What is an atom?
To determine what physics has to say about this property of the electron density one must consider not the density itself but the field one obtains by following the trajectories traced out by the gradient vectors of the density. Starting at any point, one determines the gradient of (r). This is a vector that points in the direction of maximum increase in the density. One makes an infinitesimal step in this direction and then recalculates the gradient to obtain the new direction. By continued repetition of this process, one traces out a trajectory of (r). A gradient vector map generated in this manner is illustrated in the upper diagram of Figure 4 for the same plane of the ethene molecule shown in Figure 1. Since the density exhibits a maximum at the position of each nucleus, sets of trajectories terminate at each nucleus. The nuclei are the attractors of the gradient vector field of the electron density. Because of this fundamental property, the space of the molecule is disjointly and
An atom a fundamental piece of matter. (Matter is anything that can be touched physically.) Everything in the universe (except energy) is made of matter, and, so, everything in the universe is made of atoms. An atom itself is made up of three tiny kinds of particles called subatomic particles: protons, neutrons, and electrons. The protons and the neutrons make up the center of the atom called the nucleus and the electrons fly around above the nucleus in a small cloud. The electrons carry a negative charge and the protons carry a positive charge. In a normal (neutral) atom the number of protons and the number of electrons are equal. Often, but not always, the number of neutrons is the same, too. The negative electrons are attracted to the positive nucleus by the same electrical force which causes magnets to work. That’s what keeps the atom together.
