electronegativity - miss zukowski's...

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Electronegativity is defined as the ability of an atom in a

molecule to attract electrons to itself.

Electronegativity is a function of two

properties of isolated atoms:

• The atom’s ionization energy (how strongly

an atom holds onto its own electrons)

• The atom’s electron affinity (how strongly

the atom attracts other electrons)

For example, an element which has a large (negative)

electron affinity and a high ionization (always endothermic, or

positive for neutral atoms)…

…Will attract electrons from other atoms and resist having

electrons attracted away

We say these atoms are “highly electronegative”

Electronegativity

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Electronegativity trends: across a period

Electronegativity increases across a period because:

1. The atomic radius decreases.

2. The charge on the nucleus increases without

significant extra shielding. New electrons do not

contribute much to shielding because they are added

to the same principal energy level across the period.


Electronegativity trends: down a group

2. Although the charge on the nucleus increases,

shielding also increases significantly. This is

because electrons added down the group fill new

principal energy levels.

Electronegativity decreases down a group because:

1. The atomic radius increases.


Electronegativity is dictated by:

The number of protons in the nucleus

• across a period you are increasing the number of protons, but filling electrons in the same Bohr quantized energy level. You are only filling sub-shells, so electronegativity increases from left to right

The distance from the nucleus

• down groups, you are placing electrons into new quantizedenergy levels, so moving further away from the attractivepower of the nucleus. Outer shell becomes further awayfrom the nucleus.

The amount of shielding by the inner electrons

• level of shielding upon bonding electrons increases down groups, and adds to the reduction in electronegativity. Shielding is caused by repulsion of electrons for each other.


Ions & Atomic Radius

Negative Ions

As extra electrons are added to a neutral atom

(eg O to make O-2) the ion has the same positive

nuclear charge (due to protons), and an

increased number of electrons surrounding the

nucleus.

• The electrostatic repulsion increases

• The volume occupied by the electrons

increases

NEGATIVE IONS are LARGER

than the corresponding neutral atom


Ions & Atomic Radius

Positive Ions

As electrons are removed from a neutral atom

(eg Mg to make Mg+2) the ion has the same

positive nuclear charge (due to protons), and a

decreased number of negative electrons

surrounding the nucleus.

• The electrostatic repulsion decreases

• The volume occupied by the electrons

decreases

POSITIVE IONS are SMALLER

than the corresponding neutral atom


Electronegativity & Ionic Bonds

• Formed between two atoms with large

differences in their ionization energies

and electronegativity's

• An electronegativity difference of

greater than 1.6 can be classified as an

ionic bond

• In this case we can essentially say

electrons are transferred from one

atom to another


Summary of Ionic Bonding


True or false?


Glossary

bond – A strong force that joins atoms or ions together in molecules and giant lattices.

compound ion – An ion made up of a group of atoms, rather than one single atom.

ionic bond – The electrostatic force of attraction between oppositely charged ions.

ionic compound – A compound made up of ions.

ionic lattice – A giant 3D structure of closely packed, oppositely-charged ions.

negative ion – An atom or group of atoms that has gained electrons and so has a negative charge.

noble gas – An element that has a full outer electron shell and so is very stable and unreactive.

positive ion – An atom or group of atoms that has lost electrons and so has a positive charge.


Anagrams


Multiple-choice quiz


Why do atoms form bonds?

Shared electron pair = covalent bond

Bond formation begins with atoms “colliding.”

For example as two hydrogen atoms approach each other, their

kinetic energy increases as each electron cloud is attracted

to the other’s approaching positive nucleus.

Atoms continue moving together until the repulsive forces of

the two negative electron clouds and the two positive nuclei

slow the atoms and convert their kinetic energy into potential

energy.



As the atoms get close to each other, their electron clouds

overlap enough to cause attractive forces to exceed

repulsive ones.

The two valence electrons will move into the region of space

between the atoms nuclei.

Shared electron pair = covalent bond

This force of attraction of a pair of

valence electrons between

two adjacent nuclei constitutes a

single covalent bond.



The atoms of noble gases have completely

full outer shells and so are stable.

