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4.3. Distinguishing Among Atoms A. Information in a Periodic Table Square Atomic Number Element Symbol Element Name Average Atomic Mass 1 H Hydrogen 1.008 B. Atomic Number (AN) Equals the number of protons Is the only reliable number that identifies an element!!! In the Periodic Table, elements are sorted by increasing atomic number In a neutral atom, AN equals number of electrons (#e-) Number of neutrons(# no) is similar but often not equal to the number of protons Q: how many protons does Potassium have? A: 19 Q: What is the atomic number of Phosphorus? A: 15 C. Mass number (MN) of an atom ( in amu’s) MN is not indicated on the PT. To calculate the MN, simply add up #p+ and # no each having the mass of 1 amu. Q: an atom has 5 protons and 7 neutrons, calculate the MN. A: 5 amu + 7 amu = 12 amu Q: Which element is that? (Look in the PT) A: 5 protons, it’s Boron D. Isotopes Every element exists as multiple versions, called isotopes Isotopes of the same element are identical with regard to their 1. AN 2. # of p+ 3. # of e different with regard to their 1. # no 2. MN Example: Carbon - C Comes in form of three different isotopes with either 6, 7 or 8 neutrons Q: How many protons does Carbon have? A: 6 protons – all carbon isotopes have 6 protons!!! Q. Calculate the mass number of the three Carbon isotopes? Isotope A B C #p+ 6 6 6 # no 6 7 8 MN 12 amu 13 amu 14 amu E. Writing Conventions for Isotopes Shorthand A) uses only the mass number MN Carbon-12, Carbon-13, Carbon-14 C-12, C-13, C-14 Shorthand B) indicates MN on top!!!, AN on bottom 12 13 14 6C 6C 6C F. Calculating the Number of Neutrons MN minus AN Q: How many neutrons does N-15 have? A: AN of Nitrogen is 7 so MN – AN = 15 – 7 = 8 no Q. Write the #p+, no, e- for N-15 A: 7 p+, 8 no, 7 eQ: Calculate the p+, no , e- for P-30 and 2411Na P-30 24 11Na p+ 15 11 no 15 (30-15) 13 (24-11) e15 11 4.3 Average Atomic Mass -AAM Bottom number of a Periodic Table square Average Atomic Mass This is a calculated, weighed average mass composed of a) All the element’s existing isotopes and their masses b) their natural abundance (frequency in nature) Elements all have varying number of isotopes….need list! Calculating Average Atomic Mass Example: Carbon Isotope Natural Atomic mass Abundance Carbon-12 98.89% 12.000 amu Carbon-13 1.11% 13.003 amu How to do it 1. Convert the natural abundance % into decimals (divide by 100 or slide decimal 2 x to the left) 98.89/100 = 0.9889 1.11/100 = 0.0111 2. Multiply the mass of each isotope by its natural abundance and round to three decimals: this is called Relative Mass 0.9889 x 12.000 = 11.867 0.0111 x 13.003 = _0.144 12.011 3. Add all the Relative Masses See above 4. Round to two decimals (our periodic table has 2 decimals) 12.011 → 12.01 5. Compare with the MN found in the periodic table. Do both numbers match? Yes: 12.01 amu Practice: Calculate the Average Atomic Mass for 1. Chlorine 1. Chlorine-35 75.77%/100 = 0.7577 34.969 amu 2. Chlorine-37 24.23%/100 = 0.2423 36.966 amu 0.7577 x 34.969= 26.496 0.2423 x 36.966= _8.957 35.453 round 35.45 amu 2. Oxygen 1. Oxygen-16 99.759%/100 = 0.99759 15.995 amu 2. Oxygen-17 0.037%/100= 0.00037 16.995 amu 3. Oxygen-18 0.204%/100 = 0.00204 17.999 amu 0.99759 x 15.995 = 15.956 0.00037 x 16.995 = 0.006 0.00204 x 17.999 = _0.037 15.999 round to 16.00 amu