chapter 2 jan12

7/29/2019 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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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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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

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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

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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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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


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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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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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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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

chapter 2 jan12

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