The atoms of other elements have

incomplete outer electron shells

and so are unstable.

This makes the noble gases very unreactive

and so they do not usually form bonds.

By forming bonds, the atoms of these

elements are able to have filled outer

shells and become stable.


What is a covalent bond?

Non-metal elements usually just need one or two electrons

to fill their outer shells. So how do they form a bond?

The shared electrons join the atoms together. This is called

a covalent bond.

The two non-metal atoms cannot form a bond by transferring

electrons from one to another. Instead, they share electrons.

Each atom now

has a full, stable

outer shell.

incomplete outer shells

Cl

Cl

Cl

Cl


Two common ways to represent a covalent bond are:

Only outer shells of electrons are involved in bonding, so the

inner shells do not always have to be included in diagrams.

solid

line

Cl Cl–

–Cl Cl

A covalent bond consists of a shared pair of electrons.

covalent bond

Cl

simplified dot and cross

diagram

Cl

How is a covalent bond drawn?

Cl Cl


Comparing covalent and ionic bonding


Ionic & Covalent Bond Summary:


How are covalent bonds formed?

How do non-metal atoms form covalent bonds?


Covalent bonding in hydrogen

Many non-metal elements, such as hydrogen, exist as

simple diatomic molecules that contain covalent bonds.

How is a covalent bond formed in hydrogen?

Some molecules contain double or triple covalent bonds.

How are these are formed?

H H H

Each hydrogen atom needs one more electron in its outer

shell and so each atom shares its single unpaired electron.

H

This shared pair of electrons forms a covalent bond and so

creates a diatomic molecule of hydrogen.


What are the types of covalent bonds?


Can compounds contain covalent bonds?

Covalent bonding can also occur between atoms of different

non-metals to create molecules of covalent compounds.

These covalent bonds can be single, double or triple.

How is a covalent bond formed in hydrogen chloride (HCl,

also represented as H–Cl)?

H ClH Cl

Hydrogen and chlorine both need one more electron to fill

outer shells. By sharing one electron each, they both have

a stable outer shell and a covalent bond is formed.


Covalent bonding in water

Compounds can contain more than one covalent bond.

The oxygen atom shares 1

electron with 1 hydrogen

atom, and a second

electron with another

hydrogen atom.

H H

O

Oxygen (2.6) needs 2 more electrons, but hydrogen [1] only

needs 1 more. How can these three elements be joined by

covalent bonding?

What is the name of the molecule that is formed?

H2O (or H–O–H) is water.


How is the ratio of atoms calculated?

To calculate the ratio of atoms in a stable covalent compound:

For example, how

many nitrogen and

hydrogen atoms

bond together in an

ammonia molecule?

1. Work out how many electrons are needed by each

non-metal element to complete its outer electron shell.

2. Work out the ratio of atoms that will provide enough

shared electrons to fill all the outer shells.

N H

(2.5) (1)

3electrons needed

electron configuration

ratio of atoms

element

1

1 3


H

Covalent bonding in ammonia

How do nitrogen and hydrogen atoms form covalent bonds

in a molecule of ammonia?

NH3 or H N H

H

N H

(2.5) (1)

3electrons needed


ratio of atoms

element

1

1 3

H HN


H

Covalent bonding in methane

How do carbon and hydrogen atoms form covalent bonds

in a molecule of methane?

CH4 or H C H

H

H

C H

(2.4) (1)

4electrons needed


ratio of atoms

element

1

1 4

H

H HC


Covalent bonding in carbon dioxide

How do carbon and oxygen atoms form covalent bonds in

a molecule of carbon dioxide?

C O

(2.4) (2.6)

4electrons needed


ratio of atoms

element

2

1 2

CO2 or O C O

O OC

double bonds

A double bond is when two pairs of electrons are shared.

In carbon dioxide there are two double bonds

– one between each oxygen atom and the carbon atom.


What are simple covalent structures?

Covalent molecules that contain only a few atoms are

called simple covalent structures.

weak bonds

between

molecules

strong bonds

within

molecules

Most substances that contain simple covalent molecules

have low melting and boiling points and are therefore liquids

or gases at room temperature, e.g. water, oxygen, carbon

dioxide, chlorine and hydrogen. Why?

