Atomic nucleus: The atomic nucleus is formed by the combination of a proton and an electron. Both are in charge of the nucleus’s mass and positive charge. The atomic number is equal to the number of protons in the nucleus. An atom is a sphere with a diameter of 10 10-m, but the entire positive charge and almost the entire mass of the atom is concentrated in a small central core called the nucleus. The nucleus is encircled by electrons. The electrons are dispersed throughout the remaining atom, leaving plenty of empty space. Because the atom is electrically neutral, the total positive charge of the nucleus equals the total negative charge of the atom’s electrons.