Atoms behave as if they are hard round balls. In fact, they are mostly empty space. At their centre is a very small nucleus which consists of protons and neutrons. The nucleus is surrounded by a swarm of electrons.
In some ways, an atom's electrons are quite well behaved, in other ways, they are unpredictable. The quantum theory says that it only makes sense to describe electrons in terms of probability. This is because the very act of trying to observe an electron knocks it for six. Looking for electrons is like a blind cricketer swinging his bat, he never sees the ball, all he knows is that sometimes he hears the bat hit it and feels the impact. Where the ball has gone is a complete mystery to him.
We do not accept this aspect of quantum mechanics. We believe that electrons are really well behaved and that they move according to the laws of the physics.
One of the first attempts to guess what the inside of an atom looks like pictured the electrons as orbiting the nucleus in the same way the planets orbit the sun. We believe this picture to be essentially correct, but missing one key element. We believe that the orbiting electrons generate strong magnetic fields which wrap around their orbits leaving a small tunnel through which the electrons travel. We believe that the structure of the atom is determined by the way these magnetic fields fit together.
Atoms join together to form molecules. The smallest molecules have just two atoms, but some molecules like those found in plastics and living things contain thousands, even millions of atoms. Atoms can also join together to form crystals as in salt or iron.
Atoms stick together because they swap or share electrons. Salt is made up of two types of atom, sodium and chlorine. Sodium atoms have a lone electron orbiting outside the main ball of orbiting electrons. Chlorine atoms are just one electron short of filling their electron structure, so when they meet, the lonely electron of the sodium atom jumps ship and fills the gap in the chlorine atom's structure. This leaves the two atoms electrically charged. The electron has negative charge which it adds to the chlorine atom leaving the sodium atom with a positive charge. Positive and negative charges attract one another producing the forces which hold the salt crystal together.
Water molecules consist of two hydrogen atoms and one oxygen atom. A hydrogen atom only has one electron. It would be much happier with two. Oxygen has two spare places. What happens is that they share their electrons.
Metals are different. Metal atoms have one or two loose electrons orbiting outside the main ball of orbiting electrons. They form crystals in which all of the loose electrons are shared by all of the atoms. This gives them the special property of conducting electricity.