The covalent bonds within these molecules are strong but

the bonds between molecules are weak and easy to break.


What is the structure of a molecular solid?

A few substances that contain simple covalent molecules

are solid at room temperature. These are molecular solids.

The solid is formed because

millions of iodine molecules

are held together by weak

forces of attraction to create

a 3D molecular lattice.

Two iodine atoms form a single covalent

bond to become an iodine molecule.

weak

forces of

attraction

What properties would you

expect molecular solids to have

with this type of structure?

Iodine is a molecular solid at room temperature.


What are the properties of molecular solids?

low melting and boiling points;

usually soft and brittle – they

shatter when hit.

The properties of a molecular solid, such as iodine, are:

Why do molecular solids have these properties?

The weak forces of attraction between the molecules can

be broken by a small amount of energy. This means that

the molecular solids are soft and brittle and melt and boil at

low temperatures.

Molecular solids are also unable to conduct electricity

because there are no free electrons or ions to carry a charge.

cannot conduct electricity.


Covalent bonds – true or false?


What are giant covalent structures?

In some substances, such as sand, diamond and graphite,

millions of atoms are joined together by covalent bonds.

All the bonds are covalent, so giant covalent structures have

very high melting and boiling points, and are usually hard.

The covalent bonds in these substances do not form

molecules but vast networks of atoms called giant covalent

structures.


What is the structure of sand?

Sand is mostly made of the mineral quartz, which is silicon

dioxide. It has a giant covalent structure made up of silicon

and oxygen atoms.

Each silicon atom (2.8.4) is bonded to four oxygen atoms,

and each oxygen atom (2.6) is bonded to two silicon atoms.

Si

O

O

O

O


What are the allotropes of carbon?

Diamond and graphite appear to be very different substances

but what do they have in common?

Both diamond and graphite are made up of carbon atoms.

These allotropes of carbon have different properties because

the atoms are bonded in different arrangements which create

different giant structures.

Different forms of the same element are called allotropes.


How does structure affect properties?

How do the different structures of diamond and graphite

influence their properties?


What is the structure of diamond?


What are the properties of diamond?

In diamond, all the electrons in the outer shell of each

carbon atom (2.4) are involved in forming covalent bonds.

This affects the properties of this allotrope of carbon:

Diamond is very hard – the

hardest natural substance.

Diamond has a very high

melting and boiling point

– a lot of energy is needed

to break the covalent bonds.

Diamond cannot conduct

electricity – there are no

free electrons or ions to

carry a charge.


What is the structure of graphite?


What are the properties of graphite?

In graphite, only three of the four electrons in the outer shell

of each carbon atom (2.4) are involved in covalent bonds.

Graphite is soft and slippery

– layers can easily slide over

each other because the weak

forces of attraction are easily

broken. This is why graphite is

used as a lubricant.

Graphite conducts electricity

– the only non-metal to do so.

The free electron from each

carbon means that each layer

has delocalized electrons,

which can carry charge.

This affects the properties of this allotrope of carbon:


Are there other allotropes of carbon?

A third class of carbon compounds have been discovered

in recent years. These are called fullerenes.

Buckminsterfullerene is one type of fullerene. It contains

60 carbon atoms, each of which is bonded to three others

by two single bonds and one double bond.

The atoms in this allotrope of carbon form a sphere, like the

shape of a football. The molecules can be called ‘bucky balls’.

They are large but are not classified as giant structures.

C

C

C

C


Complete the sentences


Electronegativity

values for some

common elements.

Values given here

are measured on

the Pauling scale.

In a covalent bond between two different elements, the

electron density is not shared equally.

This is because different elements have differing abilities to

attract the bonding electron pair.

This ability is called an element’s electronegativity.

What is electronegativity?


