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Parts of an Atom What is an atom? Atoms are the smallest parts to which an element can be divided yet still be that element. Atoms are the building blocks of all matter. How small is an atom? An atom is so small that a penny contains about 2 x 1022 or 20,000,000,000,000,000,000,000 atoms!!! But there’s more… Atoms consist of even smaller particles. These include: – Protons – Neutrons – Electrons Parts of an Atom The protons and neutrons make up the center or nucleus. The Nucleus The center of an atom Small, dense (packed tightly) Contains protons and neutrons: most of the atom’s mass So dense, if it were the size of a grape, it would weigh over 9 million tons! Protons Located in the nucleus Positive charge (+) Mass of 1 atomic mass unit (amu) Atoms of the same element have same number of protons! protons = positive Neutrons Located in the nucleus Have no charge - neutral Mass of 1 amu neutrons = neutral Electrons Found outside of the nucleus in electron cloud Negatively charged (-) Mass of almost zero Number of electrons equals the numbers of protons to cancel out the charges (think of -1+1=0) electrons = negative Valence Electrons Electrons in the “outer energy level” Important in determining an element’s properties and its bonding ability Review: Parts of an Atom Parts Charge Location Mass Proton Positive (+) Nucleus 1 amu Neutron No charge Nucleus 1 amu Electron Negative (-) Electron Clouds Almost zero ATOMIC NUMBER The atomic number is the number of protons in the nucleus of an atom Atomic Mass The weighted average of all the isotopes of an element that exists in nature Mass Number An element’s mass number is the atomic mass rounded to the nearest whole number To find the number of NEUTRONS in an atom: Mass Number - Atomic number = # of Neutrons ex: Fluorine atomic mass 18.998, atomic #9 19 – 9 = 10 neutrons ISOTOPES Atoms that have the same atomic number but DIFFERENT atomic masses The number of protons is the same, but the number of neutrons is not! ISOTOPES: Example Carbon-12 : Atomic # = 6 Mass # = 12 – Has 6 protons, 6 neutrons Carbon-14: Atomic # = 6 Mass # = 14 – Has 6 protons, 8 neutrons