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Nearly one hundred percent of the mass of an atom (99.94%) is contained in the nucleus. The mass of the protons and the mass of the neutrons are almost the same. Electromagnetic force binds the electron cloud to the nucleus. A nearly identical force can allow atoms to bind together, forming molecules. Atoms exist as either electrically neutral, or ions.
The nucleus is the small, dense region consisting of protons and neutrons at the center of an atom. The atomic nucleus was discovered in 1911 by Ernest Rutherford based on the 1909 GeigerâMarsden gold foil experiment.he nucleus was discovered in 1911, as a result of Ernest Rutherford 's efforts to test Thomson's  plum pudding model  of the atom. The electron had already been discovered earlier by J.J. Thomson himself. Knowing that atoms are neutral, Thomson postulated that there must be a positive charge as well.