L2 6P

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A104 Biology

Problem 02: Stable for life

6

th

Presentation

Copyright © 2015

(2)

In Today’s Problem

• Determine how the components of an atom will

determine the atomic number and mass of an

atom

• Using the Periodic Table, state and explain,



The ways which two or more atoms may

combine together



Why some atoms can exist without combining

with other atoms



The trends in atomic number and atomic

mass.

(3)

What Do You Recognise?

• All matter around us is made of atoms and that

there are different types of atoms.

• Each type of atom has its own atomic size and

mass.

• Most atoms are not able to exist on their own; they

combine with other atoms

• Chemical reaction takes place when atoms

combine together and this gives rise to most

matter around us.

• There are still exceptional cases like argon being

able to exist as single atoms.

(4)

What is your approach?

• What are atoms made up of?

• What is the atomic structure of an atom?

• What is an electron shell diagram?

• What is a periodic table?

• Why do some atoms exist by combining together

with other atoms whereas other atoms exist without

combining with other atoms?

• What are the types of bonding in which atoms can

combine together?

(5)

•

Atoms being the basic building blocks of matter

around us, are composed of

protons

,

neutrons

and

electrons

.

•

The protons are positively charged particles

and electrons are negatively charged particles,

while neutrons do not carry any charge.

•

Atoms are electrically

neutral

, as the number

of protons and electrons is the same in an

atom.

What are atoms made up of?

(6)

•

In the centre of an atom is the nucleus where the

protons and neutrons can be found. The electrons

reside in shells surrounding the nucleus.

•

Electrons are able to remain in the region around the

nucleus due to electrostatic attraction between the

positively-charged nucleus and the negatively-charged

electrons.

Proton (positively charged)

Neutron (neutral)

What is the atomic structure of an

atom?

Nucleus

Negatively charged

electrons reside in

shells surrounding

the nucleus

(7)

What is an electron shell diagram?

•

Electrons revolve along imaginary paths around the

nucleus known as shells.

•

Each

electron shell

is associated with fixed energy

level and can only hold a

fixed number of electrons

.

•

The occupied shell furthest away from the nucleus is

known as the

valence shell

with electrons residing in it

defined as

valence electrons

.

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1

st

Shell

(Maximum 2 e)

2

nd

Shell

(Maximum 8 e)

3

rd

Shell

(Maximum 18 e)

(8)

Can be represented by

p: proton

n: neutron

•2p

•2n

•Revolving

electrons

Proton (positively charged)

Neutron (neutral)

(9)

Eg. How will you draw the electron shell diagram

for sodium atom ?

•11 protons =

•11 electrons

11p 12n

•

Total: 11 electrons

•1

st

shell: 2 electrons

•2

nd

shell: 8 electrons

•3

rd

shell: Remaining 1 electron

(10)

What is a Periodic Table?

• A method where atoms of elements are

arranged based on the atomic (proton) number.

• Chart that groups the elements according to

their similar properties.

• A periodic table is useful to:

– identify the Relative Atomic mass (mass

number) and atomic number (proton number)

– identify the chemical symbol and name of the

element

(11)

•

The

Periodic Table

is a conventional way to organize

the various atoms

according to various trends

.

The Period number tells us the number of occupied electron shells

The highlighted row is the second period. The 1st_{two shells of the atom of these}

elements are occupied by electrons

(12)

•

The

Periodic Table

the various atoms

.

The Group number tells us the number of valence electrons

The highlighted column is Group III. In general, there are 3 valence electrons in the atom of the element in Group III.

(13)

•

The

Periodic Table

the various atoms

.

The relative atomic mass provides an indication on the number of protons and neutrons

The proton number tells us the number of protons (or electrons)

(14)

•

Most atoms rarely exist on their own except for the

Noble gases.

•

The valence shells of the atoms of Noble gases contain

either 2 electrons (for atoms with one electron shell i.e.

Helium) or 8 electrons (e.g. Neon and Argon).

•

This is the basis of their stability and ability to exist as

single atoms.

•

In general, an atom found in Group I to VII (first twenty

elements only) will try to achieve stability by combining

with other atoms so that it can have 2 electrons (i.e. H,

Li, Be) or 8 electrons (e.g. Al, Cl, O, C) in the outermost

shell to obtain the duplet/octet configuration.

Why do some atoms exist by combining together with

other atoms whereas other atoms exist without

combining with other atoms?

