Chem 1151: Ch. 2 Atoms and Molecules. Structure of the Atom Mass (g)Mass (u) Proton (p+)1.67 x 10...

Chem 1151: Ch. 2

Atoms and Molecules

Structure of the AtomStructure of the Atom

Mass (g) Mass (u)

Proton (p+) 1.67 x 10-24 1

Neutron (n) 1.67 x 10-24 1

Electron (e-) 9.07 x 10-28 1/1836

Most of the mass is actually in the nucleusSeager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

The origin of atomsThe origin of atoms

Following the big bangExpansion of spaceCoolingFormation of fundamental particlesFormation of low mass nuclei (H and He)Star formationFusion reactions forming heavier elements

http://www.lbl.gov/abc/wallchart/chapters/10/0.html

Synthesis of MatterSynthesis of Matter

• Nucleosynthesis: Protons and neutrons join to form nuclei.

• Fusion: Multiple nuclei join to form heavier nucleus.

• Formation of heavier elements:• Two protons collide

– Releases positron, neutrino• Nucleus with proton and neutron

collides with another proton– releases gamma ray

• Two He-3 atoms collide– Produces He-4– Releases two protons

http://en.wikipedia.org/wiki/Image:FusionintheSun.png

http://upload.wikimedia.org/wikipedia/commons/8/86/FusionintheSun.png

Periodic Table of the ElementsPeriodic Table of the Elements

• All matter in our universe categorized in periodic table.– Based on atomic

number (number of protons).

• Arranged in columns (groups) and rows (periods).– Groups have similar

properties.– Periods correspond

to filling of quantum shells by electrons.

Elements from Group 7AElements from Group 7A

chlorine

bromineiodine

Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

Elements You Should KnowElements You Should Know

ELEMENT SYMBOL ELEMENT SYMBOL ELEMENT SYMBOL Aluminum Al Fluorine F Nitrogen N Argon Ar Gold Au Oxygen O Arsenic As Helium He Phosphorous P Barium Ba Hydrogen H Potassium K Bromine Br Iodine I Silicon Si Cadmium Cd Iron Fe Silver Ag Calcium Ca Lead Pb Sodium Na Carbon C Lithium Li Strontium Sr Cesium Cs Magnesium Mg Sulfur S Chlorine Cl Manganese Mn Tin Sn Chromium Cr Mercury Hg Zinc Zn Cobalt Co Neon Ne Copper Cu Nickel Ni

Applications of Atomic and Mass NumbersApplications of Atomic and Mass Numbers

– On the periodic table, the atomic number is written as a whole number above the symbol F.

– In the written description, fluorine is said to have 9 protons (the atomic number is the number of protons).

– In the symbol, the number 9 is written in the atomic number or Z (lower left) position.

– Note: The periodic table does not show the mass number for an individual atom. It lists an average mass number for a collection of atoms!


IsotopesIsotopes• Isotopes are atoms that have the same number of protons in

the nucleus but different numbers of neutrons. That is, they have the same atomic number but different mass numbers.

• Because they have the same number of protons in the nucleus, all isotopes of the same element have the same number of electrons outside the nucleus.


Isotope SymbolsIsotope Symbols

• Isotopes are represented by the symbol , where Z is the atomic number, A is the mass number, and E is the elemental symbol.

• Isotopes are also represented by the notation: Name-A, where Name is the name of the element and A is the mass number of the isotope.

• An example of this isotope notation is magnesium-26. This represents an isotope of magnesium that has a mass number of 26.

• Since all of the mass of atom comes from the protons and the neutrons, the mass number is the total number of protons and neutrons.

• You can therefore determine the number of neutrons by subtracting the atomic number from the mass number.

AZE

Use of Elemental NotationUse of Elemental Notation

Pb20882

Q: How to represent lead-208?

Q: How many p+, e-, n? 82, 82, 126

X94

Q: How to represent element X with 4 p+ and 5 n.

