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Atomic and mass number

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Atomic theory states that the atomic number of an element is defined by the number of protons it contains, with all atoms of a given element containing the same number of protons. The mass number of an atom is equal to the sum of its protons and neutrons, and the number of neutrons can be calculated by taking the mass number and subtracting the atomic number.

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Atomic Theory

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• Atomic number is the number of Protons in an atom • All atoms of a particular element have the SAME number of protons (All Carbon atoms have 6 protons) • In a neutral atom, The atomic number =The number of electrons

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• Also called atomic weight or Mass Number • Sum weight of protons and neutrons in an atom

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Calculate the mass number of each of the following atoms

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O 8 16 Atomic number Mass number Symbol OxygenName Atomic number= Number of protons= Number of electrons Mass number= Number of protons + Number of neutrons

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Calculating Neutrons: • Mass number = protons + neutrons so….. • Neutrons = mass number – protons What is the number of neutrons of the following atom ? A 21 45

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Atomic and mass number