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Accelerated / Advanced Science 8 Helpful Science Notes Chapter 4.2 The Structure of an Atom Vocabulary: 1. Proton 2. Electron 3. Neutron 4. Atomic number 5. Mass number 6. Isotopes The Structure of an Atom Properties of Subatomic Particles Protons o Positively charged subatomic particle found in the nucleus of an atom. o Assigned a 1+ charge. Electrons o Negatively charged subatomic particle that is found in the space outside the nucleus. o Electron comes from the Greek word meaning “amber.” o Has a charge of -1, and a mass approximately 1/2000 of a proton. Neutrons o Discovered in 1932 by English physicist James Chadwick. o A neutron is a subatomic particle that is found in the nucleus of an atom. o It has a mass almost exactly equal to that of a proton. Atomic Number and Mass Number Atomic Number 1 Accelerated / Advanced Science 8 o Equal to the number of protons in the nucleus of an atom. o Atoms of different elements have different numbers of protons. o The number of electrons usually equals the number of protons. Mass Number o The sum of the number of neutrons and protons in the nucleus of an atom. o Number of neutrons = Mass number – Atomic number Isotopes Atoms of the same element that have different numbers of neutrons. For example, every atom of oxygen has 8 protons. Some have 8 neutrons for a mass number of 16. Others have 9 neutrons for a mass number of 17. Example: heavy water (Figure 12, page 112) 2