Topic 2 - Bonding

What is an ionic bond

An ionic bond happens when a metal looses electrons and non - metals gain electrons. It is a transfer of electrons

The metal atom loses electrons and becomes a positive ion. The non-metal atom gains electrons and becomes a negative ion. The oppositely charged ions attract = ionic bond

Draw electron arrangement for:

1) Mg₁₂ + O₈

2) Li₃ + Cl₁₇

3) Al₁₃ + F₉

Ions for each group

Group 1 = +

Group 2 = 2+

Group 3 = 3+

• lose electrons → becomes positive ion

Group 7 = -

Group 6 = 2-

Group 5 = 3-

• gain electrons → becomes negative ion

Complex/Molecular ions

Molecular ions are made of 2 or more atoms and stay together in a formula:

Work out ionic formula for:

1) Sodium (g1) Chloride (g2)

2) Sodium (g1) Oxide (g6)

3) Aluminium (g3) Oxide (g6)

4) Magnesium (g2) Hydroxide

Ionic properties

1) High melting points

→ very strong (electrostatic) force of attraction between oppositely charged ions

2) Form giant lattice structure

→ crystals

3) Conduct electricity when molten or dissolved

→ ions can move + carry a charge

What do all ionic compounds form

All ionic compounds form giant lattice structures

Regular giant lattice structure

• regular structure of alternating charged ions

• Increases number of folles

• Causes very high melting points

Pure metals

• rows of positively charged metal ions

• Surrounded by a sea of delocalised electrons

• The metallic bond is caused by strong forces of attraction between + ions and - electrons

What are alloys

• Is a mixture of metals

• Change properties

Alloys example

• Stainless steal (iron)

• Brass

• Bronze

• Gold

Structure:

• 2 or more sizes of atoms

• Disrupts the regular lattice structure

Metallic Bonding key properties

1) Conduct electricity

→ delocalised electrons can move and carry a charge through the structure

2) Conduct heat

→ delocalised electrons can move and transfer energy through the structure

3) Malleable/ductile

→ layers can slide/be combined

4) High mp/bp

→ opposite charges of electrons + ions

→ high (electrostatic) force of attractions

→ lots of energy required to break

Compare the bonding of sodium (metalic) + sodium chloride (ionic)

metallic:

• Rows of + ions

• Sea of delocalised electrons

• Strong force of attraction

Ionic:

• Transfer of e- from metal to non-metal

• Opposite charged ions attract

• Strong force of attraction

Describe, in terms of transfer of electrons, what is happening in the reaction between sodium & oxygen

Sodium looses 2 electrons and becomes + ion, oxygen gains 2 electrons and becomes 2- ion therefore 2 sodium required per oxygen

Describe, in terms of transfer of electrons, what happens when a magnesium atom reacts with a fluorine atom to form magnesium fluoride

Magnesium loses 2 electrons to become Mg2+ and fluorine gains 2 electrons to become f2- to become MgF2

Suggest why pure magnesium is a better conductor than pure lithium

• Magnesium has more outer shell electrons than lithium (2 vs 1)

• Therefore more electrons available to be delocalised and move/carry charge through structure

How do metals conduct electricity

Metals have delocalised electrons that are free to move throughout the structure and carry electrical charge when a potential difference is applied

Describe how sodium conducts thermal energy

sodium conducts thermal energy through the movement of delocalised electrons and the vibration of ions

Explain why steel is harder than iron

Steel I harder than iron because the carbon atoms disrupt the regular arrangement of iron atoms, preventing the layers from sliding over each other easily

How does an atom of Fe forming an Fe3+ ion occur

An atom of Fe looses three electrons from its outer shell to form a positive ion with a 3+ charge

Dot and cross diagram

Eg Sodium Fluoride

Covalent bonds

All non metal atoms want to gain electrons. How can 2 do this at the same time

By sharing electrons

Properties of covalent bonding

1) Do not conduct electricity

2) Have low mp/bp → weak intermolecular forces between molecules (imfs) we break the imfs not the strong cov bond

H-H /// H-H

Weak imf

Strong bond

Giant covalent molecules

• Most covalent molecules are small/simple however, carbon can form a variety of giant structures called allotropes

• The two most common are diamond and graphite

Diamond and Graphite

Diamond:

• Each carbon bonded to 4 other carbons

• Incredibly strong as carbuns cannot move

• High mp

• Shiny/lustrous

• Does not conduct electricity

• Use it for jewellery + tools

Graphite:

• Each carbon bonded to 3 others

• Forms layers with week imfs holding them together

• Layers can slide

• Has delocalised electrons → conducts electricity

• Use it for pencils

Compare the bonding and structure of diamond and graphite

• Diamond is bonded to 4 other carbons where as graphite is bonded to 3 others

• Diamond has a crystalline structure so each carbon is held in place where as graphite has layers for structure

• Diamond has high mp, does not conduct and is strong where as graphite has high mp (lower than diamond), does conduct because it has free electrons and is soft

Other carbon allotropes

Graphene = a single layer of graphite

→ 1 atom thick (can see through it)

→ each carbon bonded to 3 others

→ very strong

→ conducts electricity due to delocalised electricity

Nanotube = rolled up graphene

Buckyball (C60)

(Buckyminster Fullerene)

→ alternating pentagon/hexagons

→ football

→ personalised medicine

What is nanoscience

The study of particles between 1-100 nm

How small is 1nm

Why use nano particles

• Very large SA:V

• Can use more for less cost

• Can cost something without being visible → suncreme

Limitations of ionic bonding models - 2D & 3D space filling models:

2D & 3D space filling models:

• Both only show a few ions & not the huge number that are in a giant lattice

• 3D only shows the outer layer of ions

Limitations of ionic bonding models - Ball and stick giant lattice model:

Ball and stick giant lattice model:

• The ions are shown for apart when they are actually closely packed

• The sticks/lines could be confused with covalent bonds

Limitations of ionic bonding models - Dot & cross diagram:

Dot & cross diagram:

• Seem to show that there are only pairs of ions instead of the huge numbers found in a giant lattice

• Do not show relative sizes of the ions

Limitations of covalent bonding models for molecules - Molecule 2D diagrams:

Molecule 2D diagrams:

• Do not show the shape of the molecule in 3D

• Do not show which atoms the electrons in the bond have come from

Limitations of covalent bonding models for molecules - Molecule dot & cross diagrams:

Molecule dot & cross diagrams:

• Do not show relative sizes of the atoms

• Do not show the shape of the molecule