SC6 & 7 - Covalent Bonding / Types of Substance

How are covalent bonds formed?

By the sharing of pairs of electrons between non-metal atoms.

Name some common molecular elements

Hydrogen, fluorine, chlorine, bromine, iodine, oxygen, nitrogen.

Name some common molecular compounds

Water, sucrose, hydrogen chloride, carbon dioxide, ammonia

Name the bond in molecules

Covalent bond

Approximate size of atoms and small molecules

Roughly 10 to the -10 metres in diameter.

General properties of covalent, simple compounds

Low melting / boiling points, few are soluble in water, most do not conduct electricity.

Why do covalent compounds have low melting and boiling points?

There are weak intermolecular forces of attraction between molecules that can be overcome with low levels of energy.

Why do simple molecular structures not conduct electricity?

There are no mobile charge carriers.

Structure of a polymer

Long chain molecule made up of monomers joined together by covalent bonds.

Name some allotropes of carbon

Graphite, diamond, fullerenes, graphene.

Difference between simple molecules and giant covalent structures.

Simple molecules consist of small groups of atoms joined by covalent bonds and the formula is fixed. Giant covalent structures have atoms joined together in a continuous network by covalent bonds, so there is no fixed formula.

Properties and uses of diamond

Very hard, high melting point. Used in cutting tools and jewellery.

Properties and uses of fullerenes

Low melting points, often tubular or spherical. Used in medicine for delivery of drugs.

Properties and uses of Graphite

Conductor of electricity, high melting point, weak forces between layers. Used in lubrication and as electrodes.

Properties of graphene

Strong, electrical conductor

Why are metals malleable?

The layers of ions can slide over each other, and the sea of electrons holds it all together so it doesn’t break.

Why do metals conduct electricity?

The delocalised electrons are free to move and carry charge

Structure of metals

Stacked layers of ions in a ‘sea’ of delocalised electrons, packed closely to form a giant lattice.

Typical metal properties

Conduct electricity, high melting and boiling points, shiny, malleable, high density.

Typical non-metal properties

Low melting and boiling points, brittle, not usually shiny, brittle, low density, poor electrical conductor.

Ionic bonding

Formed by the loss and gain of electrons to produce oppositely charged ions, which attract.

Ionic Structure

Billions ions held together in a lattice

Simple molecular bonding

Covalent bonds formed by the sharing of pairs of electrons between atoms.

Simple molecular structure

Small distinct groups of atoms joined together in a set formula

Giant covalent bonding

Covalent bonds formed between atoms by sharing pairs of electrons

Metallic Bonding

The electrostatic force of attraction between positive metal ions and their delocalised electrons.

Metallic structure

Billions of ions held together in a giant lattice structure of positive ions in a sea of negative delocalised electrons.

Why do we use models to represent structure and bonding?

They help to explain the properties of different substances, if we know the type of bonds and structure in the model then we can predict the likely properties of the substance.

Advantages and disadvantages of dot and cross diagrams

Shows how the electrons are shared to form bonds, Formula of compound can be worked out. Only shows outer electrons, using both dots and crosses makes it look like electrons from the atoms makes it look like the electrons are different.

Advantages and disadvantages of ball and stick models.

Shows the bonds between each atom, shows 3D structure clearly. Does not accurately show the relative sizes of the atoms and the distance between them, the bonds are not really like sticks.

Advantages and disadvantages of displayed formula

Clearly shows the chemical bonds, formula of the compound can be worked out from the diagram. Does not show the relative sizes of the atoms and the bonds between them.

Advantages and disadvantages of space filling models

Shows 3D structure so we can see how atoms are arranged in relation to each other, more accurately represents the relative sizes of each atom and the disrtances between them. Does not show how the bonds are formed.

How is structure and bonding linked to a substance’s physical properties?

Substances with high melting points that require lots of energy to break and melt. Strong bonds are found in ionic, metallic and giant covalent structures, so they all have high melting points.

However, simple molecular structures have week intermolecular forces between layers so they are easily overcome and have low melting points.

Substances that conduct electricity have charged particles that can freely move.

Common simple molecular compounds

Carbon dioxide, water, and methane.

How are covalent bonds formed and how do double or triple covalent bonds come about?

Covalent bonds are formed from shared pairs of electrons, each shared pair between two atoms. Each extra pair of electrons adds to the covalent bonds number (as in double or triple).