1. Relative atomic mass - A r Just another way of saying how heavy different atoms are compared with the mass of one atom of carbon – 12 (regular carbon!) Mass number Q. What does the atomic mass number represent? A. Total number of protons and neutrons found in an atoms nucleus Atomic number Number of protons present

2. Another handy trick you can do with the periodic table Mass number Atomic number You can work out how many neutrons an element has by subtracting the proton number from the mass number!! So how many neutrons:

3. Relative atomic mass - A r ElementArAr H1 C12 O16 Mg24 Cl35. Table.1. Different elements and their different A r Relative atomic mass is easy!! It’s the same value as the mass number – it just sounds scarier! So what does this tell us about Mg and H?

4. Relative formula mass - M r To find the relative formula mass (M r ) of a compound, you just add together the A r values for all the atoms in its formula. Example 1: Find the M r of carbon monoxide (CO). The A r of carbon is 12 and the A r of oxygen is 16. So the M r of carbon monoxide is 12 + 16 = 28. Example 1: Find the M r of carbon monoxide (CO). The A r of carbon is 12 and the A r of oxygen is 16. So the M r of carbon monoxide is 12 + 16 = 28. O 16 8

5. Relative formula mass - M r Example 2:Find the M r of sodium oxide-Na 2 O The A r of sodium is 23 and the A r of oxygen is 16. So the M r of sodium oxide is (23 x 2) + 16 = 62.

6. Carbon dioxide Sulphur dioxide SO 2 Calcium carbonate CaCO 3 Sodium hydroxide NaOH Sulphuric acid H 2 SO 4 Hydrochloric acid HCl Copper sulphate CuSO 4 Magnesium chloride MgCl 2 Sodium carbonate Na 2 CO 3 Find the M r of these:

7. ATOMIC MASS AND AVERAGE ATOMIC MASS u Atomic Mass = used to numerically indicate the mass of an atom in its ground state, it is expressed in the non SI unit of u u u = refers to unified atomic mass unit (formerly known as atomic mass unit or amu) u 1 amu = 1/12 the mass of carbon-12 atom, u therefore the mass of C-12 atom is made equal to 12 amu

8. Carbon-12 atom is an isotope of carbon 1 amu = 1.66 x 10 -24 g Note that: Atomic mass of 12 C = mass of p + mass of n + mass of e Mass of e = 1/1800 of mass of p and n so it is negligible making the equation

9. Atomic mass of 12 C = mass of p + mass of n Atomic mass vs. Average atomic mass For carbon it is 12 u not 12.01 u  Used to relate the fact that the numerical value assigned to each element in the periodic table reflects the average abundances of the atoms that compose a naturally occurring element  Related to isotopes  For carbon it is 12.01 u  Chemists often will use the term “atomic mass” when they are actually referring to average atomic mass of an atom.

10. Average Mass of Isotopes Isotopes are naturally occurring. Isotopes are naturally occurring. The mass # of an element (periodic table) is the weighted The mass # of an element (periodic table) is the weighted avg. of allisotopes that exist in nature. avg. of all isotopes that exist in nature. - abundance of isotope is just as important as mass! - abundance of isotope is just as important as mass! Ex... Ex... Natural copper (Cu) consists of 2 isotopes... Natural copper (Cu) consists of 2 isotopes... Copper - 63 (mass = 62.930 g/mole) Copper - 63 (mass = 62.930 g/mole) Copper - 65 (mass = 64.930 g/mole) Copper - 65 (mass = 64.930 g/mole) 69% 69% 31% 31% To calculate avg. mass... To calculate avg. mass... mass x abundance for each isotope Step 1 : Step 2 : add the two values from step 1 together 62.93 x.69 = 64.93 x.31 = 43.42 20.13 43.42 20.13 + 63.55 g/mole

11. Ex... Ex... Three isotopes of Oxygen: Three isotopes of Oxygen: Oxygen - 18 Oxygen - 18 Oxygen - 16 Oxygen - 16 Oxygen - 17 Oxygen - 17 The avg. mass (from P.T.) is closest to 16, therefore, The avg. mass (from P.T.) is closest to 16, therefore, Oxygen-16 is the isotope that is most abundant in nature. 99. 759% 0.037% 0.204% The average mass of an element is closest to the isotope The average mass of an element is closest to the isotope that is mostplentiful in nature. that is most plentiful in nature.

12. Working with Weighted Averages & Calculating Average Atomic Mass Read page 165 of your text and make short notes.