T4. Chemical Bonding and Structure

Strength of metallic bond

*1.* Charge of the ions *2.* Ionic radius

Hydrogen bonding

*1.* Electrostatic attraction... *2.* Between polar covalent molecules which contain hydrogen(s) bonded to nitrogen/oxygen/fluorine *3.* Caused by very large differences in electronegativity

Dipole-dipole forces

*1.* Electrostatic attraction... *2.* Permanent dipoles *3.* Between polar covalent molecules *4.* Caused by relatively large differences in electronegativity

London Dispersion Forces

*1.* Electrostatic attraction... *2.* Temporary and instantaneous dipoles *3.* Between covalent molecules *4.* Caused by the random movement of electrons - The greater the molecular mass, the greater the number of London forces present

Allotropes of carbon: C60 fullerene

*C₆₀ fullerene:* Consists of a truncated icosahedral cage (soccer ball). It is spherically symmetrical with each carbon atom covalently bonded to three others. C₆₀ IS NOT a covalent network solid, it is instead composed of individual molecules with strong covalent bonds, but with weak LDF between the molecules. Does not conduct electricity

Allotropes of carbon: diamond

*Diamond:* Each carbon atom is covalently bonded to four other carbon atoms with a tetrahedral arangement (bond angle of 109.5˚). Diamond is one of the hardest substances known because of this covalently bonded interlocking structural arrangement of tetrahedra. The melting and boiling points of diamond are very high. In diamond, the valence electrons are localised in the single covalent bonds and therefore cannot move freely - does not conduct electricity.

Allotropes of carbon: graphene

*Graphene:* Consists of a single planar sheet of carbon atoms arranged hexagonally and is only one atom in thickness. As in graphite, each carbon atom is covalently bonded to three other carbon atoms. Superb electrical conductivity

Allotropes of carbon: graphite

*Graphite:* hexagonal layers of carbon atoms which can slide past each other, the layers are connected by weak LDF (think pencils). Each carbon atom adopts a trigonal planar geometry, and is covalently bonded to three other carbon atoms each at a bond angle of 120˚. Graphite is a good conductor of electricity as it has delocalized electrons

Physical properties of covalent compounds

*Melting and boiling points* - Covalent compounds have lower melting and boiling points than ionic compounds *Volatility* - Covalent compounds may be volatile *Electrical conductivity* - Covalent compounds do not conduct electricity because no ions are present to carry the charge *Solubility* - Covalent compounds are typically insoluble in water

Physical properties of ionic compounds

*Melting and boiling points* - Ionic compounds have high melting points and high boiling points because of the strong electrostatic forces of attraction between the ions in their lattice structures *Volatility* - Volatility refers to the tendency of a substance to vaporise. For ionic compounds the electrostatic forces of attraction are strong and so the volatility of such substances is very low *Electrical conductivity* - For an ionic compound in the solid state the ions occupy fixed positions in the lattice. Hence the ions are not free to move in the solid state, so solid ionic compounds do not conduct electricity. In contrast, in the molten state, the ions are free to move and conduct electricity *Solubility* - Ionic compounds dissolve in polar solvents such as water, but do not dissolve in non-polar solvents such as hexane.

How is an ionic compound formed?

1. Electron transfer to form ions 2. Electrostatic attraction between ions 3. Ionic lattice

Working method to deduce molecular polarity

1. Using VSEPR theory, deduce the molecular geometry 2. For each bond present, using the electronegativity differences, deduce the bond polarity for each bond present and draw the associated dipole moments 3. Using vector addition, sum all the dipole moments present to establish whether there is a net dipole moment for the molecule. If so, the molecule is polar.

Covalent bonding

A covalent bond is formed by the electrostatic attraction between a shared pair of electron and the positively charged nuclei. (Typically between non-metals)

Covalent network solid

A covalent network solid consists of atoms held together by covalent bonds in a giant three-dimensional lattice structure

Metallic bond

A metallic bond is the electrostatic attraction between cations and the negatively charged sea of electrons

Bond length

A single bond is longer than a double bond, which in turn is longer than a triple bond

Bond strength

A triple bond is stronger than a double bond, which in turn is stronger than a single bond

Allotrope

Allotropes are different structural modifications of the same element

Ionic bonding

An ionic bond refers to the electrostatic attraction experienced between the electric charges of a cation (positive ion) and an anion (negative ion). (Typically between metal and non-metal)

Covalent compound structure

Covalent compounds consist of molecules (though there are covalent network structures that involve lattices)

Coordinate covalent bonding

In coordinate covalent bonding, the shared pair of electrons comes from only one of the two atoms, this atoms donates both electrons to the shared pair.

Malleability of metals

Malleability is the ability of a solid to be pounded or hammered into a sheet or other shape without breaking Sea of electrons allow displacement without cation-cation contact, hence repulsion is prevented.

Resonance structures

Resonance involves using two or more Lewis structures to represent a particular molecule or ion

The octet rule

The rule states that elements tend to lose electrons (oxidation OIL), gain electrons (reduction RIG), or share electrons in order to acquire a noble gas core electron configuration.

Ionic compound structure

Under normal conditions, ionic compounds are typically solids, and have lattice-type structures that consist of three-dimensional repeating units of positive and negative ions

Polar covalent bond

When one element has a greater attraction for the shared pair of electrons in a covalent bond, a polar covalent bond is created. One atom adopts a partial negative charge and one atom adopts a partial positive charge. E.g. water

How is a covalent bond formed?

Electrostatic attraction between a nucleus and a shared pair(s) of electrons

Allotropes of carbon

Graphite, diamond and graphene and C₆₀ fullerene