Mass Relationships of Atoms · 2015-09-08 · However, two atoms of the same element can have...

Mass Relationships of Atoms

Atoms,Molecules,and Ions

Symbols of Elements

Elements are symbolized by one or two letters.

Symbols of Elements

All atoms of the same element have the same number of protons, which is called the atomic number, Z.

Symbols of Elements

The mass of an atom in atomic mass units (amu) is the total number of protons and neutrons in the atom.

Isotopes

• Isotopes are atoms of the same element with different masses.

• Isotopes have different numbers of neutrons.

Average Mass

• Because in the real world we use large amounts of atoms and molecules, we use average masses in calculations.

• Average mass is calculated from the isotopes of an element weighted by their relative abundances.

Atomic number and mass numberAtomic number (Z) = the number of protons in the nucleus.

Mass number (A) = the sum of the number of protons + neutrons in the nucleus.

Symbols for a few atoms

2He 126 C 16

8 O 6530 Zn

2He 126 C 16

8 O 6530 Zn

Mass number (A)

2He 126 C 16

8 O 6530 Zn

Atomic number (Z)

ElementAn element is a form of matter in which all of the atoms have the same atomic number.

However, two atoms of the same element can have different mass numbers.

IsotopeAtoms that have the same atomic number but different mass numbers are called isotopes.

- same number of protons in nucleus- differ in number of neutrons

Some isotopes

11H 12

6 C 168 O 235

21H 13

6 C 178 O 238

31H 14

6 C188 O

Atomic masses- synonymous with atomic weight- is a relative scale- mass-12 isotope of carbon (carbon-12) is thereference atom and assigned an atomic mass ofexactly 12- one atomic mass unit (amu) is defined as a mass exactly equal to 1/12th the mass of one carbon-12 atom

relative masses of carbon-12 and carbon-13 in a random sample carbon has a ratio of 1.0836129

= 1.0836129

= (1.0836129)(12 amu)

Since the atomic mass unit is defined such that the mass of 12C is exactly 12 atomic mass units, then

136 CMass of

= 13.003355 amu

= 1.0836129

The Mole- the fundamental SI measure of “amount ofsubstance”- the amount of substance that contains as many elementary entities as there are atoms in exactly 12 g of carbon-12- this number of atoms is 6.022045 x 1023

Avogadro’s number

Stoichiometry

Avogadro’s Number

• 6.02 x 1023

• 1 mole of 12C has a mass of 12.000 g.

The Mole vs. The Dozen

The Dozen - the amount of substance that contains 12 entities.

The Mole - the amount of substance that contains Avogadro’s number (6.022x1023) of entities.

Dozen Apples = 10 Lbs.

Mole of Helium atoms = 4.0026g

Dozen Apples = 12 Apples

Mole of Helium atoms = 6.022x1023 atoms

Example

1.3 Lbs

How many dozens of apples are represented by 1.3 Lbs. of apples.

Converting to Dozens

10 Lbsx

1 dozen.13 dozen=

Example

How many moles of He are in 6.46 g of He?

Converting to Moles

1.61 mol=6.46 g He4.003 g1 molx

The Mole

- since 6.022045 x 1023 atoms of carbon have a mass of 12 grams,

6.022045 x 1023 atoms x1 atom

12 amu12 g=

6.022045 x 1023 amu 1 g=

Stoichiometry

Mole Relationships

• One mole of atoms, ions, or molecules contains Avogadro’s number of those particles.

• One mole of molecules or formula units contains Avogadro’s number times the number of atoms or ions of each element in the compound.

Molar mass of an elementThe mass of 6.022 x 1023 atoms of an elementis equal to its atomic mass in grams.BUT: what does the periodic table tell us aboutthe atomic mass of carbon?Atomic mass is weighted average of mixture of isotopes

Atomic weight of carbon= (atomic mass ) (fraction ) 12

6 C 126 C

+ (atomic mass ) (fraction ) 136 C 13

= (12.0000 amu)(0.9889)+(13.0035 amu)(0.0111)

= 11.8670 amu + 0.1441 amu

= 12.0111 amu

ExampleCopper, a metal known since ancient times, is used in Electrical cables and pennies, among other things. The atomic masses of its two stable isotopes, (69.09%) and (30.91%), are 62.93 amu and 64.9278 amu, respectively. Calculate the average atomic mass of copper. The percentages in parentheses denote the relative abundances.

6329Cu 65

Answer

= (62.93 amu)(0.6909)+(64.9278 amu)(0.3091)

= 43.47 amu + 20.07 amu

= 63.54 amu

6329Cu= (atomic mass ) (fraction ) 63

6529Cu+ (atomic mass ) (fraction ) 65

Mass Relationships of Atoms - Extended

ExampleCalculate the number of grams of lead (Pb)In 12.4 moles of lead.

12.4 mol 2569 g=207.2 g

ExampleWhat is the mass in grams of one silver atom?

107.9 g1 mol

= 17.91 x 10–23 g 1 atom

6.022 x 1023 atoms x

ExampleCalculate the number of atoms in 0.551 g ofPotassium (K).

0.551 g39.10 g1 mol

x1 mol

6.022 x 1023 atomsx

= 8.48 x 1021 atoms K

ExampleCalculate the number of molecules in a sampleof oxygen gas (O2) with a mass of 64.0g.

1.20 x 1024 molec. O2=

64.0 g x6.02 x 1023 molec.

1 mol32.0 g

1 molx

Experimental DeterminationOf Atomic & Molecular Masses

Atomic mass is measured by mass spectrometry

Atom is bombarded by stream of highEnergy electrons.

Electron collides with atom, “bounces” off

and transfers some of its energy to it.

Atom dissipates its excess energy by expellingone of its electrons.

Ion is deflected by magnetic field

amount of deflection depends on mass to chargeratio

highest m/zdeflected least

lowest m/zdeflected most

Ions are detected after passagethrough magnetic field

++++ ++mixture of ions ofdifferent mass givesseparate peak for each m/z

intensity of peak proportionalto percentage of each atom ofdifferent mass in mixture

separation of peaksdepends on relative mass

19 20 21 22 23

90.92%

0.26%8.82%

The mass spectrum ofthe three isotopes of neon.

Atomic Mass

Atomic and molecular masses can be measured with great accuracy using a mass spectrometer.

Mass Relationships of Atoms · 2015-09-08 · However, two atoms of the same element can have...

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