H 1 Hydrogen 1.00794 | Periodic Table | He 2 Helium 4.002602 | |||||||||||||||

Li 3 Lithium 6.941 | Be 4 Beryllium 9.012182 | of the Elements | B 5 Boron 10.811 | C 6 Carbon 12.0107 | N 7 Nitrogen 14.0067 | O 8 Oxygen 15.9994 | F 9 Fluorine 18.9984032 | Ne 10 Neon 20.1797 | |||||||||

Na 11 Sodium 22.98976... | Mg 12 Magnesium 24.305 | mouse/touch for more information | Al 13 Aluminum 26.9815386 | Si 14 Silicon 28.0855 | P 15 Phosphorus 30.973762 | S 16 Sulfur 32.065 | Cl 17 Chlorine 35.453 | Ar 18 Argon 39.948 | |||||||||

K 19 Potassium 39.948 | Ca 20 Calcium 40.078 | Sc 21 Scandium 44.955912 | Ti 22 Titanium 47.867 | V 23 Vanadium 50.9415 | Cr 24 Chromium 51.9961 | Mn 25 Manganese 54.938045 | Fe 26 Iron 55.845 | Co 27 Cobalt 58.933195 | Ni 28 Nickel 58.6934 | Cu 29 Copper 63.546 | Zn 30 Zinc 65.38 | Ga 31 Gallium 69.723 | Ge 32 Germanium 72.63 | As 33 Arsenic 74.9216 | Se 34 Selenium 78.96 | Br 35 Bromine 79.904 | Kr 36 Krypton 83.798 |

Rb 37 Rubidium 85.4678 | Sr 38 Strontium 87.62 | Y 39 Yttrium 88.90585 | Zr 40 Zirconium 91.224 | Nb 41 Niobium 92.90628 | Mo 42 Molybdenum 95.96 | Tc 43 Technetium (98) | Ru 44 Ruthenium 101.07 | Rh 45 Rhodium 102.9055 | Pd 46 Palladium 106.42 | Ag 47 Silver 107.8682 | Cd 48 Cadmium 112.411 | In 49 Indium 114.818 | Sn 50 Tin 118.71 | Sb 51 Antimony 121.76 | Te 52 Tellurium 127.6 | I 53 Iodine 126.90447 | Xe 54 Xenon 131.293 |

Cs 55 Caesium 132.9054 | Ba 56 Barium 132.9054 | Hf 72 Hafnium 178.49 | Ta 73 Tantalum 180.94788 | W 74 Tungsten 183.84 | Re 75 Rhenium 186.207 | Os 76 Osmium 190.23 | Ir 77 Iridium 192.217 | Pt 78 Platinum 195.084 | Au 79 Gold 196.966569 | Hg 80 Mercury 200.59 | Ti 81 Thallium 204.3833 | Pb 82 Lead 207.2 | Bi 83 Bismuth 208.9804 | Po 84 Polonium (209) | At 85 Astatine (210) | Rn 86 Radon (222) | |

Fr 87 Francium (223) | Ra 88 Radium (226) | Rf 104 Rutherfordium (267) | Db 105 Dubnium (268) | Sg 106 Seaborgium (271) | Bh 107 Bohrium (272) | Hs 108 Hassium (270) | Mt 109 Meitnerium (276) | Ds 110 Darmstadium (281) | Rg 111 Roentgenium (280) | Cn 112 Copernicium (285) | Uut 113 Unutrium (284) | Uuq 114 Flerovium (289) | UuP 115 Ununpentium (288) | Lv 116 Livermorium (293) | Uus 117 Ununseptium (294) | Uuo 118 Ununoctium (294) | |

La 57 Lanthanum 138.90547 | Ce 58 Cerium 140.116 | Pr 59 Praseodymium 140.90765 | Nd 60 Neodymium 144.242 | Pm 61 Promethium (145) | Sm 62 Samarium 150.36 | Eu 63 Europium 151.964 | Gd 64 Gadolinium 157.25 | Tb 65 Terbium 158.92535 | Dy 66 Dysprosium 162.5 | Ho 67 Holmium 164.93032 | Er 68 Erbium 167.259 | Tm 69 Thulium 168.93421 | Yb 70 Ytterbium 173.054 | Lu 71 Lutetium 174.9668 | |||

Ac 89 Actinium (227) | Th 90 Thorium 232.03806 | Pa 91 Protactinium 231.0588 | U 92 Uranium 238.02891 | Np 93 Neptunium (237) | Pu 94 Plutonium (244) | Am 95 Americium (243) | Cm 96 Curium (247) | Bk 97 Berkelium (247) | Cf 98 Californium (251) | Es 99 Einstenium (252) | Fm 100 Fermium (257) | Md 101 Mendelevium (258) | No 102 Nobelium (259) | Lr 103 Lawrencium (262) |

If you look at the periodic table of elements, you'll see each element's atomic weight listed. Scientists call the unit of measure in which that atomic weight is expressed the atomic mass unit. Avogadro's constant -- 6.02 x 10^23 -- describes the number of atoms within a mole of an element. Weighing a sample gives you its mass in grams. Knowing all three pieces of information -- atomic weight, grams and Avogadro's number -- will tell you the number of atoms in the sample.

Express the relationship of the three pieces of information you need to calculate the number of atoms in the sample in the form of an equation. Scientists express atomic weights in terms of grams per mole, so the resulting equation looks like this: atomic weight expressed in atomic mass units = grams/mole. In scientific notation, it would appear like this: u = g/mole.

Look up the sample's atomic weight on a periodic table of the elements. For example, boron has an atomic weight of 10.811 atomic mass units which you could also express as 10.811 grams per mole of the element. Plugging that figure into the above equation would look like this: 10.811 = g/mole.

Solve the equation for the unknown quantity; if u = g/mole and you have a number for u and g, then the number of moles is your target. Multiply everything through by the divisor to isolate the unknown quantity and you will reach an equation that looks like this: mole = g/u, where g equals the sample's weight in grams and u equals the element's atomic weight in atomic mass units.

Divide the grams of your sample by its atomic weight to derive the number of moles the sample contains. If your sample of boron weighed 54.05 g, your equation would look like this: mole = 54.05/10.811. In this example, you would have 5 moles of boron.

Multiply the number of moles in the sample by Avogadro's number, 6.02 x 10^23, to derive the number of atoms in the sample. In the given example, multiply Avogadro's constant by 5 to discover that the sample contains 3.01 x 10^24 individual boron atoms.

Check your work to ensure that it makes sense. Negative numbers, small numbers and numbers that do not seem to fit with the sample size mean a mathematical error.

#### Tip

Check your units of measurement to see if you must make any conversions; if you've measured your sample in kilograms or pounds, convert those figures to grams before proceeding. Carrying decimal places out to more digits will result in a more precise answer. Scientists add the atomic weights of atoms that comprise molecules together and use this derived number, the formula unit, to calculate the number of molecules in a sample of a compound.

#### Warning

Keep an eye on your exponents when you convert your answer into scientific notation; note how the exponent in the example changed from 10^23 to 10^24.