Really! If an atom was the size of a stadium the nucleus would be the size of a pea and it would weigh as much as the stadium. Atoms are about 100,000 times bigger than their nucleus, and nearly all the mass of an atom is in its nucleus--yet almost all the space in an atom is occupied by its electrons.
Since the electron cloud around the nucleus of an atom does not have a sharp edge, the size of an atom by itself is hard to define. But for atoms that can form solid crystal lattices, the distance between the centers of adjacent atoms can be easily determined by x-ray diffraction, which gives us an estimate of the atoms' size. Using this method we find that it would take almost 20 million hydrogen atoms to make a line as long as the dash in the word "x-ray".
Atoms of different elements vary a little in size, but not by much because heavy elements have larger positive charges on their nuclei, which more strongly attracts the electrons to the center of the atom. This contracts the size of the electron cloud, so more electrons fit into a smaller volume. Atoms of lead (atomic weight 207), for example, are about the same size as aluminum atoms (atomic weight about 27), but are about 8 times denser...which is why lead is much heavier than aluminum.
The temperature of a collection of atoms, by the way, is a measure of the motion of those atoms. As the temperature increases, the kinetic energy of the particles increases, and their speed increases. At room temperature, atoms in air move at an average speed of about 1100 mph, and undergo 5 collisions every nanosecond.
p.s. A neutron walks into a bar and asks the price of a drink. The bartender, a nucleus, replies, "For you, no charge." "Are you sure," says the electron? "I'm positive," replies the bartender.