Relative Masses and Mass UnitsRelative Masses and Mass Units

• The extremely small size of atoms and molecules makes it inconvenient to use their actual masses for measurements or calculations. Relative masses are used instead.

• Relative masses are comparisons of actual masses to each other. For example, if an object had twice the mass of another object, their relative masses would be 2 to 1.

• An atomic mass unit is a unit used to express the relative masses of atoms. One atomic mass unit is equal to 1/12 the mass of a carbon-12 atom.

• A carbon-12 atom has a relative mass of 12 u because carbon-12 has 6 protons and 6 neutrons.

Mass (g) Mass (u)Proton (p+) 1.67 x 10-24 1Neutron (n) 1.67 x 10-24 1Electron (e-) 9.07 x 10-28 1/1836


Determining MassDetermining Mass

However, carbon has an actual mass listed of 12.011 u, not 12 u. Why are these values different? C6

12.011

Mass (g) Mass (u)

Proton (p+) 1.67 x 10-24 1

Neutron (n) 1.67 x 10-24 1

Electron (e-) 9.07 x 10-28 1/1836

12


How Isotopes Determine Atomic WeightHow Isotopes Determine Atomic Weight• The atomic weight of an element is the relative mass of an average atom of the

element expressed in atomic mass units.• Many elements have more than 1 isotope (e.g. – 12C, 13C, 14C). • Abundance of isotopes are not evenly distributed. • Weighted atomic mass of Carbon (12C, 13C only) = (0.98882*12u) + (0.01108 *

13.300335u) = 12.011u.

C612.011

12

C612.011

12C12

6 C13

6 C14

6 Carbon-12 Carbon-13 Carbon-14

%Abundance 98.892 1.108 1.0 x 10-10

AMU 12 u 13.300335 u

100

mass isotope% isotope weight Atomic

Determining Atomic WeightDetermining Atomic Weight

• A specific example of the use of the equation is shown below for the element boron that consists of 19.78% boron-10 with a mass of 10.01 u and 80.22% boron-11 with a mass of 11.01u.

• This calculated value is seen to agree with the value given in the periodic table.

u 81.10100

u 883.2 u 198.0

100

)u 01.11%22.80u 10.0119.78% AW


Molecular WeightMolecular Weight• The relative mass of a molecule in atomic mass units is called the

molecular weight of the molecule.• Because molecules are made up of atoms, the molecular weight of a

molecule is obtained by adding together the atomic weights of all the atoms in the molecule.

• The formula for a molecule of water is H2O. This means one molecule of water contains two atoms of hydrogen, H, and one atom of oxygen, O. The molecular weight of water is then the sum of two atomic weights of H and one atomic weight of O:

• MW = 2(at. wt. H) + 1(at. wt. O) • MW = 2(1.01 u) + 1(16.00 u) = 18.02 u


Molecular WeightMolecular Weight

• The clear liquid is carbon disulfide, CS2. It is composed of carbon (left) and sulfur (right). What is the molecular weight for carbon disulfide?

• Answer: MW = 1(atomic weight C) + 2(atomic weight S) 12.01 u + 2(32.07 u) = 76.15 u


The MoleThe Mole• THE MOLE CONCEPT APPLIED TO ELEMENTS

– The number of atoms in one mole of any element is called Avogadro's number and is equal to 6.022x1023 .

– A one-mole sample of any element will contain the same number of atoms as a one-mole sample of any other element.

– One mole of any element is a sample of the element with a mass in grams that is equal to the atomic weight of the element.

• EXAMPLES OF THE MOLE CONCEPT – 1 mole Na = 22.99 g Na = 6.022x1023 Na atoms– 1 mole Ca = 40.08 g Ca = 6.022x1023 Ca atoms– 1 mole S = 32.07 g S = 6.022x1023 S atoms


The MoleThe Mole• THE MOLE CONCEPT APPLIED TO COMPOUNDS

– The number of molecules in one mole of any compound is called Avogadro's number and is numerically equal to 6.022x1023.

