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Chem101 - Lecture 2
Atoms and Molecules
University of Wisconsin-Eau Claire Chem101 - Lecture 2 2
Elements
• Elements are pure substancescontaining only one kind of atom(homoatomic).
• There are at last count 114 elements.- 88 of them are naturally occurring.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 3
Elements as Pure Substances• As pure substances, each element has
a well defined set of physicalcharacteristics.- Hydrogen is a clear, colorless, flammable
diatomic (H2) gas.
- Nitrogen is a clear, colorless,nonflammable diatomic gas (N2).
- Sulfur is a yellow, amorphouspolyatomic (S8) solid.
- Sodium is a highly reactive, soft, metallicsolid (Na).
University of Wisconsin-Eau Claire Chem101 - Lecture 2 4
Chemical Symbols for Elements
• Each element has a unique one or twocharacter symbol (Table 2.1)- The first letter of the symbol is always
capitalized.
- The second letter, if there is one, isalways lower case.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 5
Chemical Symbols for Elements
• Most of the symbols are derived fromthe element’s English name:- H (Hydrogen), C (Carbon), Ca
(Calcium), O (Oxygen), Cl (Chlorine)
• Some of the symbols are derived fromthe element’s Greek or Latin name:- Au (Gold, aurum), Ag (Silver,
argentum), Na (Sodium, natrium), Cu(Copper, cuprum), Fe (Iron, ferrum)
University of Wisconsin-Eau Claire Chem101 - Lecture 2 6
Compounds
• Compounds are pure substanceswhose molecules contain more thanone type of atom (heteroatomic)
• There are millions of differentcompounds.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 7
Compounds as Pure Substances
• As pure substances, each compoundhas a well defined set of physicalcharacteristics.- Dihydrogen oxide (H2O) is a clear,
colorless liquid.
- Methane (CH4) is a clear, colorless,flammable gas.
- Acetic acid (C2H4O2) is a crystallinewhite solid.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 8
Chemical Formulas for Compounds
• Chemical formulas are used toindicate the chemical composition of acompound’s molecule.
• In a chemical formula, the chemicalsymbols of all of the elementscontained in the compound are listed.- Subscripts are used to indicate the
numbers of atoms of each elementcontained in each molecule of thecompound.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 9
Chemical Formulas for Compounds• For example:
- The chemical formula for water is H2OEach molecule of water contains two hydrogen
(H) atoms and one oxygen (O) atom.
- The chemical formula for methane is CH4
Each molecule of water contains one carbon (C)atom and four hydrogen (H) atoms.
- The chemical formula for acetic acid is C2H4O2
Each molecule of acetic acid contains twocarbon (C) atoms, four hydrogen (H) atoms andtwo oxygen (O) atoms.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 10
Exercise 2.5
Determine the number of each type of atom inmolecules represented by the following formulas:
a. sulfur trioxide (SO3)b. nitric acid (HNO3)c. ammonia (NH3)d. propane (C3H8)
University of Wisconsin-Eau Claire Chem101 - Lecture 2 11
Inside the Atom• Atoms are the limit of chemical
subdivision in matter.
• Each element has a different type ofatom.- Each with different chemical and
physical properties
• To understand how different types ofatoms can have different chemical andphysical properties, we need to knowwhat atoms are made of.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 12
Inside the Atom• All atoms are made of three different
subatomic particles.- These were discovered in in the latter
part of the 19th and early part of the 20th
century.
• The three subatomic particles include:- The proton
- The electron
- The neutron
University of Wisconsin-Eau Claire Chem101 - Lecture 2 13
Inside the Atom• Each of these particles have
distinguishable physical properties,the important ones being mass andelectrical charge.
Particle CommonSymbols
Charge{±} Mass {g} Relative
Mass
Electron e – 1- 9.07 x 10–28 1/1836
Proton p, p+, H+ 1+ 1.67 x 10–24 1
Neutron n 0 1.67 x 10–24 1
Characteristic Properties
University of Wisconsin-Eau Claire Chem101 - Lecture 2 14
Inside the Atom• The protons and neutrons contribute
essentially all of the mass to an atom.
• The protons contribute all of the positivecharge, while the electrons contribute all ofthe negative charge.- In a neutral atom the net charge is zero,
therefore the numbers of protons and electronsare equal.
