Hi, I'm Donald Sinclaire. I'm a science teacher with Greater London Tutors, and today we're going to be looking at a few topics in chemistry. This is how to find neutrons in an atom. The atom consists of two main parts: a dense heavy nucleus which sits in the center, and electrons which orbit around it. Here, we have an atom of lithium. Lithium is the third element. It has an atomic number of three. That means it contains three protons. Protons here are drawn in green. They live in the nucleus, or the center, of the atom, are positively charged, and are heavy compared to other particles. Also in the nucleus are neutrons, here in red. As the name suggests, neutrons have no charge, they're neutral. But they have a similar mass to protons and also exist in the nucleus. The nucleus is responsible for virtually all of the mass of an atom. It's also very small and very dense. If this diagram was drawn to scale, the nucleus would be so small as to be invisible. We draw it large here so you can see the individual particles. Virtually all the mass of an atom is concentrated in the nucleus. Orbiting the nucleus are electrons. Electrons are negatively charged, which is why they orbit the nucleus, as negative charges are attracted to positive charges. Electrons are also very light. They're approximately two thousandths the mass of a proton or a neutron, and so add very little to the mass of the atom overall. Electrons orbit in layers, which fill up. Lithium has three protons, which means since an atom is neutral, it also has three electrons. Now, the first layer surrounding the nucleus can only take two electrons. So, when you have three electrons, the third one has to go in the second layer. The electrons that are orbiting in the outermost layer are called valence electrons. These are the ones that determine the chemical properties of the atom. They are the ones that are responsible for forming bonds with other atoms to make molecules. .