Chapter 3.3 Counting Atoms

Objectives Explain what isotopes are Define atomic number and mass number, and describe how they apply to isotopes. Given the identity of a nuclide, determine its number of protons, neutrons, and electrons. Define mole in terms of Avogadro’s number, and define molar mass. Solve problems involving mass in grams, amount in moles, and number of atoms of an element

Atomic Number Symbol – Z Number of protons in nucleus of atom Determines the identity of the atom!!!!! C - 6 He - ___ F - ___ Pb - ___ Use periodic table to find these! Since atoms are neutral atomic number is also number of electrons So C also has 6 electrons He - ____ electrons 2 9 82 2

Isotopes All Hydrogen atoms contain 1 proton Many naturally occurring elements can contain different numbers of neutrons. Isotope – at0ms of the same element that have different masses. n n n Protium 99.985 % Deuterium 0.015 % Tritium Trace Three isotopes of Hydrogen

Mass Number Mass number – total number of protons and neutrons in the nucleus of an isotope Symbol – A Protium has 1 proton and 0 neutrons Mass number = #p + #n = 1 + 0 = 1 What is the mass number deuterium and tritium? Find on periodic table

Designating Isotopes Hyphen Notation Name of atom – mass number Uranium - 235 Nuclear symbol A X A : mass number, Z : atomic number, X : symbol of element 235U Z 92 Number of protons Neutrons Electrons 92 143 92 Nuclide – general term for any isotope of any element

Sample Problem A How many protons, electrons, and neutrons are there in an atom of chlorine-37? atomic number = number of protons = number of electrons mass number = number of neutrons + number of protons # protons # electrons #neutrons Atomic number from periodic table 17 17 # protons = # electrons 20 Mass # = # protons + # electrons

Practice Pg. 80 How many protons, electrons, and neutrons make up an atom of bromine-80? Write the nuclear symbol for carbon-13. Write the hyphen notation for the isotope with 15 electrons, and 15 neutrons. Answer : 35 protons, 35 electrons, 45 neutrons Answer : 13 C 6 Answer : phosphorus – 30

Relative Atomic Mass Oxygen – 16 , mass of 2.657 x 10-23 g Easier to use relative scale Need arbitrarily chosen standard All others are compared to carbon - 12 One atomic mass unit (1 amu) – exactly 1/12 the mass of a carbon – 12 atom. All others are compared to carbon -12 Hydrogen – 1 , about 1/12 the mass of carbon -12 Precise value is 1.007825 amu Magnesium – 24 , Slightly less than twice that of carbon -12 Precise value is 23.985042 amu

Average Atomic Mass of Elements Most elements occur naturally as mixtures of isotopes Average atomic mass : weighted average of the atomic masses of the naturally occurring isotopes of an element Ex. of weighted average Box contains two types of marbles, 25% have mass of 2.00g, and 75% have a mass of 3.00 g. 25 marbles x 2.00 g = 50 g 75 marbles x 3.oo g = 225 g so 50 g + 225 g = 275 g (TOTAL MASS) 275 g ÷ 100 = 2.75 g average marble mass

Calculating average atomic mass Or (2.00 g x 0.25) + (3.00 g x .75) = 2.75 g Calculate the average atomic mass of copper Use table 3-4 pg. 80 in text Copper - 63 69.17% with mass of 62.929599 amu Copper – 65 30.83% with mass of 64.927793 amu (0.6917 x 62.929599 amu) + (0.3083 x 64.927793 amu) =63.55 Match with atomic mass on periodic table Round to two decimal places in calculations Visual Concept – Click Here

Relating Mass to Numbers of Atoms The MOLE SI base unit for amount of a substance Abbreviated as mol Defined as : amount of substance that contains as many particles as there are atoms in exactly 12 g of carbon – 12 Counting unit just like a dozen We don’t order 12 or 24 eggs, we order 1 or 2 dozen In same way, a chemist may want 1 mol of carbon or 2 mol of iron. 63.55 g of copper or 1 mol of copper Visual Concept – Click Here

Avogadro’s Number The number of particles in a mole – AVOGADRO’S NUMBER 6.022 x 1023 particles in exactly one mole of a pure substance How big is this number? If every person on earth (5 billion people) counted 1 atom per second, it would take 4 million years to count all the atoms. Visual Concept – Click Here

Molar Mass Molar mass – mass of one mole of a pure substance Units : g/mol Numerically equal to atomic mass of element Mass of 6.02 x 1023 atoms of element One mole He 6.02 x 1023 atoms 4.oo g One mole Li 6.02 x 1023 atoms 6.94 g One mole Cu 6.02 x 1023 atoms 63.5o g One mole Fe 6.02 x 1023 atoms 55.78 g Visual Concept – Click Here

Gram/Mole Conversions Mass In grams Moles In mol Atoms In atoms Avogadro’s Number Molar mass Sample Problem B What is the mass in grams of 3.50 mol of the element copper, Cu? 63.55 g Cu 3.30 mol Cu x = 222 g Cu 1 mol Cu Step 1: Step 2: Step 3: Step 4: Write the given value Use molar mass from periodic table Cancel out units Calculate Always round to 2 places after the decimal

Sample Problem C Sample Problem D A chemist produced 11.9 g of Aluminum, Al. How many moles of aluminum were produced? 1 mol Al 11.9 g Al x = 0.441 mol Al 26.98 g Al Sample Problem D How many moles of silver, Ag, are in 3.01 x 1023 atoms of silver? 1 mol Ag 3.01 x 1023 atoms Ag x = 0.500 mol Ag 6.02 x 1023 atoms Ag Avogadro’s number

Sample Problem E What is the mass in grams of 1.20 x 108 atoms of copper, Cu? 63.55 g Cu 1 mol Cu x x 1.20 x 108atoms Cu 6.02 x 1023 atoms Cu 1 mol Cu = 1.27 x 10-14 g Cu Avogadro’s # ****Requires 2 conversions Molar mass

The Ten Commolements (found at http://members.tripod.com/~smalls42/commolements.html) Thou shalt not use 6.02 x 1023 in vain. Thou shalt not use the term mole if thou has no true knowledge of the term mole. Thou shalt not kill a mole. Thou shalt not covet your neighbor's mole. Thou shall always remember to celebrate Mole Day. Thou shalt not disparage Mole Day. Thou shalt not use a mole out of season. Thou shalt always honor the one who introduced thou to Mole Day. You shalt always keep sacred 10/23. Thou shalt always remember these commolments or thou will never properly celebrate Mole Day.