chapter 2 jan12
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CHAPTER 2
ATOMS, MOLECULES AND
IONS
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CONTENTS
2.1 The Modern View of Atomic Structure
2.2 The Periodic Table2.3 Molecules and Molecular Compounds
2.4 Ions and Ionic Compounds
2.5 Naming of Compounds
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Learning Outcomes
Able to differentiate between ions,molecules, ionic and molecular compound.
Able to name compounds and write theirformulas
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2.1 The Modern View of AtomicStructure
The Discovery of Atomic Structure
By 1850, scientists had begun to accumulate dataindicating that atom is composed of smaller particlescalled subatomic particles.Some of the most important experiments that led
to the current model of atomic structure are:Cathode Rays and Electrons
Radioactivity
Nuclear Atom
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Cont: 2.1 The Modern View ofAtomic Structure
Atom consists of positive (proton), negative(electron) and neutral entities (neutron).
Protons and neutrons are located in the nucleus ofthe atom (nucleus is the center of an atom). Mostof the mass of the atom is due to the nucleus.
Electrons are located outside of the nucleus. Mostof the volume of the atom is due to electrons.
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Cont: 2.1 The Modern View ofAtomic Structure
Masses of neutron, proton and electron are sosmall, that we define as the atomic mass unit,amu.
1 amu = 1.66054 10-24 g
1 g = 6.02214 1023 amu
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Cont: 2.1 The Modern View ofAtomic Structure
Comparison of the proton, neutron and electron
Particle Charge Mass (amu)
Proton Positive (+1) 1.0073
Neutron Neutral 1.0087
Electron Negative (-1) 5.486 10-4
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Cont: 2.1 The Modern View of
Atomic Structure
Atoms are extremely small, diameters between 1 10-10 m and 5 10-10 m or 100-500 pm.
Define 1 = 1 10-10 mNote : 1 = 10-10 m = 100 pm = 0.1 nm
The diameter of atomic nuclei are in the order of10-4 .
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2.1.1 Isotopes, Atomic Numbers
and Mass Numbers
All atoms of the same element have the same numberof protons.
The general symbol for an isotope: AZXX = elementA = mass number
= no.of protons(Z) + no.of neutrons (N)Z = atomic number = no. of protons = no. ofelectrons for neutral element only.The number of neutrons = A - Z.
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Cont: 2.1.1 Isotopes, AtomicNumbers and Mass Numbers
All atoms of the same element have the samenumber of protons/atomic number, Z.
Atoms that have the same number of protons butdiffer in their number of neutrons are calledisotopes.
Isotopes have the same Z but different A.E.g. 14C, 13C, 12C, 11C
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Symbol No.of Protons No. of Electrons No. of Neutrons11
C6 6 5
12C 6 6 6
13C 6 6 7
14C 6 6 8
Cont: 2.1.1 Isotopes, AtomicNumbers and Mass Numbers
Some of the isotopes of Carbon:
An atom of a specific isotope is called anuclide.
E.g: 21H nuclide (deuterium), 31H nuclide (tritium)
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2
1
3
1Example 1
Find the number of protons, electrons andneutrons in the following isotope:
The number of proton is 20;
The number of electrons in a neutral atom is
20;The number of neutrons : 40 - 20 = 20.
Ca40
20
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2
1
3
1Example 2
The atomic number sulfur is 16. Write the
nuclear-isotope symbols for the four isotopes ofsulfur with 16, 17, 18 and 20 neutrons,respectively.
All isotopes will have 16 protons. The mass
number of each isotope is the sum of its number
of neutrons plus its number of protons.
16 + 16 = 32; 16 + 17 = 33; 16 + 18 = 34; 16 + 20 = 36
SSSS36
16
34
16
33
16
32
16 ,,,
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2.2 The Periodic Table
Lists elements in order of increasing atomicnumber (at the moment there are 117 elements).
Helps in organizing trends in the chemical and
physical properties of elements.Columns are called groups (numbered from 1 to 8
and either the letter A or B).
The eight A-groups are the main group elements.The ten B-groups contain transition elements.
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Cont: 2.2 The Periodic Table
Rows are called periods.
Metals are on the left side and middle.
Non metals are on the right side.
Metalloids have properties of both metals andnonmetals. (B, Si, Ge, As, Sb and Te)
Elements in vertical columns (groups) exhibitsimilar chemical and physical properties, whereaselements in a horizontal row (period) exhibitdifferent properties.
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Cont: 2.2 The Periodic Table
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The electronic configuration of elements
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Group Name Elements1A Alkali metals Li, Na, K, Rb, Cs, Fr
2A Alkaline earth metals Be, Mg, Ca, Sr, Ba, Ra
6A Chalcogens O, S, Se, Te, Po
7A Halogens F, Cl, Br, I, At
8A Noble gases (or rare gases) He, Ne, Ar, Kr, Xe, Rn
Cont: 2.2 The Periodic Table
Names of some groups in the Periodic Table:
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2.3 Molecules and MolecularCompounds
Most matter is composed of molecules or ions.
A molecule is a small particle consisting of two or
more atoms combined together in a discrete unit.Molecules typically consist of nonmetallic
elements.
Chemical formula indicates:which atoms are found in the molecules.
in what proportion they are found.
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Cont: 2.3 Molecules and MolecularCompounds
2.3.1 Molecules
Any molecules made up of two atoms are calleddiatomic molecules.
There are seven elements that form homonuclear
diatomic molecules:N2, H2, O2, F2, Cl2, Br2 and I2
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2.3.2 Molecular Compounds
Compounds formed from molecules are molecular
compounds.
Contain more than one type of atom.
Example: a molecule of water represented by thechemical formula H2O.
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2.3.3 Molecular Formula
Chemical formulas that give the actual numbers
and types of atoms in a molecules are calledmolecular formulas.
Subscripts in a molecular formula tell you how
many atoms are actually present.E.g. C2H4 : 2 atoms of carbon, 4 atoms of
hydrogen.
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2.3.4 Empirical Formula
Chemical formulas that give only the relativenumber of atoms of each type in a molecule.
Example: Molecular formula for hydrogen
peroxide is H2O2, its empirical formula is HO.They give the lowest whole number ratio of atoms
in a molecule.
Example: C2H4 is a molecular formula; itsempirical formula, CH2, is obtained by dividing thesubscripts in the molecular formula by 2.
C2/2H4/2 = CH2
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2.3.5 Picturing Molecules
Molecules occupy three dimensional space.We often represent them in 2-D.
Structural formula shows how atoms are joined
together that is the relative orientation andposition of bonded atoms.
Structural formula does not show the actual
geometry of the moleculeVarious models are used to visualize the 3-D shape
of the molecule:perspective drawing, ball and stickmodel or space-filling model.
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Cont: 2.3.5 Picturing Molecules
Perspective drawing: dashed line, wedges lineBall-and-stick: atoms as spheres and the bonds as
sticks.
Space-filling models: shows the relative sizes ofatoms.
C
H
H
H
H C
H
HHH
Structural
formula
Perspective
drawing
Ball-and-stickdrawing
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2.4 Ions and Ionic Compounds
If electrons are removed or added to a neutralatom, a charged particle called an ion is formed.
- Ion with positive charge is calledcation.
- Ion with negative charge is calledanion.
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2-
43-
Cont: 2.4 Ions and IonicCompounds
E.g. Na atom Na+ ion[11p+ 11e-] -e [11p+ 10e-]
Cl atom Cl- ion[17p+ 17e-] +e [17p+ 18e-]
In general, metal atoms tend to lose electrons andnonmetal atoms gain electrons.
When molecules lose or gain electrons, polyatomic
ions are formed (e.g. SO42-
, NO3-
).
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Example 3
What is the symbol for an ion with 26 protons, 30neutrons and 24 electrons?
Mass number of isotope = 26 + 30 = 56
Atomic number 26
Check with periodic table: Fe
Ions has two more protons than electrons, it
has a net charges of 2+.
Complete symbol for this ion = Fe2+
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2.4.1 Predicting Ionic Charges
An atom or molecules can loose more than oneelectron.
Many atoms gain or lose enough electrons to havethe same number of electron as the nearest noblegas.
Member of the noble gas are chemically very non
reactive (stable).
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Cont: 2.4.1 Predicting IonicCharges
Example: Loss of one electron from an atom ofsodium leaves it with the same number of electronsas the neon atom.
(i) Na+ ion [11p+ 10e-] similar to neon 10Ne(ii) 8O + 2e- O2- [8p+ 10e-] similar to 10Ne
The number of electrons an atom loses is related toits position on the periodic table.
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2.4.2 Ionic Compounds
Majority of chemistry involves the transfer ofelectrons between species.
Example: To form NaCl, the neutral sodium atom,
Na, must loose an electron to become a cation:Na+.
Electron cannot be lost entirely so it is transferred
to a chlorine atom, Cl
-
, which then become ananion: Cl-.
Na+ and Cl- ions are attracted to form an ionicNaCl lattice which crystallizes.
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2.4.3 Ionic Compounds VsMolecular Compounds
Ionic compounds are generally combinations of
metal ions (positively charged) and non-metal ions(negatively charged).
We cannot use molecular formulas to describeionic compounds, therefore we write empiricalformula.
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Cont: 2.4.3 Ionic Compounds
Vs Molecular Compounds
Molecular compounds are nonmetals only.
E.g.N2O - Molecular compound
Na2O - Ionic compound
CaCl2 - Ionic compoundSF4 - Molecular compound
P4O6 - Molecular compound
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2.5 Naming of Compounds
2.5.1 Names and Formulas of IonicCompounds
Naming of compounds, nomenclature is dividedinto organic compounds and inorganic compounds.
Organic compounds - those containing carbon
in combination with hydrogen, oxygen, nitrogenor sulfur.
Other compounds are called inorganic
compounds.
N i f i i
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Naming of ionicCompounds
Cations Anions
metal
Polyatomicnon-metals
Monoatomic anions
Polyatomic anionscontaining O
- ate, - ite
Per.-ate, -ate,-ite, hypo.-ite
Prefix hydrogen or
dihydrogen
One oxidation state
More than oneoxidation state
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2.5.2 Names and Formulas for Cations
1. Cations formed from a metal have the samename as the metal.
Na+ is a sodium ion
Zn2+ is a zinc ion
K+
is a potassium ion
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Cont: 2.5.2 Names and Formulas forCations
2. If the metal can form more than one cation, thenthe charge is indicated in parentheses after the
name of the metal.Cu+ = Copper (I) ; Cu2+ = Copper (II)
Fe2+ = Iron (II) ; Fe3+ = Iron (III)
Note: Older method (still widely used)
Cu+ cuprous ion ; Cu2+ cupric ion
Fe2+
ferrous ion ; Fe3+
ferric ion
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Cont: 2.5.2 Names and Formulas forCations
3. Cations formed from non-metal atoms (polyatomic)have names that end in - ium
Polyatomic - Atoms that are joined together, asin a molecule, but carry a net charge.
NH 4+ ammonium ion
H3O+ hydronium ion
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2.5.3 Names and Formulas for Anions
1. Monatomic anions (with only one atom) are called -ide.
H
hydride ionO2 oxide ion
N3 nitride ion
Exceptions: Simple polyatomic anions ;hydroxide, cyanide and peroxide ions.
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Cont: 2.5.3 Names and Formulas forAnions
2. Polyatomic (many-atom) anions containing oxygen,having names ending in - ate or - ite. These anions
are called oxyanions. The one with more oxygen iscalled - ate.
NO3
nitrate ; NO2
nitriteSO42 sulfate ; SO32 sulfite
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Cont: 2.5.3 Names and Formulas forAnions
3. Polyatomic anions containing oxygen with morethan two members in the series are named asfollows (in order of decreasing oxygen):
per-ate; -ate ; -ite ; hypo-.-ite
E.g.
ClO4
perchlorate (one more O than chlorate)
ClO3 chlorate ion
ClO2 chlorite ion (one less O than chlorate)
ClO
hypochlorite (one less O than chlorite)
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Cont: 2.5.3 Names and Formulas forAnions
4. Polyatomic anions derived by adding H+ to anoxyanion are named by adding as a prefix thewordhydrogen ordihydrogen, as appropriate.
CO32 carbonate ionHCO3 hydrogen carbonate or (bicarbonate) ion
PO43 phosphate ionH2PO4 dihydrogen phosphate ion
Note: Each H+ reduces the negative charge.
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Cont: 2.5.3 Names and Formulas forAnions
Note:
The monoatomic ions of group 7A always have a
1 charge:E.g: F (Fluoride), Cl(Chloride), Br(Bromide)
The monoatomic ions of group 6A always have a2 charge:
E.g: O2(oxide) and S2(sulfide)
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2.5.4 Ionic Compounds
Names of ionic compounds - the cation namefollowed by the anion name: BaBr2 : barium bromideCu(ClO
4
)2
: copper (II) perchlorateK2SO4 : potassium sulfateFeCl3 :Charge of Fe is 3+, anion is Cl-
: Iron (III) chloride
Note:Parentheses followed by the appropriatesubscript are used when a compound contains two
ore more polyatomic anions.
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2.5.5 Names and Formula of Acids
An acid is composed of anion connected to enoughH+ ions to totally neutralize or balance the anionscharge.
E.g. SO4
2-
ion requires two H+ ions to form H2
SO4
. The names of acids are related to the names of
anions, whose names end in -ide, -ate, -ite:
-ide becomeshydro.- icacid-ite.becomes -ous acid-atebecomes .-icacid
2 5 6 N d F l f Bi
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2.5.6 Names and Formula of BinaryMolecular Compounds
Binary molecular compounds have two elements. The most metallic element is usually written first.The prefix mono may be omitted for the first
element. E.g. PCl3 is named phosphorous trichloride Greek Prefixes are used.
Mono- = 1, Hexa- = 6Di- = 2, Hepta- = 7Tri = 3, Octa- = 8Tetra- = 4, Nona- = 9
Penta- = 5, Deca- =10
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Cont: 2.5.6 Names and Formula ofBinary Molecular Compounds
N2O4 dinitrogen tetroxide
NF3 nitrogen trifluorideP4S10 tetraphosphorus decasulfideSO2 sulfur dioxide
SO3 sulfur trioxideNO2 nitrogen dioxide
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Cont: 2.5.6 Names and Formula ofBinary Molecular Compounds
Exceptions to the use of Greek prefixes formolecular compounds containing hydrogen.
B2H4 diboraneH2S hydrogen sulfide
CH4 methaneSiH4 silaneNH3 ammonia
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2.5.7 Hydrates
Hydrates - compounds that have a specific numberof water molecules attached to them.
E.g. CuSO4.5H2O - Copper (II) sulfatepentahydrate
The water molecules can be removed by heating.
When this occurs, the resulting compound isCuSO4 - called anhydrous copper (II) sulfate.
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Exercise 2.1
Name the following compounds:(a) CaSO4(b) PF5
(c) KBr(d) KHSO4(e) Na2S(f) H
2SO
4
(g) CO(h) HClO4(i) NaClO3
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Exercise 2.2
Give the systematic names for the formulas or theformulas for the names of the following compounds:
a) Tin (II) fluorideb) CrI3c) Ferric oxided) MnSe) Lead (IV) oxidef) FeBr2
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Exercise 2.3
Explain what is wrong with the name or formula inthe second part of each statement and correct it:
a) SF4 is monosulfur pentafluoride
b) Dichlorine heptaoxide is Cl2O6
c) N2O3 is dinitrogentrioxide
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END of CHAPTER 2
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END of CHAPTER 2
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