4.3 How Atoms Differ4.3 How Atoms Differ

Atomic NumberAtomic Number- Atomic Number - Atomic Number = # protons in an = # protons in an

atomatom- - # of protons determines kind of # of protons determines kind of

atomatom (atoms of an element always (atoms of an element always have the SAME number of protons)have the SAME number of protons)

- The The atomic numberatomic number == the number of the number of electronselectrons in a neutral atom in a neutral atom

- Written just Written just above the atomic above the atomic symbolsymbol on the periodic table on the periodic table

Complete the Table:Complete the Table:

Element Atomic # # Protons # Electrons

6

N

Pb

3

19

Br

Element Atomic # # Protons # Electrons

C 6 6 6

N 7 7 7

Pb 82 82 82

Li 3 3 3

K 19 19 19

Br 35 35 35

IsotopesIsotopes• All atoms in an element have

the same number of protons

• The number of neutrons vary

• Atoms of an element with different numbers of neutrons = ISOTOPES

IsotopesIsotopes• Elements in nature = a mix of isotopes

• The abundances of different isotopes are constant

• EX: in ANY banana, 93% K has 20 neutrons, 7% have 22 neutrons

http://www.youtube.com/watch?v=Jdtt3LsodAQ&feature=related

Mass of atomsMass of atoms

• One atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12 atom.

• One amu is nearly, but not exactly, equal to one proton or one neutron.

Mass of subatomic Mass of subatomic particlesparticles

Mass NumberMass Number

# PROTONS + # # PROTONS + # NEUTRONS = MASS NEUTRONS = MASS NUMBERNUMBEREXAMPLE: What is the mass

number for manganese if there are 30 neutrons in the nucleus?

Element SymbolsElement Symbols

Top is mass Top is mass numbernumber

Bottom is Bottom is atomic atomic numbernumber

PracticePractice

SymbolsSymbolsEXAMPLE 1:EXAMPLE 1:

Find the Find the – number of protonsnumber of protons– number of neutronsnumber of neutrons– number of electronsnumber of electrons– Atomic numberAtomic number– Mass NumberMass Number

F19 9

Answer…

- 9 protons- 10 neutrons- 9 electrons- Atomic # = 9- Mass # = 19- What would the mass number be of

a fluorine isotope with two additional neutrons?

SymbolsSymbolsEXAMPLE:EXAMPLE:

Find the Find the

–number of protonsnumber of protons

–number of neutronsnumber of neutrons

–number of electronsnumber of electrons

–Atomic numberAtomic number

–Mass NumberMass Number

Br80 35

Answer

– Number of protons: 35Number of protons: 35– Number of neutrons: 45Number of neutrons: 45– Number of electrons: 35Number of electrons: 35– Atomic number: 35Atomic number: 35– Mass Number: 80Mass Number: 80

SymbolsSymbolsIf an element has an atomic number If an element has an atomic number

of 34, a mass number of 78, find…of 34, a mass number of 78, find…

–number of protonsnumber of protons

–number of neutronsnumber of neutrons

–number of electronsnumber of electrons

And WRITE the Complete symbolAnd WRITE the Complete symbol

Answer…

- 34 protons- 34 electrons- 44 neutrons- Symbol: Se

78

34

SymbolsSymbolsIf an element has 91 protons and If an element has 91 protons and

140 neutrons what is the……..? 140 neutrons what is the……..?

–Atomic numberAtomic number

–Mass numberMass number

–number of electronsnumber of electrons

–Complete symbolComplete symbol

Pa91

231

SymbolsSymbolsIf an element has 78 electrons and If an element has 78 electrons and

117 neutrons what is the ………?117 neutrons what is the ………?

–Atomic numberAtomic number

–Mass numberMass number

–number of protonsnumber of protons

–Complete symbolComplete symbol

Pt78

195

Naming IsotopesNaming Isotopes

Put the mass number after Put the mass number after the name of the elementthe name of the element

carbon- 12carbon- 12 carbon -14carbon -14 uranium-235uranium-235

Mass of the atomMass of the atom

• Atomic mass on the periodic table is NOT an even #

• The atomic mass of an element is the weighted average mass of the isotopes of that element

• Depends on abundance of each isotope

Atomic MassAtomic Mass- How heavy is an atom of oxygen?How heavy is an atom of oxygen?- The The MASS NUMBERSMASS NUMBERS we’ve been using are we’ve been using are

measured in amumeasured in amu- 1 amu 1 amu (atomic mass units)(atomic mass units) is is ABOUT ABOUT the mass of 1 the mass of 1

proton or 1 neutron (or 1/12 of a carbon -12 atom)proton or 1 neutron (or 1/12 of a carbon -12 atom)- On the periodic table, the MASS of each element is On the periodic table, the MASS of each element is

NOT reported as a whole number…NOT reported as a whole number…- We can’t have HALF a proton or HALF a neutron in We can’t have HALF a proton or HALF a neutron in

an atom… so why aren’t these masses whole an atom… so why aren’t these masses whole numbers??numbers??

Atomic Mass- Chlorine-35 is MORE common in

nature than chlorine-37…so the mass of 1000 Cl atoms will be CLOSER to 35000 amu

- To take the average of the two isotopes, we need to use the WEIGHTED AVERAGE:

- In nature, 75.53% of Cl atoms are chlorine-35, and 24.47 % are chlorine- 37…

- (% is called % abundance)

Calculating the WEIGHTED AVERAGE

- Write each percent as a decimal- Multiply that decimal by the isotope’s mass- Find the average by adding together

Average = % as decimal x mass + % as decimal x Average = % as decimal x mass + % as decimal x massmass

- Average MassAverage Mass of a Chlorine atom: of a Chlorine atom:

(0.7553(0.7553)(35))(35) + (0.2447 + (0.2447)(37))(37) = = 35.4894 amu35.4894 amu% abundance

isotope 1Mass

isotope 1

% abundance isotope 2

Mass isotope 2

Atomic Mass: EXAMPLEAtomic Mass: EXAMPLE

- Calculate the atomic mass of - Calculate the atomic mass of copper if copper has two isotopes. copper if copper has two isotopes. 69.1%69.1% have a mass of have a mass of 62.93 amu62.93 amu and the rest have a mass of and the rest have a mass of 64.93 64.93 amu… amu…

ANSWER…Mass of Cu = (.691)(62.93) + (.309)(64.93) =

63.548 amu

Atomic Mass: EXAMPLE Atomic Mass: EXAMPLE 22- Magnesium has three isotopes. - Magnesium has three isotopes.

78.99% magnesium-24,78.99% magnesium-24, 10.00% 10.00% magnesium-25magnesium-25, and the , and the rest isrest is magnesium-26magnesium-26 amu. amu. What is the What is the atomic mass of magnesium?atomic mass of magnesium?

Answer…

Mass of Magnesium =

(.7899 x 7899 x 24) + (.24) + (.1000 x 1000 x 25) + 25) + (.(.1101 x1101 x 26) = 26) =

24.3202 amu24.3202 amu