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Distinguishing Between Atoms Remember, atoms consist of three subatomic particles: Atoms of different elements contain different numbers of protons • Hydrogen has only one proton while oxygen has 8 protons in the nucleus. • The atomic # on the periodic table describes the number of protons in the nucleus Atomic # So, the question is: How do atoms of different elements differ from each other? O 8 15.9994 Atomic Symbol Average Atomic Mass Remember, protons have a positive (+) charge. Therefore, for neutral atoms, the number of electrons, having a negative charge (-) must equal the number of protons. So, for neutral atoms the atomic number describes the number of protons and electrons in the atom. Symbols of Elements Elements are symbolized by one or two letters. Atomic Number All atoms of the same element have the same number of protons: The atomic number 1 Atomic Mass • The mass of an atom in atomic mass units (amu) is the total number of protons and neutrons in the atom. • Called the mass number Nuclear symbols vs. Hyphen notation •Nuclear symbols, as shown above, uses the symbol of the element preceded by a superscript mass # and subscript atomic number. •The same atom can be described by hyphen notation which lists the atom name or symbol followed only by the mass number hyphenated. Carbon-12 or C-12 To find the number of neutrons in an atom, you subtract the atomic number (number of protons in the nucleus) from the mass number (total number of protons and neutrons in the nucleus) Number of Neutrons = mass number – atomic number 238 How many neutrons are in 92 U ? How many protons are in Sulphur-33? How many neutrons are in Zn-60? Isotopes: • Atoms of the same element with different masses. • Isotopes have different numbers of neutrons. MOVIE 2 Atomic Mass The Atomic mass is different for each isotope of an element because each isotope has a different number of neutrons in the nucleus. Atomic and molecular masses can be measured with great accuracy with a mass spectrometer. Since the mass of atoms is so increadibly small, a unit known as an atomic mass unit (amu) was defined to make the masses of atoms easier to work with. •The amu is defined by the carbon-12 isotope of carbon. •1 amu is equal to 1/12 the mass of the carbon-12 isotope. (roughly the mass of a neutron or proton) Notice, the mass of electrons is so much less than that of a neutron or proton that its mass is negligible. Now the question becomes: Since there are different isotopes for each element, how do we report the mass on the periodic table to reflect all of the isotopes of an element in a sample of that element? Average Atomic Mass Atomic # O 8 15.9994 Atomic Symbol Average Atomic Mass • Because in the real world we use large amounts of atoms and molecules, we use average masses in calculations. • Average mass is calculated from the isotopes of an element weighted by their relative abundances. 3 Calculating the Average Atomic Mass To calculate the average atomic mass of an element, multiply the mass of each isotope by its natural abundance, expressed as a decimal, then add the products. 4