chapter 2 the structure of the atom bch

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Topic 2: The Structure of the At

Topic 3: Chemical Bonding in Simple Molecu

Cambridge Internationa

Le

By: Ms. Nitya



Who are these men?

In this lesson, we’ll learn about themen whose quests for knowledgeabout the fundamental nature ofthe universe helped define ourviews.



Flashback on Atoms

• Democritus (Greek) used the term “atom”- cannot be cut

• John Dalton (1808) – Atomic Theory – atoms are indivisible matter – Atoms is smallest part of element exist – Each element has different atom – Cant be broken/destroyed

• J.J Thompson (1897)- discovered 1st sub-atomic particle – electron (-charged particle)

• Ernest Rutherford (1908) - atom has a positively charged central part(nucleus) – Proton. Mass is concentrated in nucleus.

• Sir James Chadwick (1932) – Discovered neutron, a neutral particle. – Neutron has a relatively equal mass to the proton



Atoms• Atom: An atom is a smallest

part of an element that cantake part in a chemicalreaction

• An atom consists of a cloudof electrons surrounding a

small, dense nucleus ofprotons and neutrons

• Very small

• H atoms weighs: 1.67 x 10-27Kg



Cathode Ray Tube• Used to produce beams of electro

– Cathode –heated to high temp. whlow voltage applied

– E- from heated wire (cathode rayare attracted to + charged metalplates

– e- are deflected by the magneticand electric fields

– Fluoresent screen glows when e- h

• Hence, shows that – e- have negative charge : attracte

to the + charged anode

–

e- have very small mass: easilydeflected by electromagnetic fiel



Proton, Electron & Neutron

• Protons (p) –

carry a positive charge of +1 – positive electrical charge of 1.602 x 10-19 coulomb – Mass: 1.673 x 10-24 g, 1,840 times the mass of an electron – Mass/amu: 1.007

• Electron (e) –

carries a negative electric charge -1 – The negative electrical charge is –1.602 x 10-19 coulomb – mass of 9.109 x 10-28 g = lightest particles with a known mass. – Mass/amu: 5.485 x 10-4

• Neutron (n) –

electrically neutral, undeflected, charge 0 – Electrical charge = 0 – Mass: 1.675 x 10-24 g, slightly heavier than a proton – Mass/amu: 1.009



ELEMENTS

• Element: substance that made up only fromone type of atoms

• Cannot be broken down into simpler substance

• Have its own symbol to indicate the element inthe Periodic table



Symbol of Element,

A-mass numberZ-atomic numberz = p = e- ; n=A-Z

•

Mass Number (A): the total number of protons & neutrons; nucleonnumber

• mass depends on the number of protons & neutronsAtomic

• Number (Z): the number of protons in the nucleus and, since atomsare neutral, it is also the number of electrons in the atom; proton

number

• atomic number is same for every atom of that element

XA

Z



Isotopes

• Atoms or the isotopes can be converted tocharged particles, ion, by removing or gainingelectron(s)

Isotopes Helium-4 Helium-5 Helium-6 Helium-7 Helium-8

Symbols 24 2

5 26 2

7 28

Proton 2 2 2 2 2

Neutron 2 3 4 5 6

Electron 2 2 2 2 2



Isotopes• Number or proton and neutron remain but number of

electron would be differ – Removing electron(s): deduction of number of electron from its

neutral atom; – Gaining electron(s): addition of number of electron its neutral

atom

• Example:

Exercise: pg. 30 – 32

Species 36 3

7 37 + 17

37 1737 −

Proton 3 3 3 17 17

Neutron 3 4 4 20 20

Electron 3 3 2 17 18



TOPIC 3: CHEMICAL BONDING INSIMPLE MOLECULES



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What is Chemical Bond?

• A chemical bond is defined: –

As the force of attraction which holds various constituentsentities (atoms/ ions) together in different chemical substances

• Bonding involves the valence electrons

• Atoms of other elements tend to achieve the stable noblegas electron arrangement. – Duplet electron arrangement - outermost shell occupied with two v.e.

– Octet electron arrangement -outermost shell occupied with eight v.e.

• Can be formed by the transfer of electron or sharingelectrons.

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Stability of Noble Gases

•

Atom of noble gas does not gain, lose nor share electrons withother atoms.

• Noble gas atoms do not combine with atoms of other elements toform compounds or with each other to form molecules.

