is analogous to current flow and the motion of water particles can be analogous to electrons in current flow.
Now, river flows continuously without any resistance but it also contains stones in the path which cause resistance.
See these rocks as the molecules or atoms in conductors. Now, water particles moves random as electron in conductors (thermal motion) and rocks or molecules in conductors causes resistance to its motion. (thermal resistance)
Hope this cleared your doubt. Physics is very fascinating, just relate everything to what you see.
Perhaps part of the issue is the terminology of atom, molecule, and nuclei. A conductor such as a metal can be imagine as a grid of nuclei (protons + neutrons) surrounded by a "sea of electrons"; this model is known as metallic bonding (as opposed to ionic or covalent bonding).
I'm not an expert, but I'm going to guess the electrons are colliding with the nuclei. Imagine a metal atom is a little rock surrounded by a bit of water. Now put billions of them next to each other, and let the water fuse into a river, with a grid of little rocks throughout it. As the water flows (electrical current), it will collide with the little rocks (nuclei), which impart resistance.
So unless somehow some stray molecules somehow wound up inside the metal lattice (not likely), the electrons are probably colliding with "their own" nuclei.