Bonding
Ionic bondingis when positively and negatively chargedionsstick to each other through
electrostatic force. These bonds areslightly weaker than covalent bonds^1 and stronger than
Van der Waals bonding or hydrogen bonding.
In ionic bonds the electronegativity^2 of the negative ion is so much stronger than the elec-
tronegativity of the positive ion that the two ions do not share electrons. Rather, the more
electronegative ion assumes full ownership of the electron(s).
Sodium chloride forms crystals with cubic symmetry. In these, the larger chloride ions are
arranged in a cubic close-packing, while the smaller sodium ions fill the octahedral gaps
between them. Each ion is surrounded by six of the other kind. This same basic structure
is found in many other minerals, and is known as the halite structure.
Perhaps the most common example of an ionically bonded substance is NaCl, or table
salt. In this, the sodium (Na)^3 atom gives up an electron to the much more electronegative
chlorine (Cl)^4 atom, and the two atoms become ions, Na+and Cl-.The electrostatic bonding
force between the two oppositely charged ions extends outside the local area attracting other
ions to form giant crystal structures. For this reason most ionically bonded materials are
solid at room temperature.
8.2 Covalent Bonding
Covalent bondingis close to the heart of organic chemistry. This is where two atoms
share electronsin a bond. The goal of each atom is tofill its octetas well as have a
formal charge of zero. To do this, atomic nuclei share electrons in the space between them.
This sharing also allows the atoms to reach alower energy state, whichstabilizesthe
molecule. Most reactions in chemistry are due to molecules achieving a lower energy state.
Covalent bonds are most frequently seen between atoms with similar electronegativity. In
molecules that only have one type of atom, e.g. H 2 or O 2 , the electronegativity of the
atoms is necessarily identical, so they cannot form ionic bonds. They always form covalent
bonds.
Carbon is especially good at covalent bonding because its electronegativity is intermediate
relative to other atoms. That means it can give as well as take electrons as needs warrant.
Covalently bonded compounds have strong internal bonds but weak attractive forces be-
tween molecules. Because of these weak attractive forces, the melting and boiling points
of these compounds are much lower than compounds with ionic bond^5 s. Therefore, such
compounds are much more likely to be liquids or gases at room temperature than ionically
bonded compounds.
In molecules formed from two atoms of the same element, there is no difference in the
electronegativity of the bonded atoms, so the electrons in the covalent bond are shared
1 Chapter8.2on page 34
2 Chapter 6 on page 23
3 https://en.wikipedia.org/wiki/sodium
4 https://en.wikipedia.org/wiki/chlorine
5 https://en.wikibooks.org/wiki/Organic%20Chemistry%2FFoundational%20concepts%20of%20organic%20chemistry%2FIonic%20bonding