Structure and Bonding: Types of chemical bond

Question 1

What is a metallic structure?

Answer 1

The metallic structure consists of a giant lattice of positively charged ions in a sea of delocalised electrons.

Question 2

What is a metallic bond?

Answer 2

The metallic bond is the electrostatic force of attraction between positively charged ions and the delocalised electrons.

Question 3

Why do atoms bond?

Answer 3

Atoms bond to achieve a full, stable outer electron energy level.

Question 4

What is bonding?

Answer 4

Bonding is the mechanism which holds atoms or particles together.

Question 5

What is structure?

Answer 5

Structure is the way in which these atoms or particles are arranged.

Question 6

What are properties?

Answer 6

Properties are the resulting physical and chemical characteristics.

Question 7

In a metallic structure what holds the positive metal ions together?

Answer 7

Delocalised electrons are the “glue” that holds positive metal ions together. This attraction occurs in all directions.

Question 8

What are the properties of metals which can be related to their structure and bonding?

Answer 8

The properties of metals which can be related to their structure and bonding:

• They are electrical conductors as the electrons are delocalised.
• The strong metallic bond has to be broken on melting so, generally metals have high melting points.
• They are malleable and ductile as the metallic bond acts in all directions.
• They are good thermal conductors as the atoms are very closely packed and can pass on heat (kinetic energy/vibrations) through conduction.

Question 9

What happens to the electrical conductivity of metals across a period?

Answer 9

Periodicity of metals across a period:

The electrical conductivity increases as there are more delocalised electrons.

Question 10

What happens to the melting point of metals across a period?

Answer 10

Periodicity of metals across a period:
The melting point of metals increases as the positive core is increasing in charge and the number of delocalised electrons increases, creating a stronger metallic bond.

Question 11

What happens to the melting point of metals as you descend a group?

Answer 11

Periodicity of metals down a group:
The melting point of metals decreases as you descend a group. This is because the strength of the metallic bond decreases as there are more occupied energy levels and so the nucleus is further away from the delocalised electrons. The inner energy levels also screen the nucleus from the delocalised electrons which also decreases the strength of the metallic bond.

Question 12

Describe a covalent bond

Answer 12

In a covalent bond, atoms share pairs of electrons. The covalent bond is a result of two positive nuclei being held together by their common attraction for the shared electrons. The covalent bond is strong.

Question 13

What is a pure or non-polar covalent bond?

Answer 13

When atoms of the same element form a covalent bond the electrons are shared equally. This is because the atoms have the same electronegativity and pull on the bonding electrons with equal force. Atoms of different elements with the same electronegativity value will also share electrons equally and form a pure or non-polar covalent bond (this is quite rare).

Question 14

How is a polar covalent bond formed?

Answer 14

Polar covalent bonds are formed in compounds when atoms of different elements share electrons. Atoms with different electronegativity values will share the pair of bonding electrons unequally forming partial charges or a permanent dipole

Question 15

What notation is used to indicate partial charges?

Answer 15

Delta positive (δ+) and delta negative (δ-) notation is used to indicate the partial charges on atoms in a polar covalent bond. The atom with the highest electronegativity will attract the bonding electrons closer towards it, becoming slightly negative (δ-). The other atom is left slightly electron, becoming slightly positive (δ+) .

Question 16

What are ionic bonds?

Answer 16

Ionic bonds are the electrostatic attraction between positive and negative ions. Ionic bonds are strong.

Question 17

What is ionic formula used for?

Answer 17

Ionic formula gives the simplest ratio of each type of ion in the substance.

Question 18

What do ionic compounds form?

Answer 18

Ionic compounds form lattice structures of oppositely charged ions.

Question 19

Are compounds formed between metals and non metals ionic or covalent?

Answer 19

Compounds formed between metals and non-metals are often but not always ionic. This is because the greater the difference in electronegativity between two elements the less likely they are to share electrons. Atoms of elements with high electronegativity attract electrons towards themselves whereas those with low electronegativity are more likely to lose electrons. Hence due to periodic trends in electronegativity, elements far apart in the periodic table are more likely to form ionic rather than covalent bonds.

Question 20

What are the properties of ionic compounds?

Answer 20

The bonding in ionic compounds gives rise to the following properties:

• The strong electrostatic attraction between oppositely charged ions (the ionic bonds) have to be overcome on melting which means that ionic compounds are high melting point solids.
• Ionic compounds do not conduct electricity when solid as the ions are not free to move (the ions are struck in the crystal lattice).
• Ionic compounds do conduct electricity when molten (in liquid state) or in solution as the ions are free to move.
• Many ionic compounds are soluble in water e.g. salt, sodium chloride

Question 21

What are the properties of covalent network substances?

Answer 21

Covalent network substance:

• Have very high melting points as strong covalent bonds have to be broken on melting.
• form very hard substances as the atoms are held together by strong covalent bonds in a giant network e.g. carbon in the form of diamond, silicon dioxide siO2 (sand-from rock), silicon carbide SiC (used as an abrasive, and in grinding and cutting tools).
• Do not conduct electricity (exception is graphite) as there are no free charged particles.
• are insoluble in water.

Question 22

What are van der Waals forces?

Answer 22

The forces of attraction that hold discrete atoms (noble gases) and discrete molecules together in the liquid and solid states are called van der Waals forces of attraction. All van der Waals forces are weak but some are stronger than others.

Question 22

What are van der Waals forces?

Answer 22

The forces of attraction that hold discrete atoms (noble gases) and discrete molecules together in the liquid and solid states are called van der Waals forces of attraction. All van der Waals forces are weak but some are stronger than others.

Question 23

What are the types of van der Waal forces?

Answer 23

There are two types of van der waal forces:

• London dispersion forces of attraction
• Permanent dipole-Permanent dipole forces of attraction (including hydrogen bonding).

Question 24

Where can london dispersion forces be found?

Answer 24

London dispersion forces are forces of attraction that operate between all atoms and molecules.

Question 25

What type of van der Wall forces do the noble gases have and why?

Answer 25

The monotamic elements are the most stable elements in the periodic table. They have a filled outer layer of electrons and so do not form bonds between their atoms. They occur as single atoms and therefore their structure is described as monatomic. Noble Gases form liquids when cooled to very low temperatures. This suggests that there must be some forces between the atoms that can hold them close together. These forces are called London dispersion forces and they are very weak, resulting in the noble gases having very low melting and boiling points.

Question 26

What is a temporary or instantaneous dipole?

Answer 26

In the case of noble gases, one side of the atom can have an excess of electrons and the other side can have a deficiency of electrons. This makes one side of the atom have a slight negative charge and the other side a slight positive charge. This is called a temporary or instantaneous dipole. Oppositely charged instantaneous dipoles cause the atoms to attract each other when they approach closer at low temperatures.

Question 27

What type of van der Waal forces can be found in discrete molecules (Covalent molecular)?

Answer 27

In molecules the bonding electrons move constantly round and between the atoms. Most of the time they are between the atoms but this is not always the case. London dispersion forces are formed as a result of electrostatic attractions between temporary (instantaneous) dipoles and induced dipoles caused by the movement of electrons in all discrete molecules.

Question 28

Why do discrete molecules have low melting and boiling points?