Atomic Mass and Isotope Objectives: SWBAT calculate the average atomic mass of an element when given the relative abundance of the isotopes. Bellringer: What does the atomic mass tell you about the atom?

RELATIVE MASSES Chemists more than 200 years ago used a relative scale to compare weights of atoms to each other. Dalton assigned a H atom a mass of 1. According to his scale a helium atom has a relative mass of 4 because it is 4 times as heavy.

RELATIVE MASSES Using Dalton’s scale a carbon atom has a relative mass of 12 because a carbon atom is twelve times heavier than a hydrogen atom

Comparing Masses In 1961 Dalton’s method of comparing masses of atoms was replaced by IUPAC- International Union of Applied Physics and Chemistry.

IUPAC RELATIVE MASSES IUPAC decided that the most common isotope of C which is 12C would be used as a reference standard and assigned an atom of 12C a mass of 12 exactly. Using this scale the helium isotope is assigned a relative mass of 4 Comparing a helium atom to a carbon atom The He atom is 3 times lighter

RELATIVE ATOMIC MASSES A Krpton atom that is given a relative mass of 36 A Kr atom is three times heavier than a 12C atom

Atomic Symbols atomic number 11 Show the mass number and atomic number Give the symbol of the element mass number 23 Na sodium-23 atomic number 11

Isotopes Atoms with the same number of protons, but different numbers of neutrons. Atoms of the same element (same atomic number) with different mass numbers Isotopes of chlorine 35Cl 37Cl 17 17 chlorine - 35 chlorine - 37

RELATIVE ATOMIC MASSES All isotopes of elements are given a relative isotopic mass compared to the 12C isotope. There are 3 isotopes of Mg 24Mg 25Mg 26Mg These 3 atoms are different because they have different numbers of neutrons

Abundances of Isotopes In a sample of pure Mg you will find the isotopes of Mg always occur in the following quantity 78.7% 24Mg 10.13% 25Mg 11.17% 26Mg Like Magnesium most elements exist as a mixture of isotopes. Eg 1H, 2H and 3H Deuterium (heavy hydrogen) is used in heavy water moderated fission reactors, usually as liquid D2O (heavy water), to slow neutrons without high neutron absorption of ordinary hydrogen. During World War II, Nazi Germany was known to be conducting experiments using heavy water as moderator for a nuclear reactor design. Such experiments were a source of concern because they might allow them to produce plutonium for an atomic bomb. Ultimately it led to the Allied operation called the "Norwegian heavy water sabotage", the purpose of which was to destroy the Vemork deuterium production/enrichment facility in Norway. At the time this was considered important to the potential progress of the war.

Guided Practice 1 Calculate the average relative atomic mass of Magnesium given: Isotope Relative Mass Abundance 24Mg 23.99 78.7% 25Mg 24.89 10.13 % 26Mg 25.98 11.17% Assume we have 100 atoms of Mg. mass contributed by the 24Mg isotope is 23.99 x 78.7 mass contributed by the 25Mg isotope is 24.89 x 10.13 mass contributed by the 26Mg isotope is 25.98 x 11.17 Total mass of 100 Mg atoms = (23.99 x 78.7) + (24.89 x 10.13) + (25.98 x 11.17) Step one: multiply each relative mass by the relative abundance %. (this will give us the total mass of 100 mg atoms)

Finding Ar Total mass of 100 Mg atoms = (23.99 x 78.7) + (24.89 x 10.13) + (25.98 x 11.17) Ar(Mg) = (23.99 x 78.7) + (24.89 x 10.13) + (25.98 x 11.17) 100 Ar(Mg) = 24.3 This is not the true mass of a Mg atom but its relative mass compared to a 12C atom. Step 2: Divide by 100. Ar stands for relative atomic mass

Finding Ar The general rule is: Ar = Σ(relative isotopic mass x abundance) 100

Guided Practice 2 Find the relative atomic mass of Chorine. Isotope Relative Mass Abundance 35Cl 34.969 75.80% 37Cl 36.966 24.20% Ar(Cl) = (34.969 x 75.8) + (36.966 x 24.2) 100 Ar(Cl) = 35.45

Find Ar(O) Isotopes Relative Isotopic Mass Abundance 16O 15.995 99.76% 17O 16.999 0.04% 18O 17.999 0.20% Ar(O) = (15.995 x 99.76) + (16.999 x 0.04) + (17.999 x 0.2) 100 Ar(O) = 16

Atomic Mass of Magnesium Isotopes Mass of Isotope Abundance 24Mg = 24.0 amu 78.70% 25Mg = 25.0 amu 10.13% 26Mg = 26.0 amu 11.17% Ar(Mg) =(24 x78.7) + (25 x10.13) + (26 x 11.17) 100 Ar(Mg) = 24.3

HW: Practice Quiz tomorrow on: Atom History Atomic Structure Drawing Bohr Models Finding Relative Abundance (?)