Atoms: Ionic Bonding, Covalent Bonding, Covalent Molecular Bonding And Network Structures

Ionic Bonding and Network Structures

Ionic compounds are hard but brittle, do not conduct electricity in solid-state, good conductors of electricity when dissolved in water or as a liquid, have high melting and boiling points, they also vary in solubility but are not soluble in solvents that are non-polar for example oil, fats and greases. When electrons transfer from one atom to another, usually from a metal to a non-metal, an ionic bond is formed. This produces a pair of ions with one positively charged and the other negatively charged, creating a strong electrostatic attraction in the form of a lattice.

Atoms are most stable when their valence shell is full (contains eight electrons). For example, sodium chloride (NaCl), where the sodium atom has one electron in its outer shell in sodium chloride which it passes to the chlorine atom which has seven electrons in its outer shell. Therefore, the valence shells are now full when these electrons are transferred. The electronegativity of metals is less than electronegativity and ionisation energy of non-metals, therefore metal ions become cations (positively charged) whereas non-metals gain electrons and form anions (negatively charged). The chlorine anion and the sodium cation form a stable compound. The property that ionic bonds possess is a high melting point as the bond is strong and in order to break the bond, higher temperatures are needed. In an ionic bond, between particles, the forces are strong, there are no free-moving electrons. Ionic compounds are arranged in the form of a crystal lattice as the strong electrostatic forces are distributed in this arrangement holding the ions together.

Covalent Bonding and Network Structures

A covalent bond is formed from the electrostatic attraction of two nonmetals sharing a pair of electrons, essentially filling out and stabilising their outer electron shells. Each atom is supplied with one of the electrons in the covalent bond. Covalent substances have very low melting and boiling points and do not conduct electricity in any phase. This shows that some bonds of covalent substances are weak and covalent bonds do not contain ions or delocalised electrons (free-moving electrons). Non-metallic atoms have high numbers of electrons in their outer shells and share electrons instead of transferring to form a covalent bond.

Some elements have several structural arrangements called allotropes. An example of an allotrope consists of oxygen and ozone molecules made up of only oxygen atoms. Covalent network structures are molecules that exist as continuous three-dimensional structures. Moreover, the allotropes of carbon include graphite (layers of carbon covalently bonded) and diamond (each carbon atom is bonded with four single covalent bonds in a tetrahedral arrangement) molecules that are structured in different forms and therefore are covalent network structures. Other carbon allotropes are examples of nanomaterials.

Nanomaterials include fullerenes, graphene and nanotubes. Fullerenes are spherical shaped, consisting of atoms arranged in a series of pentagons and hexagons with three covalent bonds to each carbon atom. Graphene is similar to the structure of graphite but is one single layer sheet with the same arrangement as the layers stacked in graphite. Graphene is also stronger than graphite as there are no weak dispersion forces between layers. A carbon nanotube is a long, hollow, cylindrical structure enclosed with half a fullerene on each end whereby its walls are formed by graphene.

Covalent molecular bonding

Covalent molecular bonds are a type of covalent bonds. Including covalent bonds consisting of single, double and triple bonds specifically. For example, in a chlorine molecule, the electrons are attracted to the positively charged nuclei. For another example, two hydrogen atoms form a hydrogen molecule (diatomic molecule). As one atom has one electron in its outer shell, it bonds with another hydrogen atom to have a full shell with two electrons, forming a single covalent bond (H2). The electrons in a hydrogen molecule are attracted to the proton and move around the two nuclei rather than orbiting around its own.

Molecules that are made up of more than two atoms are called polyatomic molecules. An example of this is a water molecule (H2O). In this case, the oxygen atom shares two of its six electrons with each of the two hydrogen atoms.

Double covalent bonds occur when two pair of electrons (ie. four electrons) are shared. An example of this is an oxygen molecule (O2), where each oxygen atom requires two electrons to become stable. Therefore, when two oxygen atoms bond, each atom shares two electrons to have a full outer shell of eight electrons.

Additionally, triple bonds occur when three electron pairs are shared (ie. six electrons). For example, a nitrogen molecule (N2) where each atom contains five electrons, requiring three electrons to become stable. When one nitrogen atom bonds to another each contributes the remaining three electrons needed forming a triple covalent bond.

Metallic Bonding and Network Structures

Metallic bonding is a type of chemical bonding that arises from the electrostatic attractive force between conduction electrons (in the form of an electron cloud of delocalised electrons) and positively charged metal ions. Metals are hard, have high boiling points, are good conductors in both solid and liquid states of electricity and heat, malleable, ductile, reflective and lose electrons when involved in chemical reactions. These properties inform that there is a strong electrostatic force between particles, free movement of charged particles, attractive forces are stronger than repulsive forces, particles are closely packed, quick transfer of energy, presence of free electrons and electrons can be easily lost from metal atoms. Metallic bonds have a basis of cations arranged in a lattice and where electrons move freely throughout the lattice (delocalised electrons from the outer shells of atoms). As the electrons are free to move, transmit energy rapidly through the lattice as they vibrate rapidly when heated.