Chem 1151: Ch. 2 Atoms and Molecules. “It’s a theory not a fact” Hypothesis: A possible...

Chem 1151: Ch. 2Chem 1151: Ch. 2

Atoms and Molecules

“It’s a theory not a fact”• Hypothesis: A possible explanation.• Theory: A proposed explanation that is consistent with all

available evidence that has been repeatedly tested.• A theory evolves from interpretation of data (facts).• “Theory” is frequently misunderstood by the general public.• Gravity (Newton and Einstein) is “just a theory”, but I predict

that if you jump from the roof of your house, you will fall.• The scientific method is a sequence of steps for systematically

analyzing the natural world.

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 elements up to Iron-56

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

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

4 Fundamental Forces in the Universe4 Fundamental Forces in the Universe

http://hyperphysics.phy-astr.gsu.edu/hbase/forces/funfor.html#c4

These forces generally considered manifestations of the same force. Unified.

http://hyperphysics.phy-astr.gsu.edu/hbase/grav.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

Abundance of Elements in the UniverseAbundance of Elements in the Universe

Mg

K

Cd

http://en.wikipedia.org/wiki/File:SolarSystemAbundances.png

• Fuse 3 4He atoms together 12C, basic building block of all organic compounds life.

• Elements with FW or AMU multiples of 4 are more abundant: (C, O, Si, Ca, etc.)

• Fusion Process continues up through Fe.

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

Elemental NotationElemental Notation

http://hyperphysics.phy-astr.gsu.edu/HBASE/pertab/pertab.html

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!


Elemental NotationElemental 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.

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.

AZE

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

12AMU 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 numerically 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


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

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

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 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

• Any two of these nine quantities can be used to provide factors for use in solving numerical problems.


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Chem 1151: Ch. 2 Atoms and Molecules. “It’s a theory not a fact” Hypothesis: A possible...

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