– A one-mole sample of any compound will contain the same number of molecules as a one-mole sample of any other compound.

– One mole of any compound is a sample of the compound with a mass in grams equal to the molecular weight of the compound.

• EXAMPLES OF THE MOLE CONCEPT– 1 mole H2O = 18.02 g H2O = 6.022x1023 H2O molecules– 1 mole CO2 = 44.01 g CO2 = 6.022x1023 CO2 molecules– 1 mole NH3 = 17.03 g NH3 = 6.022x1023 NH3 molecules


Relationships: Mass, Moles, Molecular WeightRelationships: Mass, Moles, Molecular Weight

1 mol C atoms = 6.022 x 1023 atoms C6.022 x 1023 atoms C = 12.01 g C1 mol C atoms = 12.01 g CC has atomic weight of 12.01 u or 12.01 g/mol

1 mol S atoms = 6.022 x 1023 atoms S6.022 x 1023 atoms S = 32.1 g S1 mol S atoms = 32.1 g SS has atomic weight of 32.1 u or 32.1 g/mol

ProblemsProblems1. What is the mass in g of 1.35 mol of S? Mass S? Mol g 1.35 mol S X 32.1 g S = 43.3 g S

1 mol S

2. How many S atoms are in 98.6 g of S? 98.6 g S X 1 mol S X 6.022 x 1023 atoms S = 1.85 x 1024 atoms S

32.1 g S 1 mol S

3. What is the mass in g of 1 atom of S? 1 atom S X 1 mol S X 32.1 g = 5.33 x 10-23 g S

6.022 x 1023 atoms S 1 mol S

Moles of MoleculesMoles of Molecules

1. What is the mass in g of 1.62 mol of O2 molecules?

MW of O2 = 2 x (atomic weight of O) = 2 x (16.0 u) = 32.0 u1 mol O2 molecules = 6.022 x 1023 molecules O2

6.022 x 1023 molecules O2 = 32.0 g O2

1 mol O2 molecules = 32.0 g O2

1.62 mol O2 molecules X 32.0 g O2 = 51.8 g O2

1 mol O2 molecules 1 mol O2 molecules = 12.04 x 1023 atoms O

Compound (Molecular) FormulasCompound (Molecular) Formulas

The compound (molecular) chemical formula represents the numerical relationships that exist between atoms in a compound. This also applies to moles.

Compound formula: all elements and number of each in a compound

Examples:Urea 1C, 4H, 2N, 1OHydrofluoric acid 1H, 1FSodium bicarbonate 2H, 1C, 3OSodium Azide 1Na, 3N

1 molecule of H2SO4 contains2 atoms of H1 atom of S4 atoms of O

1 mol of H2SO4 contains2 mol of H1 mol of S4 mol of O

Mole Calculations (continued)Mole Calculations (continued)• The mole concept applied earlier to molecules can be applied

to the individual atoms that are contained in the molecules.• An example of this for the compound CO2 is:

1 mole CO2 molecules = 1 mole C atoms + 2 moles O atoms

44.01 g CO2 = 12.01 g C + 32.00 g O

6.022x1023 CO2 molecules = 6.022x1023 C atoms +

(2) 6.022x1023 O atoms


Finding the Molecular WeightFinding the Molecular Weight*Use the mole relationship of the compound to find the MWEx. 1 H2SO4

Atomic WeightsH = 1.008 g/molS = 32.06 g/molO = 16.00 g/mol MW of 1 mol of H2SO4 = (2 * H = 2.016 g/mol) + (1 * S = 32.06 g/mol) + (4 * O = 64.00 g/mol) = 98.22 g/mol

Ex. 1 C3H8O (isopropyl alcohol)

Atomic WeightsC = 12.01 g/molH = 1.008 g/molO = 16.00 g/mol MW of 1 mol of C3H8O = (3 * C = 36.03 g/mol) + (8 * H = 8.064 g/mol) + (1 * O = 16.00 g/mol) = 60.09 g/mol

