Counting Atoms

Counting Atoms

1. Key Terms

   • Atomic number: the number of protons in the nucleus of an atom; the atomic number is the same for all atoms of an element.
   • Isotope: an atom that has the same number of protons (or the same atomic number) as other atoms of the same element do but that has a different number of neutrons (and thus a different atomic mass,
     • Isotopes can be made in a laboratory (artificial isotopes). Although the isotopes have different masses, they do not differ significantly in their chemical behavior
   • Mass number: the sum of the numbers of protons and neutrons in the nucleus of an atom
   • Nuclide: an atom that is identified by the number of protons and neutrons in its nucleus
   • Unified atomic mass unit: a unit of mass that describes the mass of an atom or molecule; it is exactly 1/12 of the mass of a carbon atom with mass number 12.
   • Average atomic mass: the weighted average of the masses of all naturally occurring isotopes of an element
     • Unlike atomic number, average atomic mass is a statistical calculation. Different samples of the sam element can differ in their relative abundance of isotopes.
   • Mole: the SI base unit used to measure the amount of a substance whose number of particles is the same a the number of atoms of carbon in exactly 12g of carbon-12
   • Avogadro's number: 6.02 x 1023, the number of atoms or molecules in 1 mol
     • Used to find the number of atoms of an element from the amount in moles or to find the amount of an element in moles from the number of atoms
       • It is expressed in units of atoms per mol
   • Molar mass: the mass in grams of 1 mol of a substance

2. All atoms of an element must have the same number of protons but not neutrons