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) |

The number of protons in an atom determine what element it is, but atoms can have different numbers of neutrons to give it a different mass. When two atoms of the same element have different numbers of neutrons, they are called isotopes. Some isotopes occur naturally, and it is possible to calculate the percent abundance of two isotopes in nature if atomic masses and the element's average atomic mass are already known.

Determine the atomic masses of the isotopes, as well as the element's average atomic mass. The units of these values will be in amu, which stands for "atomic mass unit." One amu is approximately the mass of one proton. For example, boron has two naturally occurring isotopes: B-10 with a mass of 10.013 amu and B-11 with a mass of 11.009 amu. The average atomic mass of boron, according to the periodic table, is 10.811 amu.

Enter the values into the following formula: a = b (x) + c (1 - x). In the equation, "a" is the average atomic mass, "b" is the atomic mass of one isotope, "c" is the atomic mass of the other isotope, and "x" is the abundance of the first isotope. For example, 10.811 = 10.013 (x) + 11.009 (1 - x)

Factor the equation. For example, 10.811 = 10.013x + 11.009 - 11.009x

Add the negative x factor to both sides of the equation. For example, 10.811 + 11.009x = 10.013x + 11.009 - 11.009x + 11.009x, which reduces to 10.811 + 11.009x = 10.013x + 11.009

Subtract the non-x factor from both sides of the equation. For example, 10.811 + 11.009x - 10.811 = 10.013x + 11.009 - 10.811, which reduces to 11.009x = 10.013x - 0.198

Subtract 10.013x from both sides of the equation. For example, 11.009x - 10.013x = 10.013x - 0.198 - 10.013x, which reduces to 0.996x = 0.198

Divide both sides by the coefficient of the x factor. For example, 0.996x/0.996 = 0.198/0.996, which reduces to x = 0.1988. This is the abundance of B-10.

Multiply your answer by 100 to get a percentage. For example, 0.1988 x 100 = 19.88 percent.

Subtract this value from 100 percent to find the abundance of the other isotope. For example, 100 - 19.88 = 80.12 percent. This is the percent abundance of B-11.

#### Tip

This formula only works for two unknown percentages. For elements with three or more isotopes, this formula can only be used if all but two of the percent abundances are already known.