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Atomic Structure Modern Atomic Theory • Atom – smallest particle of an element that retains the properties of the element • Subatomic Particles – Electrons – Protons – Neutrons Electrons • Negative Charge (-1) • Located outside the nucleus • Mass = 1/1836 of a proton (basically mass less) • Symbol: e- or e-1 Nucleons • Particles located inside the nucleus – Protons and Neutrons • All mass is located in the center of the atom (inside the nucleus) Protons • Positive Charge (+1) • Located in the nucleus • Mass = 1 amu (atomic mass unit) – 1 amu = 1.66 x 10-24 grams • Symbol: p+ or H+ Neutrons • • • • Neutral Charge Located inside the nucleus Mass = approximately 1 amu Symbol: n Atomic Number • The number of protons in an atom • Bold number on the periodic table • For a neutral atom: Number of protons = number of electrons Why??? Atomic Mass/Mass Number • The mass of an atom (expressed in amu) • Equal to the number of protons + the number of neutrons in an atom • Why aren’t electrons included in the mass? Nuclear Symbol • The nuclear symbol consists of three parts – the symbol of the element – the atomic number of the element – the mass number of the specific isotope Examples Indicate how many protons, neutrons and electrons are in each of the following atoms. 1. Aluminum-27 2. Xe-131 3. 56Fe Gram Atomic Mass • The mass of 1 mole of the element – 1 mole = 6.02 x 1023 particles • Expressed in grams instead of amu Mass Examples 1. 1 atom of K = 2. 1 mole of K = 3. 1 CO2 molecule = 4. 1 mole of CO2 = Isotopes • Same element, same atomic number, number of protons are the same • Different masses, because the number of neutrons varies Example: indicate the number of protons and neutrons in carbon-12 and carbon-14 Weighted Average • Mass found on the periodic table is the weighted average of the naturally occurring isotopes • Accounts for the percent abundance and mass of each of the elements isotopes How to Calculate • Each exact atomic mass is multiplied by its percent abundance (expressed as a decimal). Then, add the results together and round off to an appropriate number of significant figures. Example: Nitrogen Mass Number Exact Mass Percent Abundance 14 14.003074 99.63 15 15.000108 0.37 Examples • Calculate the weighted average atomic mass for Magnesium. Mass Exact Number Mass Percent Abundance 24 23.985043 78.99 25 24.985837 10.00 26 25.982593 11.01 Ions • Occur when a neutral atom gains or loses a charge (electrons) • Result from the transfer of electrons • Results in a charged atom • Protons are never lost or gained Cation • Positive Ion • Results from the loss of an electron (loss of negative charge) • Elements on the left side of the table (metals) tend to lose electrons to form cations Anion • Negative ion • Results from the gain of an electron (gain of negative charge) • Elements on the right side of the table (nonmetals) tend to gain electrons to form anions Ion Examples Indicate the number of protons and electrons in each: 1. 2. 3. 4. 5. 6. Cl atom: Cl- ion: K atom: K+ ion: Ca atom: Ca2+ ion: