Covalent Bonds - LeFonch

The Structure of Covalent Bonds

A covalent bond is a bond between atoms, mainly non metals.
The bonding is caused by a similar electrical force as in an ionic bond.
In ionic bonding however, one atom gives away electrons while another atom receives them, thus creating opposite charges which cause them to attract one another.
In a covalent bond neither of the atoms want to give up electrons, and thus they share electrons between themselves.
The atoms holding onto the electrons is what keeps the bond together.
Non-metals try to have a octet shell, similarly to noble gasses (although hydrogen only has 1 valence electron, so it needs only 1 more for a full shell), so they share atoms between each other.
When non-metals share electrons in either single, double or triple bonds (and so forth), they form covalent bonds.
A single covalent bond consists of 2 electrons, one from each atom.
This means that double bonds have 4 electrons, triple bonds have 6 electrons, and so on.
Covalent bonds can be expressed through the Lewis formula, which shows covalent bonds between atoms via lines.
One line represents a single covalent bond.

Examples

Hydrogen atoms have 1 valence electron each, meaning that they need 1 more electron for a full valence shield.
Two hydrogen atoms can form a covalent bond, which would be written as H–H.
Oxygen has 6 valence electrons, so two oxygen atoms can share 4 electrons between themselves (two from each) to achieve a full valence shield.
In the Lewis formula this can be written as O=O.
Nitrogen has 5 valence electrons, meaning to form an octet valence shield, they need to form a triple bond with one another, written as N≡N.

Polarity

Different pulling powers of different atoms can make covalent compounds polar, as electrons might be shifted towards one atom.
This phenomenon is called electronegativity.
Electronegativity describes the electron pulling force of an atom in a compound, and is written as χ (the symbol is a lowercase chi from the Greek alphabet).
The magnitude of electronegativity is affected by the size of an atom and its nuclear charge.
Atoms with less electron shields (because the protons are closer to the valence electrons) and the most electrons on their outer shield have the highest electronegativity.
For example fluorine is more electronegative than oxygen, as its atoms have 7 valence electrons (thus 7 protons which pull in electrons), while oxygen has 6 valence electrons.
However fluorine is also more electronegative than chlorine, even though they have the same amount of valence electrons.
This is because chlorine has one more electron shield than fluorine, meaning that its valence electrons are pulled less strongly.
The polarity of covalent compounds can be found from the Pauling scale, which ranks all elements according to their electronegativity.
Polarity can be found from the difference of the ratings on the Pauling scale.
If the difference between electronegativity between the atoms in the bond is between 0.4 and 1.7, the compound is polar.
A polarity of 0.4 is rather weak whereas a polarity of 1.7 is strong.
Only ionic compounds have a polarity greater than 1.7, although keep in mind that ionic compounds don't have partial charges as they don't share electrons.

Polar Molecules

If atoms have differing levels of electronegativity, they form a polar covalent bond.
Electron density forms around the more electronegative atom, whereas there is electron deficiency around the less electronegative atom.
Electron density is denoted by a negative partial charge, written as 𝛿- (the symbol is called a lowercase delta).
Electron deficiency however, is written as 𝛿+.
These partial charges less powerful than electrons and form between covalent compounds.
A polar molecule is called a dipole, and it has permanent partial charges.

Nonpolar Molecules

Molecules can be polar or non polar, depending on the electronegativity of the atoms.
Elemental molecules, molecules with only one type of atom are nonpolar as they all have the same electronegativity.
Nonpolar molecules don't have any partial charges.
If a molecule has no polar covalent bonds, then it is always nonpolar.
If a molecule has polar covalent bonds, it can be polar or nonpolar.
If the molecule has only one bond, which is polar, then it's always polar.
A molecule with polar bonds can be nonpolar if the negative partial charges cancel each other out.
Imagine people of equal strength playing tug of war. If they pull equally from all sides then the rope doesn't move at all.
For example the linear carbon dioxide is nonpolar, even though the double bonds between carbon and oxygen are polar.
This is because the oxygen on both sides cancel each other out.
Molecules that are drawn as straight or on a flat plane are nonpolar.
If they are drawn as crooked or bent then they are polar.
If a molecule has equally or very similarly electronegative atoms on symmetrical sides of a non electronegative atom, then the compound is nonpolar.
Such as if they're at a 120° angle to one another or at the corners of a tetrahedron.
Otherwise it is polar.

Covalent Lattices

Covalent compounds form lattices, similarly to ionic compounds and metals.
These are called covalent lattices and are held together by the atoms holding one another through their shared electrons.
They can be formed from one or more types of atom.
They usually have repeating patterns or layers and are usually very hard but brittle.
They have high melting points.
For example diamond and graphite are made from covalent lattices of carbon.

Formulae and Naming of Covalent Compounds

Elements are usually listed in order of electronegativity from smallest to largest.
The first part of the name comes from the significant element and the last part ends in "-ide".
The number of atoms is multiplied by the prefix.
Prefixes are not needed when there is only one compound between substances.
A list of the prefixes:
Mono - 1
Di - 2
Tri - 3
Tetra - 4
Penta - 5
Hexa - 6
Hepta - 7
Octa - 8
Nona - 9
Deca - 10
For example carbon dioxide means that there's one carbon atom and two oxygen atoms.