# CHEM 3.3ST

```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
```
Oxygen compounds

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