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Chapter Two
ATOMS, MOLECULES AND IONS
Section 2.3 The modern view of atomic structure:
Subatomic particles in the atom:
Proton, neutron, and electron.
General Chemistry A: First
Charge of electron = - 1.602  10-19 C
-19
2013/2014
Charge of proton = + 1.602  10 C
Neutrons are not charged.
Every atom has an equal number of electrons and protons, so atoms are electrically neutral.
Electrons reside in a vast space in the atom.
Protons and neutrons reside in the nucleus of the atom.
Electrons are attracted to the protons in the nucleus by electrostatic forces (between particles of
opposite electrical charges)
Atoms have extremely small masses. The mass of the heaviest known atom, for
example, is approximately 4 10-22 g . Because it would be cumbersome to express
such small masses in grams, we use the atomic mass unit (amu), where 1 amu = 1.66054 10-24 g
A proton has a mass of 1.0073 amu, a neutron 1.0087 amu, and an electron 5.486  10-4amu (See
TABLE 2.1)
Most of atoms have diameters between 1  10-10 m and 5  10-10 m and . A convenient ‫متفق عليه‬
non–SI unit of length used for atomic dimensions is the angstrom (Å), where
1 Å = 110-10 m . Thus, atoms have diameters of approximately 1-5 Å .
SAMPLE EXERCISE 2.1 Atomic Size
The diameter of a US dime is 17.9 mm, and the diameter of a silver atom is 2.88 Ǻ . How many
silver atoms could be arranged side by side across the diameter of a dime? silver atoms could be
arranged side by side across the diameter of a 0.2 mm dime?
Solution:
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Practice exercise 2.1
The diameter of a carbon atom is 1.54 Ǻ
(a) Express this diameter in picometers.
(b) How many carbon atoms could be aligned side by side across the width of a pencil line that is
0.20 mm wide?