Finding the Number of Atoms in a CompoundFinding the Number of Atoms in a Compound

Ex. 1 How many C atoms in 5.2 g of C3H8O (isopropyl alcohol)

Atomic WeightsC = 12.01 g/molH = 1.008 g/molO = 16.00 g/mol MW of 1 mol of C3H8O = (3 * C = 36.03 g/mol) + (8 * H = 8.064 g/mol) + (1 * O = 16.00 g/mol) = 60.09 g/mol

1 mol of C3H8O contains 3 mol of C

Percent CompositionPercent Composition

Mass relationships can be used to determine percent compositions 1 mol of H2SO4 % H = 2.0 g x 100 = 2.0 %

98.1 g % S = 32.1 g x 100 = 32.7 %

98.1 g % O = 64.0 g x 100 = 65.2 %

98.1 g

% Mass of element in compound% Mass of element in compound

% mass N in HNO3? N = 14.01 g/mol H = 1.008 g/mol O = 16.00 g/mol

HN03 % mass= part X 100 total Total= (1 x H) + (1 x N) + (3 + O) = (1 x 1.008) + (1 x 14.01) + (3 x 16.00) = 63.018 g/mol HN03 % mass= 14.01 g/mol X 100 = 22.23%

63.018 g/mol

% Mass of element in compound% Mass of element in compound

% mass N in NaN3? N = 14.01 g/molNa = 22.99 g/mol

NaN3 % mass= part X 100 total Total= (1 x Na) + (3 x N) = (1 x 22.99 g/mol) + (3 x 14.01 g/mol) = 65.02 g/mol NaN3 % mass= 42.03 g/mol X 100 = 64.64%

65.02 g/mol

Atomic Weight of Element with Multiple IsotopesAtomic Weight of Element with Multiple Isotopes

Element X has 3 isotopes (10)X 70% 41.00 u (11)X 20% 42.00 u (12)X 10% 43.00 u

What is atomic weight of element X?

(70% x 41.00 m) + (20% x 42.00 u) + (10% x 43.00 u) =

(70 x 41.00 m) + (20 x 42.00 u) + (10 x 43.00 u) = 41.40 u100

IonsIons Ion: Atom or molecule that has either lost or gained electrons from

valence shell resulting in a net charge (positive or negative) compared to the number of protons.

Ions of element have the same number of protons, but a different number of electrons.

For example compare the following:

Common atomic ions you should know: H+, Na+, K+, Mg2+, Ca2+, Fe2+, Fe3+, Ag1+, Pb2+, N3-, P3-, O2-, S2-, F-, Cl-, Br-

24420CaCa44

20 vs.

ExercisesExercisesWhat are the charge, mass (u) in the following nuclei?1. 5 p+, 6 n2. 10 p+, 12 n3. 11 p+, 14 n, 1e-

What are the p+, n, e-, charge?

Ne2210

Ca4420

24020Ca

What are the MW values in u?

C6H12O6 (glucose)

O3 (ozone)

Which is denser, U-235 or U-238? Why?

What these numbers mean?What these numbers mean?Mass of 1 atom of Mg = 4.037 x 10-23 gMg atomic weight = 24.31 u How many atom of Mg in24.31 g Mg? 24.31 g Mg x 1 atom Mg = 6.022 x 1023 atoms Mg

4.037 x 10-23 g

Mass of 1 atom of C = 1.994 x 10-23 gC atomic weight = 12.01 u 12.01 g C x 1 atom C = 6.022 x 1023 atoms C

1.994 x 10-23 g

The number of atoms of an element, in a mass equivalent to it’s atomic weight, is equal to Avogadro’s number (mole).

or

1 u = 1 g/mole

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Chem 1151: Ch. 2 Atoms and Molecules. Structure of the Atom Mass (g)Mass (u) Proton (p+)1.67 x 10...

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