Valence Electrons, Ionization Energy, and Effective Nuclear Charge - Prof. Kelly Marville, Study notes of Chemistry

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Periodic

Trends

Lose

1e

‐^ in

chemical reactions Group

1A

/^1

/^ I

1 valence

electron

Valence

electrons

are

those

in

the

outermost

principal

quantum

level

For

elements

of

the

Periodic

Table:

number

of

valence

electrons

(ve)

=^

Group

number

Group

2A

/^2

/^ II

2 valence

electrons

Lose

2e

‐^ in

chemical reactions

Examples:

(^2) 1s

(^1) 2s

(^2) 1s

(^2) 2s

Group

7A

/^17

/^ VII

7 valence

electrons

Gain

1e

‐^ in

chemical reactions

(^2) 1s

(^2) 2s 2p

5

Protons are located in the nucleus and thus the nucleus has an overall positive

charge

equal to the number of protons present

This

charge

is^

called

the

nuclear

charge

(Z).

Example

:^ Nuclear

charge

of

the

hydrogen

atom

is^

The greater the nuclear charge, the greater the attraction

of^

the

nucleus

for

its

electrons.

Nuclear

Charge

2 s 1 s 3+

‐

‐ ‐

Inner shells of electrons interfere with the attraction between thenucleus and valence electrons. Example:

(^2) 1s 1 2s

The

two

inner

(core)

electrons

shield

the

2s

electron

from

the

nucleus

since

they

repel

each

other,

as

well

as

the

2s

electron.

This

prevents

the

2s

electron

from

“feeling”

the

full

nuclear

charge.

This

diminished

charge

is^

called

the

effective

nuclear

charge:

Zeff

Z

Shielding

and

Effective

Nuclear

Charge

(Z

)eff

Bohr

model of^ Li

Z^ =

The first ionization energy (I

) is the energy needed to remove the highest 1

energy electron of a

gaseous

atom and move it infinitely far away (i.e.

remove one electron from a neutral atom and form an ion)

The calculation of the first ionization energy can be performed using theequation:

where

nf

=^

First

Ionization

Energy

(I

x^

‐^18

J

1 atom

x^

23 atoms 1 mol

•^

To

remove

an

‐ e from

the

ground

state

in

hydrogen

to

form

H

+^ :

ni^

=^1

E

First

Ionization

Energy,

I^1 =^ 2.

x^

‐^18

J

nf^

=^ 

Calculation

of

I^1

for

Hydrogen

Electron

is completely removed

x

J

1 kJ

x

=^

1 mole

(mol)

hydrogen

atoms

=^ 6.

x^10

23 atoms

2+

2.^

Shielding

electrons:

Li^

1 s^

2 2 s

1

Z^ =

= 520

kJ/mol

‐

‐

He

1 s^

2

Z^ =

I^1 (helium)

=^2372

kJ/mol

Bohr

model

of

He

I^1 (lithium)

Why

is^

I^ of^1

lithium

significantly

lower?

Factors

that

Affect

I^1

Helium

vs.

Lithium

The

core

electrons

in^

Li

shield

the

2 s

electron

from

the

full

nuclear

charge,

making

the

2 s

electron

easier

to

remove.

I^1

is

significantly

lower.

2 s 1 s 3+

‐

‐ ‐

Bohr

model

of

Li

Zeff

of valence

‐^ e =^ +

Zeff

of^

valence

‐^ e =^ +

Across a period, both the magnitude of Z and number of electrons possessedby an element increase.Therefore, as a period is crossed, valence electrons are “feeling” a greaterattraction for the nucleus and thus more energy will be required to removethese electrons.

As

you

can

see

from

this

chart,

there

is

a^ general increase

in

I^1

across

a period.

I^ increases across^1

a^

period

Why

the

deviation?

-^

Electron

configuration

of

boron:

1s

2 2s

2 2p

1

1s^

2s^

2p

Notice

that

the

2p

subshell

is

not

full,

it

contains

only

one

electron.

-^

Electron

configuration

of

beryllium:

1s

2 2s

2

1s^

2s

Notice

that

these

subshells

are

completely

full.

A large quantity of energy is required to remove anelectron from a full subshell, so I

is high.^1

Less energy is required to remove an electron from a partially filledsubshell, so

I^1

of boron is less than the I

of beryllium 1

DEVIATION

IN

I^1

TREND:

N

to

O

Period

Deviations

in

the

trend

of

I^1

across

period

As

a^

group

is^

descended,

there

is^

an

increase

in

the

number

of

shells.

Valence

electrons

can

be

easily

removed

because

they

are

not

as

strongly held by the positive attractiveforce of the nucleus

.

As

atoms

get

larger,

valence

electrons

are

increasingly

further

away

from

the

nucleus.

H Li

Na

Group

(^1) 1s (^2) 1s 1 2s

(^2) 1s 2 2s

2p

6 3s

1

Valence

electrons

are

those

in^ the

outermost

principal

quantum

level.

Idecreases down^1

a^

group

Problem

Rank the elements, I, Xe and Cs, in order of

decreasing

first ionization

energy (I

Problem

Using the following successive ionization energies in kJ/mol, determinewhich Group of the Periodic Table this element belongs.

I^1

I^2

I^3

I^4

896

1752

14,

17,

The atomic radius is equal to the distance from the nucleus to the outermostelectron of that atom.

Trend

in

Atomic

Radius

Notice

exclusion

of^

transition

metals

atomic

radii (in^ picometers)