a) pm 1.54 A 
1012 pm
154 pm

1
m
10
10 A
1m


1m
10 A 1C atom
b) No. of C atoms across the line  0.2 mm  3


1.310 6 C atoms

10 mm 1 m
1.54 A
Atomic Numbers, Mass Numbers, and Isotopes
Atomic Number: Number of protons in the nucleus = Number of electrons ( in a neutral atom)
Mass Number: total number of protons and neutrons in the atom.
Isotopes: Atoms of identical atomic numbers but different mass numbers (different number of
neutrons).
Carbon has four isotopes: 116C , 126C , 136C , and 146C all have the same number of electrons = same
number of protons but they have different numbers of neutrons.
supercript
subscript
X
Sample exercise 2.2
How many protons, neutrons, and electrons are in (a) 197Au
(b) strontium-90
Solution:
(a) Atomic number of Au = number of es = number of protons = 79 (from periodic table), and
the mass number of Au = 197
Number of neutrons = 197 – 79 = 118
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(b) Atomic number of Sr = number of es = number of protons = 38 (from periodic table), and
the mass number of Sr = 90
(c) Number of neutrons = 90 – 38 = 52 neutrons.
Atomic Weights
Unit : amu = 1.66054  10-24 g
Mass of a hydrogen atom = 1.66054  10-24 g
Mass of an oxygen atom = 2.6560  10-23 g
1.008 g H atom  6.022 10 23 atom
?g
 1 atom
? 1.6735  10 23 g
Average atomic mass = % of the first isotope  its at.wt. + % of the second isotope  its at.wt.
For example, Carbon has two isotopes
98.93 %of 12C (12 amu), and 1.07% of 13C (13.00335 amu)
The average atomic mass of C = (0.9893)(12 amu) + (0.0107)(13.00335 amu) =12.01 amu
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AMPLE Calculating
the Atomic Weight of an Element from Isotopic Abundances
Naturally occurring chlorine is 75.78% 35Cl (atomic mass 34.969 amu) and 24.22% 37Cl (atomic
mass 36.966 amu). Calculate the atomic weight of chlorine.
Solution:
Atomic weight = (0.7578)(34.969 amu) + (0.2422)(36.966 amu)
= 26.50 amu + 8.953 amu
= 35.45 amu
The atomic weight, which is actually the average atomic mass, is between the masses of the two
isotopes and is closer to the value of 35Cl,
the more abundant isotope (75.78 %).
Problem (2.92)
There are two different isotopes of bromine atoms. Under normal conditions, elemental bromine
consists of Br2 molecules, and the mass of a Br2 molecule is the sum of the masses
of the two atoms in the molecule. The mass spectrum of Br2
consists of three peaks:
Mass
(amu)
157.836
159.834
161.832
Relative
Size
0.2569
0.4999
0.2431
(a) What is the origin of each peak (of what isotopes does each consist)?
(b) What is the mass of each isotope?
(c) Determine the average molecular mass of a Br2 molecule.
d) Determine the average atomic mass of a bromine atom.
(e) Calculate the abundances of the two isotopes.
Solution
a) Bromine molecule (Br2) consists of two bromine atoms (Br-Br). These two atoms may be
originated from the same type of isotopes Br2(11) and Br2(22) or from two types of isotopes,
Br2(12).
The first peak will represent the lighter Br2 molecule, the third peak will represent the heavier
Br2 molecules and the middle peak will represent the intermediate Br2 molecule which is
Br2(12) .
b) Mass of Br(1) is 157.836/2 = 78.918 amu.
Mass of Br(2) is 161.832/2 = 80.916 amu.
c) Average molar mass of Br2 molecules
= 0.2569 (157.836) + 0.4999 (159.834) + 0.2431 (161.832) = 159.79 amu
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d) Average atomic mass of a bromine atom = Average molecular weight of Br2/2
Average atomic mass of a bromine atom = 159.79/2 = 79.895 amu
e) Let x = Abundance of Br(1) and (1-x) is the abundance of Br(2).
79.895 = x (78.918) + (1-x) (80.916) x = 0.5110
The abundance of Br isotope (type 1) is 51.10 % and the abundance of the ther isotope is 48.90 %
The periodic table
Is the most significant tool that is used by chemists to organize and to remember the chemical
facts: similarities, differences, reactivities, properties, …etc.
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Periods: Seven horizontal rows labeled by Arabic numerals (1-7).
Period
No. of elements in the period
First
2
Second
8
Third
8
Fourth
18
Fifth
18
Sixth
32 = 18 +14
(to fit in a page)
Incomplete
Seventh
12+... + 14
Groups: Eight vertical groups, of specific names, and are labeled as:
1A, 1B, 2A, 2B, …, 8 A, and 8B. or, IA, IB, IIA, IIB …, VIII A
Roman numeral rather than Arabic ones are often used in this scheme.
Elements in a group have similar properties. Because they have the same arrangement of es at
the periphery of their atoms.
Metallic elements: share characteristic properties, such as: luster, high electrical and heat
