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Chemistry 11

Chemical Bonding


Electrostatic Forces

An electrostatic force is a

forces existing as a result of

the attraction or repulsion

between 2 charged particles

All bonding is based on the experimentally-derived

relationships of electrostatics:• Opposite charges attract (+ -)

• Like charges repel (+ + or - -)

• The greater the distance between two charged particles, the smaller

the attractive force

• The greater the charge on two particles, the greater the force of

attraction (or repulsion) between them

© Boardworks Ltd 20073 of 44

Electron Shells REVISITED!

What links atomic number and the properties of elements?

The periodic table shows that patterns in the properties of

elements are linked to atomic number.

atomic number = number of protons

atomic number = number of electrons

number of protons = number of electrons

Electrons!

As atomic number increases by one, the number of electrons

also increases by one.

This means that the elements in the periodic table are

also arranged in order of the number of electrons.


What is periodicity?

The term periodicity describes a repeating pattern in

properties of elements across periods of the periodic table.

The Russian chemist Dmitry

Mendeleev is credited with being the

creator of the first version of the

periodic table. He observed that when

the elements are arranged in order of

atomic mass, there are recurring

patterns in certain properties.

The modern periodic table can be used

to analyse trends in properties such as

Atomic Radius across periods and

down groups.


Atomic Radius Trends


What is atomic radius?

The atomic radius of an element is difficult to precisely define

because of the uncertainty over the size of the electron cloud.

Several definitions are used.

For non-metallic elements, the

covalent radius is often used

as the atomic radius. This is half

the internuclear distance

between two identical atoms in

a single covalent bond.

covalent

radius

One definition is half the shortest internuclear distance found

in the structure of the element.


More on atomic radius

For non-bonded adjacent atoms (e.g. in a covalent crystal of

a non-metallic element), the van der Waals radius is used as

a value for atomic radius. This is half the shortest internuclear

distance between two similar non-bonded atoms.

van der

Waals radius

For metallic elements, the metallic radius is often used as

the atomic radius. This is half the shortest internuclear

distance between two adjacent atoms in a metallic bond.


Trends in atomic radius in period 3


Trends in atomic radius in period 3

The atomic radius of the elements

across period 3 decreases.ElementAtomic

radius (nm)

Na

Mg

Al

Si

P

S

Cl

Ar

0.190

0.145

0.118

0.111

0.098

0.088

0.079

0.071

However, more than 99% of the

atom is empty space – the

nucleus and electrons themselves

occupy a tiny volume of the atom.

This might seem counter-

intuitive, because as the

numbers of sub-atomic particles

increase, the radius might be

expected to also increase.


WHY? Increase in proton number

The number of protons in the nucleus of the atoms

increases across period 3.

This increase in the number of protons increases the

positive nuclear charge of the atoms. The nucleus has

stronger attraction for the electrons, pulling them in

closer and so the atomic radius decreases across the

period.increased

nuclear charge

pulls electrons

closer

11Na 12Mg 13Al 14Si 15P 16S 17Cl 18ArElementproton

number


What is shielding?


Explaining atomic radius in period 3

Proton number increases

across period 3, but

shielding remains

approximately constant.0.190

0.145

0.118

0.111

0.098

0.088

0.079

0.071

ElementAtomic radius (nm)

Proton number

11

12

13

14

15

16

17

18

Na

Mg

Al

Si

P

S

Cl

Ar

This pulls these electrons

closer to the nucleus and

results in a smaller radius.

This causes an increase

in effective nuclear

charge, leading to a

greater attraction

between the nucleus and

the outermost electrons.


How are electrons arranged?

Electrons are arranged in shells around an atom’s nucleus. (also called energy levels-principle quantum number n1).

This electron arrangement is written as 2,8,18.

1st shell holdsa maximum of

2 electrons

2nd shell holds

a maximum of

8 electrons

3rd shell holds

a maximum of

18 electrons

Each shell has a maximum number of electrons that it can

hold. Electrons will fill the shells nearest the nucleus first.


Electrons in period 1

Elements in period 1 only have electrons in the first shell.

Why are there only two elements in period 1?

The first shell can only hold a maximum of two electrons,

so period 1 only includes the elements hydrogen and helium.

What is special about the outer shell of helium?

1 2

H He

1 2 43 5 6 07

1



Elements in period 2 all have a complete first shell.

What happens to electrons in the second shell in period 2?

The second shell is completed one electron at a time going

across the period from left to right.

What is special about the outer shell of neon?

2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8

Li Be B C N O F Ne

1 2 43 5 6 07

2



2,8,1 2,8,2 2,8,3 2,8,4 2,8,5 2,8,6 2,8,7 2,8,8

Elements in period 3 have complete first and second shells.

What happens to electrons in the third shell in period 3?

The third shell is completed one electron at a time going

across the period from left to right.

Na Mg Al Si P S Cl Ar

1 2 43 5 6 07

What is special about the outer shell of argon?

3


What are the patterns of electron arrangement?

Consider the electron arrangements of the first 20 elements

in the periodic table.

What is the pattern of outer shell electrons in a group?

2,8,1 2,8,2 2,8,3 2,8,4 2,8,5 2,8,6 2,8,7 2,8,8

1 2 43 5 6 07

1

2

3

4

2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8

1 2

2,8,8,1 2,8,8,2

What is the pattern of outer shell electrons across a period?

What is the pattern of full electron shells in a group?

*Remember…will fill 4s2 before we complete the 3d subshell


Electron trends in the periodic table

Trends down a group:

The point at which a new period starts is the point at

which electrons begin to fill a new shell.

The number of a group is the same as the number of

electrons in the outer shell of elements in that group,

except for group 0.

the number of outer shell electrons is the same;

the number of complete electron shells increases by one.

the number of outer shell electrons increases by one;

Trends across a period:

the number of complete electron shells stays the same.


Valence Electrons Revisited

An open shell is a shell containing less than it’s

maximum number of electrons

All electrons in an atom except those in the [core] or in

filled d- or f- subshells / orbitals

All electrons in ‘open shells’

Valence electrons are:

Valence electrons are considered reactable electrons


Electrons and groups


Glossary

atomic number – The number of protons in an atom. Sometimes called the proton number.

electron arrangement – A shorthand way of writing the number of electrons in an atom’s electron shells.

element – A substance made up of only one type of atom.

group – A column in the periodic table containing elements with the same number of outer shell electrons and similar chemical properties.

period – A row in the periodic table containing elements with the same number of full electron shells.

periodic table – The table that lists all the elements in order of increasing atomic number, arranged into groups and periods.

property – Any characteristic of an element.


Complete Assignment #1

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Chemical Bonding - misszukowski.weebly.commisszukowski.weebly.com/.../9.0_chemical_bonding_1_notes_key.pdf · 1 of 43 © Boardworks Ltd 2009 Chemistry 11 Chemical Bonding. 2 of 43