Some Information about Radiometric Dating

Element: fundamental substance into which material can be separated by chemical means, with characteristic atomic properties and behavior (examples: He, H, Fe, O, Si)

Atom: smallest particle that retains all properties of a given element

Atom = protons + neutrons + electrons

Number of protons defines an element: the atomic number

Number of protons + neutrons defines the mass: the mass or isotopic number

One element can have several isotopes (# neutrons)

Different isotopes (=masses) measured in a mass spectrometer

Some isotopes fall apart spontaneously (unstable isotopes): RADIATION
Parent Daughter+ product
14/6 C 14/7 N
40/19 K40/18 Ar
235/92 U 207/82 Pb
238/92 U 206/82 Pb
87/37 Rb87/38 Sr

The time during which half of the parent isotopes decay to daughter isotopes is constant for each unstable isotope: the half life

ParentDaughter Half life
14 C 14 N 5,730 year
40 K 40 Ar 1,300,000,000 (1.3 x 109 y)
235 U 207 Pb 700,000,000 year (.7 x 109 y)
238 U 206 Pb 4,500,000,000 year (4.5 x 109 y)
87 Rb 87 Sr 48,800,000,000 year (48.8 x 109 y)

Basic radiometric age calculation:

1 measure parent now (P)

2 measure daughter (D)

3 P + D = original amount of P (Po)

4 D/Po ratio gives age. The larger the ratio, the older the sample.