Particle CommonSymbols
Charge{±} Mass {g} Relative
Mass
Electron e – 1- 9.07 x 10–28 1/1836
Proton p, p+, H+ 1+ 1.67 x 10–24 1
Neutron n 0 1.67 x 10–24 1
Characteristic Properties
University of Wisconsin-Eau Claire Chem101 - Lecture 2 15
Inside the Atom• The protons and neutrons are tightly
bound together and located in thecenter of the atom in a region calledthe nucleus.
• The electrons surround the nucleus.
Nucleus(comprising the
protons and neutrons)Electrons
Are located outside ofthe nucleus
University of Wisconsin-Eau Claire Chem101 - Lecture 2 16
Inside the Atom• The popular model of an atom shown on
the previous slide is somewhat misleading.- The electrons are not hard spheres that whirl
about the nucleus, but rather should bethought of as a cloud that surrounds thenucleus.
Nucleus(comprising the
protons and neutrons) ElectronsAre located outside of
the nucleus
University of Wisconsin-Eau Claire Chem101 - Lecture 2 17
Inside the Atom• The popular model of an atom shown on
the previous slide is also misleading.- The volume occupied by the nucleus, which
contains essentially all of the mass, is quitesmall compare to that occupied by theelectrons.
- The diameter of the nucleus is approximately1/100,000 the radius of the electron cloud.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 18
Nucleus(mass and
positive charge)
Electron Cloud(negative charge)
If the nucleus were1 cm in diameter
The electron cloudwould extend 1km to
Water Street!
Inside the Atom
University of Wisconsin-Eau Claire Chem101 - Lecture 2 19
Inside the Atom• The number protons an atom contains
determines which element it is, thisnumber is called the atomic number and isrepresented by the letter Z.
• For an electrically neutral atom, thenumber of electrons has to equal thenumber of protons.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 20
Exercise 2.13
Determine the number of electrons and protonscontained in an atom of the following elements:
a. sulfurb. Asc. element number 24
University of Wisconsin-Eau Claire Chem101 - Lecture 2 21
Isotopes
• The number of neutrons an atom hasdoes not affect its electrical charge,- nor does it affect which element an atom is.
• The number of neutrons in an atom isapproximately equal to the number ofprotons, but it can vary for the differentatoms of a given element.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 22
Isotopes• The number of neutrons does affect an
atom’s mass.- The sum of the number of protons and
neutrons an atom contains is called theatom’s atomic mass number.
- The atomic mass number is represented bythe letter A.
• The different forms of atoms thatelements have due to differences in theirnumber of neutrons are called isotopes.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 23
Isotopes• Each element has its own
characteristic number of isotopes andrelative abundance of each.
• For example- Phosphorus (P) has just one naturally occurring
isotope; it contains 16 neutrons.
- Carbon (C) has two naturally occurringisotopes: one has 6 neutrons and represents98.93% of all naturally occurring carbon, theother has 7 neutrons and represents theremaining 1.07% of all naturally occurringcarbon
University of Wisconsin-Eau Claire Chem101 - Lecture 2 24
Isotopes- Chlorine (Cl) also has two naturally occurring
isotopes: one has 18 neutrons and represents75.78% of all naturally occurring chlorine, theother has 20 neutrons and represents the remaining24.22% of all naturally occurring chlorine.
- Magnesium (Mg) has three naturally occurringisotopes: one has 12 neutrons and represents78.99% of all naturally occurring magnesium,another has 13 neutrons and represents 10.00% ofall naturally occurring magnesium, and the last has14 neutrons and represents the remaining 11.01%of all naturally occurring magnesium.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 25
Isotopes• The number of naturally occurring isotopes
an element has, and their natural abundance,are characteristic properties of eachelement.- They have been determined experimentally.
- They can be looked up in the CRC Handbook ofChemistry and Physics.
- They can also be found on-line at theWebElements web site:(http://www.shef.ac.uk/chemistry/web-elements/)
University of Wisconsin-Eau Claire Chem101 - Lecture 2 26
Isotopes• The convention used to distinguish one
isotope of an element from another ismodify the element’s chemical symbol:
ZAE
- Where E is the element’s chemical symbol,
- Z is the element’s atomic number,
- and A is the element’s atomic mass number.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 27
Isotopes• For our previous examples:
- Phosphorus:
- Carbon:
- Chlorine:
- Magnesium:
• Since an element’s atomic number isunique, it is not necessary to includeit, for example, for magnesium we canwrite:- Magnesium:
1531P
612
613C and C
1735
1737Cl and Cl
1224
1225
1226Mg Mg and Mg,
24 25 26Mg Mg and Mg,
University of Wisconsin-Eau Claire Chem101 - Lecture 2 28
Isotopes• Another way to represent the different
isotopes of an element is to write outthe name of the element followed bythe atomic mass number:- phosphorus-31
- carbon-12 and carbon-13
- chlorine-35 and chlorine-37
- magnesium-24, magnesium-25 andmagnesium-26
University of Wisconsin-Eau Claire Chem101 - Lecture 2 29
Exercise 2.15
Determine the number of protons, number ofneutrons, and the number of electrons in atoms ofthe following isotopes:
a.
b.
c.
