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The Atom AC Physical Science 2009 How Small is an Atom? Ø Three One-Hundred-Millionths of a Centimeter! Scientists know that aluminum is made of average-sized atoms. An aluminum atom has a diameter of about 0.00000003 cm. What is an Atom Made of? The Nucleus: Protons are positively charged particles in the nucleus. Neutrons are the particles of the nucleus that have no electrical charge. Outside the Nucleus: Electrons are the negatively charged particles in atoms. Electrons are found around the nucleus within electron clouds. All the structures of the atom can be seen on the next slide. Parts of an Atom How do Atoms of Different Elements Compare? Starting Simply: The hydrogen atom has one proton and one electron. Now for Some Neutrons: The helium atom has two protons, two neutrons, and two electrons. How do Atoms of Different Elements Compare? Building Bigger Atoms: For bigger atoms, simply add protons, neutrons, and electrons. Protons and Atomic Number: The number of protons in the nucleus of an atom is the atomic number of that atom. All atoms of an element have the same atomic number. Forces in Atoms Four Basic Forces: Four basic forces are at work everywhere, even within the atom. These forces are gravitational force, electromagnetic force, strong force, and weak force. These forces work together to give an atom its structure and properties. Homework! Answer the following questions… 1. What is an atom’s mass number equal to? 2. How is the atomic mass of an element calculated? 3. How do isotopes differ from one another? 4. How does an atom become positively charged? 5. Why is gravitational force in the nucleus so small?