The electronegativity of an element depends on a

combination of two factors:

1. Atomic radius

As radius of an atom increases, the bonding pair of

electrons become further from the nucleus. They are

therefore less attracted to the positive charge of the

nucleus, resulting in a lower electronegativity.

higher

electronegativity

lower

electronegativity

Electronegativity and atomic radius


Electronegativity, protons and shielding

2. The number of unshielded protons

The greater the number of protons in a nucleus, the

greater the attraction to the electrons in the covalent

bond, resulting in higher electronegativity.

However, full energy levels of electrons shield the

electrons in the bond from the increased attraction of the

greater nuclear charge, thus reducing electronegativity.

greater nuclear

charge increases

electronegativity…

…but extra shell of

electrons increases

shielding.


Types of Covalent Bonds:

We have discussed the two extreme cases of

bonding: complete electron transfer- ionic

bonding

And covalent bonds with completely equal

electron sharing.

Between these extremes are covalent bonds

which involve unequal electron sharing.

When atoms with different electronegativities form covalent bonds,

those ΔEN values may be minimal or significant.


Non-polar bonds

If the electronegativity of both atoms in a covalent bond is

identical, the electrons in the bond will be equally attracted

to both of them, and form a non-polar covalent bond.

This results in a symmetrical

distribution of electron

density around the two

atoms.

Bonding in elements (for

example O2 or Cl2) is always

non-polar because the

electronegativity of the atoms

in each molecule is the same.

both atoms are

equally good at

attracting the

electron density

cloud of electron density


Mostly Covalent bonds

both atoms are

equally good at

attracting the

electron density

Such bonds are designated

as being “mostly

covalent” because ΔEN

appears to be insignificant.

Another way to

characterize this is to say

that these bonds have very

little “ionic character.”

If ΔEN is less than 0.4, the bonding electrons

between the two atoms spend no more of their time

nearer one nucleus than the other.


Electronegativity and Covalent Bonds

• As ΔEN increases beyond 0.4, the pair of

bonding electrons will be drawn closer

and closer to the nucleus of the atom

with the higher electronegativity.

• This unequal distribution of electron

density will give that end of the bond a

partially negative “pole” and the other a

partially positive “pole.”

• A bond “dipole” is said to exist and the

bond itself is known as a polar covalent

bond.


Polar bonds


Effect of electronegativity on polarization

The greater the electronegativity difference between the two

atoms in a bond the greater the polarization of the bond.

decreasing polarization

This can be illustrated by looking at the hydrogen halides:

H F Cl Br I

Molecule

Electronegativity

difference

between atoms

H–F H–Cl H–Br H–I

1.8 1.0 0.8 0.5

Pauling

elecronegativities

Element

2.2 4.0 3.2 3.0 2.7


Ionic or covalent?

Rather than saying that ionic and covalent are two distinct

types of bonding, it is more accurate to say that they are at

the two extremes of a scale.

Less polar bonds have

more covalent

character.

increasing polarization

More polar bonds have more

ionic character. The more

electronegative atom attracts the

electrons in the bond enough to

ionize the other atom.


Polar molecules

Molecules containing polar bonds are not always polar.

If the polar bonds are

arranged symmetrically,

the partial charges cancel

out and the molecule is

non-polar.

Non-polar molecules

If the polar bonds are

arranged asymmetrically,

the partial charges do not

cancel out and the

molecule is polar.

Polar molecules


Practice Problems: Covalent Bonding


Identifying polar molecules


Glossary (1/2)

allotrope – A structurally different form of an element with

different physical properties.

covalent bond – A strong bond between two atoms in

which each atom shares one or more electrons with the other.

covalent compound – A compound containing atoms

joined by covalent bonds.

double bond – A covalent bond in which each atom

shares two of its electrons.

giant structure – A structure containing millions of atoms

or ions bonded together. The structure extends in three dimensions until all available atoms are used up.


Glossary (2/2)

molecule – A small group of atoms which are held together

by covalent bonds.

molecular solid – A solid substance made up of

molecules held together by weak forces of attraction, forming a lattice.

single bond – A covalent bond in which each atom shares

one of its electrons.

triple bond – A covalent bond in which each atom shares

three of its electrons.


Anagrams


Multiple-choice quiz


Assignment #3

• Covalent Bonds

electronegativity - miss zukowski's...

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