Note: The duplet/octet configuration may not apply beyond the first 20 elements.

(15)

What are the types of bonding in

which atoms can combine together?

Atom Metallic/

non-metallic

Number of electrons to be gained/ lost per

atom Result ion Na Cl Li O

Atoms of metallic elements have the tendency to lose their

valence electrons to achieve stability

Atoms of non-metallic elements have the tendency to gain extra

electrons to achieve stability

Na has one valence electron

Cl has seven valence electrons

Metallic Non-metallic

Lose 1 electron Na+

Cl-

The total number of electrons lost by atoms of one element

must be equal to the total number of electrons gained by the atoms of the other element,

i.e. balanced electron transfer

with the resulting ionic compound being overall

electrically neutral

Hence, Na combines with Cl by transferring its only

valence electron to Cl Gain 1 electron

(16)

What are the types of bonding in

The total number of electrons lost by atoms of one element must be equal

to the total number of electrons gained by the

atoms of the other element, i.e. balanced electron transfer with the resulting ionic compound being overall electrically

neutral

One Na+_{ion must be}

combined with one Cl-_ion

to form an overall electrically neutral ionic

compound.

Hence, the balanced chemical formula of this ionic compound is NaCl.

Na

+

Cl

=

Na

+

Cl

-Atom Metallic/

non-metallic

Atoms of metallic elements have the tendency to lose their

Metallic Non-metallic

Lose 1 electron Na+

Cl- Gain 1 electron

(17)

What are the types of bonding in

Thus, in simpler terms:

Na

+



Group 1(Metal)

Cl

-



Group 7 (Non metal)

Na

1+

Cl

1-

Na

1

Cl

1

Chemical formula: NaCl

Na

Cl

=

Na

+

Cl

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(18)

What are the types of bonding in

Atom Metallic/

non-metallic

Li has one valence electron

O has six valence electrons

Metallic

Non-metallic

Na

+

Cl

-

Metallic

Non-metallic

Li

+

O

2-

The total number of electrons lost by atoms of one element must be equal

to the total number of electrons gained by the

atoms of the other element, i.e. balanced electron transfer with the resulting ionic compound being overall electrically

neutral

Two Li+_{ions must be}

combined with one O2-_ion

to form an overall electrically neutral ionic

compound.

Hence, the balanced chemical formula of this ionic compound is Li₂O. Atoms of metallic

elements have the tendency to lose their

Hence, Li combines with O by transferring

its only valence electron to O

Lose 1 electron

Gain 1 electron

Lose 1 electron

Gain 2 electrons

(19)

Li

+



Group 1 (Metal)

O

2-



Group 6 (Non-metal)

Li

1+

O

2-

Li

2

O

1

Chemical formula: Li

2

O

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(20)

What are the types of bonding in

Atom Metallic/

non-metallic

Number of electrons to be gained/ shared

F

Cl

F

O

F has seven valence electrons

Cl has seven valence electrons

Non-metallic

The total number of electrons gained/shared by atoms of one

element must be equal to the total number of electrons gained/shared by atoms of the

other element, i.e. balanced electron sharing.

It is unlikely for electrons to be transferred as both

atoms prefer to gain electrons  sharing of

electrons occur

Share 1 electron

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(21)

What are the types of bonding in

Atom Metallic/

non-metallic

F Cl F O

Non-metallic

One F atom must combine with one Cl atom by sharing a total

of two electrons. Each atom contributes one electron in this

sharing.

Hence, the balanced chemical formula of this covalent

molecule is FCl. It is unlikely for electrons

to be transferred as both atoms prefer to gain electrons  sharing of

electrons occur

Share 1 electron

F

+

Cl

=

F

Cl

(22)

F

-



Group 7

Cl

-



Group 7

F

-

Cl

-

F

1

Cl

1

Chemical formula: FCl

F

Cl

=

F

Cl

Note: This ‘cross’ method may not work for all covalent compounds.

Copyright © 2015

(23)

What are the types of bonding in

Atom Metallic/

non-metallic

F

Cl

F

O

F has seven valence electrons

O has six valence electrons

Non-metallic

electrons occur

Share 1 electron

Share 2 electrons

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(24)

What are the types of bonding in

Atom Metallic/

non-metallic

F Cl F O

Non-metallic

Two F atoms must combine with one O atom by sharing a total of four electrons. Each F atom contributes one electron and each O atom contributes

two electrons in this sharing.

