Atoms are defined as fundamental building blocks. The first people to suggest the idea of an atom were the Ancient Greeks. Democritus was deeply interested in the divisibility of matter and theorized that if matter were divided into finer and finer pieces, it would eventually reach a point where it could no longer be divided. This smallest bit of matter was called an atom, from a Greek word meaning "not cuttable." In 1803, John Dalton changed this idea into a firmly structured principle; a theory of atoms. He believed that all elements are made up of atoms which cannot be destroyed, split, or created; that all atoms of the same element are exactly alike with the same mass; that atoms of different elements are different with a different mass; and that compounds are formed by joining the atoms of two or more elements. Atoms are very tiny and can be seen only with powerful electron microscopes, however, they are made up of even smaller particles. Inside the atom is the nucleus (central core) and tiny, almost weightless, negative electrical charges called electrons. The nucleus of an atom consists of several different kinds of particles, packed closely together. The basic particles of the nucleus are protons and neutrons. These particles are called nucleons. Around the atom's nucleus, at fantastic speeds, whirl electrons. The whirling electrons complete billions of trips around the nucleus in a millionth of a second. The neutral atoms of each element have a definite number of electrons. The number of electrons determines the chemical behavior of the element. Electrons have almost no mass. Protons are positive charged baryons in the nuclei of atoms. All the atoms of a given element have the same number of protons. This number differs from the number of protons in the atoms of any other element. It equals the number of electrons outside the atom's nucleus, so that the charges balance and the atom is electrically neutral. Neutrons resemble protons in size and mass. But they have no electrical charges. The neutron is unstable outside the nucleus. This means it breaks down to form a proton, an electron, and a neutrino. Atoms of a given element may have different number of neutrons in the nucleus. Because atoms consist mostly of empty space, it may seem wrong to speak of the "size" of an atom as though it were a marble. But the electrons travel around the nucleus so fast that they give form to the atom. Atoms are so small that they measure less than the thickness of a page. Scientists can see individual atoms with an electron microscope, but most of what is known about atoms has come from studies with other instruments such as X rays and spectroscopes.