The only other force that appears to exist in the universe is gravity, but it is so weak (more than 10 -37 times weaker than the electrostatic force), that we can ignore it from the perspective of chemistry (although it does have relevance for the biology of dinosaurs, elephants, whales, and astronauts). Since the nucleus is much smaller than the atom itself, we can (and will) ignore the weak and strong forces when we consider chemical interactions. The other force involved in nuclear behavior, the "weak force", plays a role in nuclear stability, specifically the stability of neutrons, but it has an even shorter range of action (10 -18 m). This is the strongest of all known forces in the universe, approximately 137 times stronger than the electrostatic force, but it acts only at very short ranges, approximately 10 -15 m, or about the diameter of the nucleus. There is an attractive force between neutrons and protons, known as the strong nuclear force, that holds these particles together in the nucleus. This simple model captures important features that enables us to begin to consider how atoms interact with one another to form molecules and how those molecules can be rearranged – real chemistry! One thing we can ignore (for now) are the interactions involved in holding the nucleus together. We can think of them as if they were a cloud of electron density – rather than particles whizzing around. For now let’s just assume the electrons are outside the nucleus and moving. Where the electrons actually are, however, is a trickier question to answer, because of quantum mechanical considerations, specifically the Heisenberg uncertainty principal (which we will return to in the next chapter). Surrounding the atomic nucleus are electrons, the same number as there are protons. Since we talk about biology now and again, take care not to confuse the nucleus of an atom with the nucleus of a cell, they are completely different besides being of very different sizes - for example, there is no barrier round the nucleus of an atom, an atomic nucleus is a clump of protons and neutrons. This model has a very small, but heavy nucleus that contains both protons and neutrons. At this point, we have arrived at a relatively simple model of the atom (not to worry, we will move to more complex and realistic models in the next chapter).
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