Hence, the balanced chemical formula of this covalent

molecule is OF₂.

F

O

F

electrons occur

Share 1 electron

Share 2 electrons

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(25)

What are the types of bonding in

F

-



Group 7 (Non-metal)

O

2-



Group 6 (Non-metal)

F

-

O

2-

F

2

O

1

Chemical formula: F

2

O

Note: This ‘cross’ method may not work for all covalent compounds.

(26)

•

Combination of atoms results in the formation of

chemical bonds

.

•

Atoms involved in the bonding are able to stay

close to each other due to the

attraction

between

the positive nuclei of one atom and the negative

valence electrons of the other that are used for

bonding.

•

The transfer of electrons results in

ionic bonds

while the sharing of electrons results in

covalent

bonds

.

(27)

•

Extra points to take note of:

̶ Atoms can gain or lose more than one

valence electron e.g. O

2-

and Mg

2+

.

̶ An atom can combine with more than one

atom at the same time e.g. MgCl

2

and NH

3

.

̶ Two atoms may form multiple bonds with

each other e.g. in O

2

.

O

(28)

•

Atomic radius

is measured by the distance

between its nucleus and its valence electron.

•

Nuclear charge

refers to the total charge of

the protons and it measures the attractive

forces between the nucleus and the electrons.

•

This implies that the valence electrons would

be drawn closer to the nucleus if an atom has

higher nuclear charge (i.e. a smaller atomic

radius).

What are the factors that influence the

atomic radius of an atom?

(29)

•

For the atoms with electrons occupying more than

one electron shell, the inner electrons

tend to

repel

the valence electrons, i.e. shielding effect.

•

Hence, the valence electrons experience a

weaker attraction if an atom has higher shielding

effect (i.e. a bigger atomic radius).

•

Nuclear charge and the number of inner electrons

have

opposing effects

on the atomic radius.

•

Relating this back to the Periodic Table, it can be

deduced that the atomic radius increases down

the Group but decreases across the Period.

(30)

•

An atom which loses electron(s) to achieve stability

will form a positive ion. As a result of the losing of

electron(s), an electron shell will be lost as well.

Hence, the ionic radius is smaller than its original

atomic radius.

̶

E.g. the ionic radius of Na

+

is smaller than the atomic

radius of Na atom.

•

An atom which gains electron(s) to achieve stability

will form a negative ion. As a result of the gaining of

electron(s), an overall increase in electron-electron

repulsion will cause the ionic radius to be bigger

than its original atomic radius.

̶

E.g. the ionic radius of Cl

-

is bigger than the atomic

radius of Cl atom.

(31)

For example, let’s look at the carbon atom (C) and nitrogen

atom (N) on the second period.

What are the factors that influence the

+ +

+

+ +

+

C

Both C and N have two occupied shells.

Both have one inner shell giving the same shielding

effect

But N has one extra proton than C, making the

electrostatic attraction of its nucleus stronger (i.e.

higher nuclear charge)

N

+ +

+

+ +

(32)

For example, let’s look at the carbon atom (C) and nitrogen

atom (N) on the second period.

+ +

+

+ +

+

C

N

+ +

+

+ +

As a result of of the higher

nuclear charge, the

valence shell of N is pulled in more. Thus, the atomic radius of N is smaller than the atomic radius of C.

(33)

In general, due to the increasing attractive force from the

nucleus, the atomic radii of atoms decrease across the

Period. The shielding effect is less significant as compared

to increased nuclear charge effect across the Period.

Atomic radii of atoms

decrease from left to

right

(34)

In general, due to the increase in number of electron shells,

the atomic radii of atoms increase down the Group. The

increased shielding effect is more significant as compared to

nuclear charge effect down the Group.

Atomic radii of atoms

increase from top to

bottom

(35)

What you have learnt

•

An atom is composed of protons and neutrons

found inside the nucleus with electrons revolving

around this nucleus.

•

Most atoms attain stability by forming chemical

bonds which takes place either through the

transfer of electrons or sharing of electrons.

•

The atomic radius of an atom can be influenced by

the nuclear charge and the number of inner

electrons present.

(36)

Some information is provided for two unknown

compounds

AB

2

and

XY

2

:

•

AB

2

is formed by ionic bonding

•

XY

2

is formed by covalent bonding

From the information given above, what could be

deduced about elements A, B, X and Y?

Discussion