Understanding Atomic Mass
What does a deck cards and the atomic mass on the periodic table have in common.?
Let s Review Significant Figures and Digits All numbered digits are significant. i.e. 632 g = 3 sig. digits A zero between numbers is significant i.e. 6032 g = 4 sig. digits Zeros to the right of decimal are significant i.e. 6032.0 = 5 sig. digits Initial zeros are not significant i.e. 0.0000632 = 3 sig. digits Final zeros may or may not be significant Rules When multiplying and dividing, the number with the fewest amount of significant digits determines the answer When adding or subtracting, the number with the fewest decimal places determines the answer
Isotopes Atoms that have the same number of protons but a different number of neutrons. Hydrogen 1 H, 2 H, 3 H Magnesium 24 Mg, 25 Mg, 26 Mg, Carbon 12 C, 13 C, 14 C Most elements have natural occurring isotopes
Symbols Contain the symbol of the element, the mass number and the atomic number.
Symbols Contain the symbol of the element, the mass number and the atomic number. Mass number Atomic number X
Naming Isotopes Put the mass number after the name of the element. carbon- 12 carbon -14 uranium-235
Symbols Find the number of protons number of neutrons number of electrons Atomic number Mass Number Name 24 11 Na
Symbols Find the number of protons number of neutrons number of electrons Atomic number 80 35 Br Mass Number Name
Symbols if an element has an atomic number of 34 and a mass number of 78 what is the number of protons number of neutrons number of electrons Complete symbol Name
Symbols if an element has 91 protons and 140 neutrons what is the Atomic number Mass number number of electrons Complete symbol Name
Symbols if an element has 78 electrons and 117 neutrons what is the Atomic number Mass number number of protons Complete symbol Name
Atomic Mass How heavy is an atom of oxygen? There are different kinds of oxygen atoms. More concerned with average atomic mass. Based on abundance of each element in nature. Don t use grams because the numbers would be too small.
So atomic mass should be Atomic Mass= 12 C + 13 C + 14 C = (12 + 13 + 14) / 3 = 13 u
Calculating averages You have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks? Total mass = 4 x 50 + 1 x 60 = 260 g Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 5
Calculating averages Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 5 Average mass =.8 x 50 +.2 x 60 80% of the rocks were 50 grams 20% of the rocks were 60 grams Average = % as decimal x mass + % as decimal x mass + % as decimal x mass +
So atomic mass should be Atomic Mass= 12 C + 13 C + 14 C = (12 + 13 + 14) / 3 = 13 u
Think back to the deck of cards There were more chances to pick a heart compared to any other suit Therefore the largest majority of cards are hearts The atomic mass of carbon, according to the periodic table is 12.01 u Therefore, there must be a majority of carbon-12 atoms.
Isotopic Abundance The relative abundance of the isotopes will determine the atomic mass of the element. The atomic mass on the periodic table is in fact a weighted average for all of the atomic masses of the element s isotopes.
Calculating an Average Atomic Mass In order to calculate the atomic mass as recorded in the periodic table, we must take into account the relative abundances of the element s isotopes. Atomic Mass= 12 C (x) + 13 C (y) + 14 C (z) X = 0.989 Y = 0.011 Z = 1 x 10-12 12 (0.989) + 13 (0.011) + 14 (1 x 10-12 ) = 12.01 u
Determining the Isotopic Abundance General Formula: Average Atomic Mass= (Isotope Mass x Relative Abundance) + (Isotope Mass x Relative Abundance) By the same method, we can determine what the relative abundance is of each isotope. i.e. Boron exists as: Boron-10 and Boron-11; 10.01 u and 11.01 u According to the periodic table, the average atomic mass for boron is 10.81 u. 10.81 = 10 B(x) + 11 B(y) x + y = 1 = 10.01 (x) + 11.01 (y) x = 1 y = 10.01 (1-y) + 11.01 (y) = 10.01 10.01y + 11.01y Therefore: The abundance of boron-11 is 80% The abundance of boron-10 is 20% 10.81 = 10.01 10.01y+11.01y 10.81 10.01 = -10.01y + 11.01y 0.80 = y 80%
Atomic Mass Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest has a mass of 64.93 amu.
Atomic Mass Magnesium has three isotopes. 78.99% magnesium 24 with a mass of 23.9850 amu, 10.00% magnesium 25 with a mass of 24.9858 amu, and the rest magnesium 25 with a mass of 25.9826 amu. What is the atomic mass of magnesium? If not told otherwise, the mass of the isotope is the mass number in amu
Atomic Mass Is not a whole number because it is an average. are the decimal numbers on the periodic table.