37Li
1022Ne
2044Ca
University of Wisconsin-Eau Claire Chem101 - Lecture 2 30
Exercise 2.17
Write symbols like those given in Exercise 2.15for the following isotopes:
a. cadmium-110b. cobalt-60c. uranium-235
University of Wisconsin-Eau Claire Chem101 - Lecture 2 31
Relative Masses
• The mass of an atom cannot bedetermined by simply adding up themass of its constituent protons,neutrons and electrons.- This is because some of the mass is
converted to energy, which is used tohold the nucleus of the atom together.
• The mass of the atoms for eachelement have therefore beendetermined experimentally.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 32
Relative Masses
• By convention, atomic masses aredetermined by comparing them to themass of the carbon-12 isotope.
• The unit of mass that is used is calledthe atomic mass unit and isrepresented by the symbol u.
• The atomic mass unit is equal toexactly 1/12 the mass of the carbon-12isotope.- A carbon-12 atom weighs exactly 12 u.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 33
Relative Masses• The atomic mass is also called the atomic
weight.
• The atomic weights of the elements are givenon most periodic tables of the elements.
• The atomic weights given reflect an average ofthe weights of the naturally occurring isotopesof an element.- For example, the atomic weight given for carbon is
12.01 u instead of exactly 12 u.This is because naturally occurring carbon comprises
98.93 % carbon-12 and 1.07% carbon-13.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 34
Relative Masses• Chlorine has two naturally occurring isotopes:
chlorine-35 (75.53%) and chlorine-37(24.47%).- The atomic weight for chlorine-35 is 34.97 u.
- The atomic weight for chlorine-37 is 36.97 u.
• The average atomic weight for chlorine is
75 53 34 97 24 47 36 97100
35 45. . . .
.( )( ) + ( )( ) =u u
u
• This is the mass reported for chlorine on theperiodic table.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 35
The Mole
• The atomic mass unit is a very smallunit of mass.
• In the lab we typically use grams asour unit of mass.
• A mole is defined as the number ofatoms of an element who’s mass ingrams is numerically equal to theatom’s mass in atomic mass units.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 36
The Mole
• For example:- The mass of 1 mole of carbon is 12.01 g.
- The mass of 1 mole of chlorine is 35.45 g.
• The number of atoms in a mole of atomsis 6.022 x 1023.- This number is called Avogadro’s number.
- You can have a mole of any object, like1 dozen (12) or 1 gross (144), it is just anumber.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 37
The Mass of a Mole of Molecules.
• Most pure substances are made ofmolecules.
• The mass of a mole of molecules iscalled the molar mass or molecularweight.
• The molecular weight for a moleculecan be determined from its chemicalweight and the atomic weights of itsconstituent atoms.
University of Wisconsin-Eau Claire Chem101 - Lecture 2 38
The Mass of a Mole of Molecules.
• To determined the molecular weight ofH2O:- O:
- H:
• Added together these give the molecularweight for H2O:
11
16 001
16 001
mol
mol
g
mol
g
mol
OH O
OO
OH O2 2
=
. .
21
1 0081
2 0161
mol
mol
g
mol
g
mol
HH O
HH
HH O2 2
=
. .
16 001
2 0161
18 021
. . .g
mol
g
mol
g
mol
OH O
HH O
H OH O2 2
2
2
+
=
University of Wisconsin-Eau Claire Chem101 - Lecture 2 39
Exercise 2.29
Determine the molecular weights of the followingin u:
a. oxygen (O2)b. carbon monoxide (CO)c. chloric acid (HClO3)d. glycerine (C3H8O3)e. sulfur dioxide (SO2)
University of Wisconsin-Eau Claire Chem101 - Lecture 2 40
Exercise 2.57
Urea (CH4N2O) and ammonium sulfate(N2H8SO4) are both used as agriculturalfertilizers. Which one contains the higher masspercentage of nitrogen?