Section 3-3: Counting Atoms A. Atomic Number 1. atoms of same element have same number of protons 2. atoms of different elements have different number of protons 3. atomic number (Z) – number of protons in the nucleus of an atom B. Isotopes 1. isotopes – atoms of same element that have different masses 2. isotopes of an element have same number of protons and electrons, but different numbers of neutrons 3. most elements are mixtures of isotopes; Fig. 9, p. 78, top; hydrogen isotopes 4. mass number (A) – number of protons and neutrons in the nucleus of an isotope C. Designating Isotopes; Table 3, p. 79, middle 1. hyphen notation – mass number is written with a hyphen after the name of the element; uranium-235 2. nuclear symbol – a left superscript indicates the mass number and the left subscript indicates the atomic number 3. number of neutrons equals the mass number minus the atomic number: mass number – atomic number = number of neutrons; A – Z 4. nuclide – general term for a specific isotope of an element Sample Problem A; p. 79, bottom 3-3b D. Relative Atomic Masses 1. standard used by scientists to compare units of atomic mass is the carbon-12 atom; arbitrarily assigned a mass of exactly 12 atomic mass units; 12 amu 2. atomic mass unit (amu) – exactly 1/12 the mass of one carbon-12 atom 3. atomic mass of any other atom is determined by comparing it with the mass of the carbon-12 atom E. Average Atomic Masses of Elements 1. average atomic mass – weighted average of the atomic masses of the naturally occurring isotopes of an element 2. weighted average – average that reflects the value and the frequency of each entry 3. Calculating average atomic mass a. depends on both the mass and the relative abundance of each of the element’s isotopes b. multiply atomic mass of each isotope by the relative abundance expressed as a decimal and add the results F. Relating Mass to Numbers of Atoms 1. mole – amount of a substance that contains as many particles as there are atoms in exactly 12 g of carbon-12; SI unit for amount of substance 2. Avogadro’s number – number of particles in exactly one mole of any pure substance: 6.022 x 1023 3. molar mass – mass of one mole of a pure substance; Fig. 10, p. 83 a. units are g/mol b. molar mass of an element is numerically equal to the atomic mass of the element in atomic mass units 4. Gram/Mole Conversions; molar mass used as a conversion factor 5. Conversions with Avogadro’s Number; also used as a conversion factor Sample Problems B,C,D, & E