Counting Atoms

Counting Atoms

1. Atomic Number

   • The number of protons in the nucleus of an atom; the atomic number is the same for all atoms of an element.

2. Mass number

   • The sum of the numbers of protons and neutrons that make up the nucleus of an atom.

3. Nuclide

   • An atom that is identified by the number of protons and neutrons in its nucleus.

4. 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 (abbreviation, amu).

5. Average atomic mass

   • The weighted average of the masses of all naturally occurring isotopes of an element.
   • Calculating Average atomic mass
     • the average atomic mass of an element on mass & the relative abundance of each of the element's isotopes.
     • can be calculated by multiplying the atomic mass of each isotope by its relative abundance and adding the results

6. Mole

   • The SI base unit used to measure the amount of a substance whose number of particles is the same as the number of atoms of carbon in exactly 12 g of carbon-12.
   • is a counting unit

7. Avogadro's number

   • The law that states that equal volumes of gasses at the same temperature and pressure contain equal numbers of molecules.
   • = 6.022 141 79 x 10^23
     • is rounded to = 6.022 x 10^23
       • the number of particles in one mole of a pure substance.
   • Conversions with Avogadro's number
     • Avogadro's number can be 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.

8. Molar mass

   • The mass in grams of 1 mol of a substance.
   • usually written in units of g/mol
     • chemists usually use molar mass as a conversion factor in chemical calculations
       • eg. the molar mass of helium is 4.00 g He/mol He. To find out how many grams of helium there are in 2 moles of helium, multiply by the molar mass.