•

Noble gases are chemically unreactive, Inert

• Noble gases exist as monoatomic

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•Can classified as intramolecular attraction and intermolecularattraction

CHEMICAL BONDING

Intramolecular attraction Intermolecular attractionStrong interaction Weak interaction

Strongly hold individual atomtogether through sharing ortransfer of electrons

Interaction between molecule

through weak attraction forces

The bond requires large amountof energy to break

The attraction force do notake a large amount of energto break

Ionic bonding, covalent bondingVan der Waals, hydrogen bonddipole-dipole forces



IONIC BONDING @ELECTROVALENT BOND

Def: Type of chemical bond that involves the electrostatic attraction

between oppositely charged ions. These ions represent atoms that havelost one or more electrons (known as cations) and atoms that have gainedone or more electrons (known as anions).

1



• interaction between metal and non-metalthrough electron transfer

• Electron transfer forms ion; – metal loses/donate electron form cation (+ ions) – non-metal gain electron form anion (- ions)

•

Attracted each other by strong electrostaticforce to form ionic compound

IONIC BONDING



• Ionic compound shows characteristics as below:

– The name of the metal comes first followed by thenon-metal when naming the compound

– Easily dissolve in water and in polar solvents – can conduct electricity

– tend to form giant lattice structure with high meltingtemperatures and high boiling temperatures

– Ionic crystal are brittle

IONIC BONDING



Dot and Cross Diagram• Only show electrons from the outer-most shell (valance

electron)

When Mg atom react with oxygen atom to forMgO, the 2 valence electron from Mg atom artransferred to incomplete filled oxygen atom

• Losing 2 electrons, Mg atom achieve stablenoble gas confiq by becoming Mg2+ ion

• Accepting 2 electrons, oxyen atom achievestable noble gas config by becoming O2- ion



• sharing of electrons between two or more

non-metals atoms to achieve stable noblegas configuration.• The number of electrons sharing must be

equal for both side• The covalent compound formed is usually

not charged

• The non-shared electrons at the outer-most shell is known as lone pairs

• One pair electron sharing forms singlebond which can be represent as a line;

known as structural formula, two pair – double bond, etc.

COVALENT BONDING



•

Covalent compound shows the properties below:

– tends to exist as liquid or gas or sometimes solid with avery low melting and boiling point

– Do not conduct electricity because they are neutral –

Generally insoluble in water but soluble in organic solvent – They are volatile

• Three types of covalent bonding – Single Bond –

Double Bond – Triple Bond

COVALENT BONDING



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Drawing Lewis Dot Structures

1. Predict the location of the atoms

a. Hydrogen is a terminal atomb. The central atom has the smallest electronegativity

2. Count the valence electrons.

3. Draw a single covalent bond between the central atomand the surrounding atoms.

4. Subtract the number of electrons in the single covalentbonds from the total number of electrons in 2.

5. Use the remaining electrons to complete the octets ofeach atom.

6. If the central atom does not have a complete octetthen try double or triple bonds.



Drawing Lewis Dot Structures

• PH3

• H2S

• HCl

• CCl4

• SiH4

• CH2Cl2• CH4

• HCl

• Cl2• C02

• O2

• N2

• C2H2

• H20

• CS2

Draw Lewis Dot Structures for



Octet Rule Exceptions

• Molecules with an odd number of electron (NO)

• Elements without octets (BF3 and BeF2) -possessless than 8 electrons (e- deficient compound)

• Elements with more than an octet (SF6) – possessmore than 8 electrons.



Co-ordinate covalent Bond @ DativeCovalent Bond

”A covalent bond in which both electrons comefrom the same atom. Also known as dativecovalent bonding.”



EXAMPLES OF CO-ORDINATE BOND

Boron hydride-ammonia complex.



• Here, the nitrogen atom becomes the donor. The hydrogen atom become

the acceptor.

• The linkage between N and H atoms is called coordinate bond. It isrepresented by an arrow →.

AMMONIUM ION CO-ORDINATE BOND:



• Compounds with an incomplete outer shell tendto bond datively and form dimer (eg. Group IIcompound, BeCl2 and Group III, Al/B)

Dative Covalent Bond



PROPERTIES

•Has all the characteristic of the covalent bond.

• Have low boiling and melting point.

• Does not conduct electricity in the liquid or in thedissolved state.

• Compounds are that much soluble in water.• As strong as other covalent bonds.



Dot and Cross Diagram

Draw the dot and cross diagram for the followingcompound.

a) BaCl2

b) K2Oc) Na3N

d) Ca2Ce) Al2O3

f) CaH2 g) MgBr2 h) CaS

i) AlN j) Mg3N2

k) AlCl3 l) AlH4

–

m) NF3

n) PCl3 o) HCNp) COq) SO2 r) SO3

s) O3

t) H2O2

chapter 2 the structure of the atom bch

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