what is the difference between a cell and a an atom

The cell is the basic unit of life: it is the smallest entity that can be considered a living thing. Atoms themselves, however, are not alive. But cells could not be cells without atoms.
Atoms are the building blocks of everything you see (or can't see!) around you. The Periodic Table of Elements lists all known types of atoms. These different types of atoms, called elements, have very different ways of behaving. They can interact with each other and bond together to form molecules (2 or more atoms).

Millions of certain types of molecules combine in specific ways to make the four types of macromolecules found in all cells: proteins, nucleic acid (DNA, etc.), carbohydrates (sugar, starch), and lipids (fat and oils). These larger building blocks, made up of atoms, combine in very specific ways to form a cell.
In addition, a cell can be seen with a microscope, whereas individual atoms cannot.

Cells are basically the smallest building block of living tissues. Put enough of the right cells together and you have biological matter. Some cells could be seen under a strong magnifying glass, most can only be seen under a microscope.

Atoms, on the other hand, are usually thought of as the smallest building block of an element - even if that element is a hunk of copper or the helium in a balloon. You cannot see individual atoms with a magnifying glass or even a powerful optical microscope. In fact, no one has actually "seen" an atom with their eyes - scientists have to study them using techniques that leave traces behind or trigger instuments.

When individual atoms (which make up an element) combine with others, this forms a molecule. Molecules of various substances are present in large numbers even in the smallest cell. So, atoms are much, much smaller than cells.