Isotopes & Atomic Mass Ms. Klarke, 2011

Objectives In this presentation, you will learn … How the atomic mass of one proton or neutron is determined What the unit of atomic mass is What an isotope is How average atomic mass of an element is determined, including isotopes

How the unit of atomic mass is determined

Atomic Mass Unit Protons and neutrons are approximately the same mass. Each proton is about the same mass as every other proton. Each neutron is about the same as every other neutron.

Atomic Mass Unit Consider a carton of eggs, brown and white. The eggs are similar but not identical in mass. carton.png

Atomic Mass Unit How do you find the average mass of an egg? carton.png

Atomic Mass Unit You add all the masses and then divide by carton.png

Atomic Mass Unit In just the same way, the mass of Carbon-12, the isotope of carbon with 6 protons and 6 neutrons, is used to find an average. atom_e1.gif

Atomic Mass Unit Scientists add the mass of the 6 neutrons and 6 protons, then divide by 12, to get 1 atomic mass unit. 1 atomic mass unit = 1 amu = 1 u Both amu and u are symbols for atomic mass unit, the approximate mass of one proton or neutron. atom_e1.gif Drawing not to scale- electrons would be too small to see, and further away.

What an isotope is

Isotopes Every atom of a particular element, such as the element carbon, has the same number of protons.

Isotopes The neutron number in atoms of an element can vary, however. Carbon can have 6, 7 or 8 neutrons. Each atom above is a different isotope of carbon.

Isotopes Each atom above has the SAME atomic number, 6. Each atom above has a DIFFERENT atomic mass number, 12, 13 or 14.

Isotopes The different isotopes of an atom do not appear with the same frequency; some isotopes are common and some are rare. Above, carbon-12 is the mass number of 98.9% of all carbon atoms, while carbon-14 is less than 1/1000 of all carbon atoms.

Isotopes Below, you see three isotopes of the element hydrogen.

Isotopes Hydrogen-1 is called Hydrogen, hydrogen-2 is called Deuterium and hydrogen-3 is called Tritium.

Isotopes Only Hydrogen has special names for its isotopes; the others just have a number after the element, like carbon-14.

Average Atomic Mass

We have learned about atomic mass number, the number of protons and neutrons in an atom.

Average Atomic Mass Now we consider mass of the average atom of an element, not a specific atom. Since there are a mixture of isotopes, the average mass of an element is somewhere in between the lightest and heaviest isotope.

Average Atomic Mass For example, the average atomic mass of the element helium is amu.

Average Atomic Mass amu suggests that helium-3 must be rare, or the average mass would be closer to amu.

The End