conductivity, all (except Hg) are solids.
In the periodic table: on the left and in the middle.
Non metals: Generally differ from the metals in appearance, and in other physical properties.
Some of the non metals are gaseous, some are solid, and one is liquid (bromine).
Nonmetallic elements are separated from the metallic ones, by a stepped line started from Boron
(B) to Astatine (At).
Metalloids: lie along the stepped ‫ الخط المتعرج‬line that runs from boron (B) to Astatine (At)
Have properties fall between those of metals and those f non metals.
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Molecule: an assembly of two or more atoms tightly bound together.
Diatomic molecule: is made up of two atoms. O2, H2, Cl2, …etc
Triatomic molecule: is made up of three atoms. O3
Polyatomic molecule: is made up of more than three atoms.
Molecular compounds: are composed of molecules contain more than one type of atom. For
example: H2O, C2H5OH, Na2CO3, … etc
Molecular formula: a formulum that indicates the actual numbers and types of atoms in the
molecule.
Molecular formula: that formula which indicates the actual number and type of atoms in a
molecule.
Empirical formula: indicates the relative number atoms in a molecule.
CH is the empirical formula of benzene (C6H6).
HO is the empirical formula of hydrogen peroxide (H2O2).
BH3 is the empirical formula of diborane (B2H6).
CH2O is the empirical formula of glucose (C6H12O6) .
Picturing molecules:
Structural formula: shows the arrangement of bonded atoms, which atom is attached to which
within the molecule.
There are different representation for structural formula can be used :
CH4
(molecular formula)
Structural formula Perspective drawing
ball and stick
space filling
Ionic compounds: combination of metal and non metal as: NaCl, BaCl2, Na2CO3, …
Molecular compounds: combination of non metals only as : NO, H2O, CH4, …
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Sample exercise 2.9
Which of the following compounds would you expect to be ionic?
N2O, Na2O, CaCl2, SF4.
Ionic: Na2O and CaCl2. The other two compounds are molecular.
Names and formulas of ionic compounds:
1. Positive Ions (cations)
(a) Cations formed from metal atoms: Na+, Zn2+, Al3+, …
(b) Cations with different charges:
Cu(I), and Cu(II)
Fe(II), and Fe(III)
Older method: add …ous, or …ic to the elements Lattin name.
(c) Cations formed from nonmetal atoms: ended by … ium.
NH4+: ammonium ion
H3O+: hydronium ion
See table 2.4
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2. Negative Ions (Anions):
(a) monatomic anions ended by … ide.
H- Hydride ion, O2- : Oxide ion,
N3- nitride: ion.
A few simple polyatomic anions also having names ending in ide, such as:
OH- hydroxide
CN- cyanide
O22- peroxide
(b) polyatomic anions (oxyanions)
these anions containing oxygen have names ending in ate or ite
NO3- nitrate ion
NO2- nitrite ion
SO42- sulphate ion
SO32- sulphite ion
SeO42- selenate ion
SeO32- selenite ion
ClO4- perchlorate ion
ClO3- chlorate ion
ClO2- chlorite ion
ClO- hypochlorite ion
BrO4- perbromate ion.
BrO3- bromate ion.
BrO2- bromite ion.
BrO- hypobromite ion.
(c) Anions derived by adding H+ to an oxyanion named by adding the word hydrogen or
dihydrogen:
HCO3- hydrogen carbonate
H2PO4- dihydrogen phosphate
Common anions are listed in table 2.5
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3. Ionic compounds:
The cation name is followed by the anion name
CaCl2
calcium chloride
Al(NO3)3
aluminium nitrate
Cu(ClO4)2
copper perchlorate (or cupric perchlorate)
SAMPLE EXERCISE 2.12
Name the following ionic compounds: (a) K2SO4 (b) Ba(OH)2 (c) FeCl3
(a) K2SO4 : potassium sulphate
(b) Ba(OH)2 : barium hydroxide
(c) FeCl3 : Iron(III) chloride (or Ferric chloride)
PRACTICE EXERCISE
Give the formula for (a) magnesium sulphate, (b) silver sulphide, (c) lead(II) nitrate
(a) magnesium sulphate: MgSO4
(b) silver sulphide: Ag2S
(c) lead(II) nitrate: Pb (NO3)2
Names and Formulas of acids:
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Names and formulas of binary compounds
Cl2O dichlorine monoxide NF3 nitrogen trifuoride
P4O10 tetraphosphorus decasulphate
N2O4 Dinitrogen tetroxide
SAMPLE EXERCISE
Name the following compounds: (a) SO2, (b) PCl5, (c) N2O3
(a) SO2: sulfur dioxide
(b) PCl5: phosphorus pentachloride
(c) N2O3: dinitrogen trioxide
SOME SIMPLE ORGANIC COMPOUNDS
Alkanes: structural formula
Derived alkanes
Different forms of propanol:
Selected questions:
Start from 2.17
2.18, 23, 25, 32, 35, 49, 62, 65-72.
HW2 : 65-72‫في ورقة واحد‬
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