 Since all the points on that line are the same zero age, the line is called an isochron (iso “same,” chron “age”), in this case the zero isochron. But as both samples are the same age, they will plot on the same line on the graph (see Figure 2).However, the isochron line has simply rotated upwards from time zero (horizontal) to time 1 (sloping).Within the nucleus of nearly all atoms are protons and neutrons, positively-charged and neutral particles respectively.All the atoms in each chemical element have the same number of protons in their nuclei.The result is different atoms, with seven protons and seven neutrons in their nuclei, and seven orbiting electrons.Those characteristics define element 7 atoms, which are stable N (nitrogen).Atoms of the same element that have different atomic weights are called isotopes of that element.For example, carbon (symbol C) is element 6 because it has six protons in every one of its atoms.

However, the number of neutrons in the nucleus of each atom is not always the same.

Again the isochron line has simply rotated from time 1 to time 2.

As it turns out, from the math describing the isochron line, the slopes of these isochron lines equate to the ages of the rock unit at times zero, 1, and 2.

Often there are equal numbers of neutrons and protons, but sometimes there are more neutrons than protons.

Thus every element has atoms with the same atomic number, but its atoms can have several different atomic weights because of the different numbers of neutrons.   This method is called radioactive, radiometric, or radioisotope dating.2 The radioisotope, which decays, is often referred to as the parent isotope; while the stable isotope, which is the end result of the radioactive decay, is called the daughter isotope.