Unit 4. Covalent Bonding Flashcards
When does covalent bonding occur?
occurs between elements with a difference in electronegativity of fewer than 1.8 units. It occurs between non metal elements
What is a covalent bond
A covalent bond is the electrostatic attraction between positively charged nuclei and shared pairs of bonding electrons.
How are diatomic molecules bonded
Single Covalent bonds for Halogens
Oxygen and nitrogen are also diatomic but the atoms are via double and triple bond.
A double covalent bond consists of four shared electrons, while a triple covalent bond involves the sharing of six bonding electrons.
How does bond length change across single, double and ripple bonds
Consider the group 17 elements, which form diatomic molecules. Atomic radius increases down a group, so the length of the bond between the atoms also increases.
multiple bonds are shorter and stronger than single bonds as they share more electrons between the atoms, so the electrostatic attraction between the bonded nuclei is greater. This greater attraction brings the nuclei closer together
How do covalent bonds weaken?
Longer bonds are weaker than shorter bonds due to the increased distance between the nuclei and the shared pairs of bonding electrons, which results in a weaker electrostatic attraction between the atoms
How do we know of differences in bond lengths in strength
by comparing multiple bonds between atoms of the same element - for example, by comparing the lengths of the carbon-to-carbon bonds in different compounds.
How does bond strengths affect the reactivity of a compound
Bond strength can affect the reactivity of a compound. Nitrogen is unreactive because it has a triple bond, N≡N. It takes a lot of energy to break this bond: 945 kJ mol−1. Oxygen is much more reactive as it only takes 498 kJ of energy to break a mole of O=O bonds.
What is a coordinate covalent bond and give an example
A coordinate covalent bond differs from a ‘regular’ covalent bond because both the bonding electrons come from one atom.
Ex. CO
How do you represent a coordinate covalent bond
With an arrow on the bonds in a Lewis structure. The arrow comes from to atom from which the electrons originate from.
Give an example of a coordinate covalent bond in a polyatomic ionic compound
the hydronium ion (H3O+) and the ammonium ion (NH4+). In the hydronium ion, the coordinate covalent bond is formed between a hydrogen ion and a non-bonding pair of electrons on the oxygen atom. In the ammonium ion, it is formed between a hydrogen ion and the non-bonding pair of electrons on the nitrogen atom.
When do coordinate covalent bonds form? Give an example
Atoms in molecules that are electron-deficient (lacking in electrons) are able to form coordinate covalent bonds. An example of such a molecule is aluminium chloride. Despite being a metal and non-metal atom bonded together, the bonding in aluminium chloride is actually polar covalent, not ionic.
What is a dimer
A dimer is a larger molecule composed of two identical smaller molecules and can be linked by coordinate covalent bonds or by hydrogen bonds.
What is bond order
The bond order is the number of bonds between a pair of atoms. Single bonds have a bond order of 1, double bonds have a bond order of 2, etc.
The higher the bond order, the greater the strength of the bond. Fractional bond orders are possible
How do you calculate bond order
A simple calculation of bond order is to divide the sum of the individual bond orders by the number of bonding groups.
Ex: bond order of CO3 2–.
The carbonate ion is composed of two single bonds and one double bond. The sum of the individual bond orders is 1+1+2 = 4. There are 3 bonding groups in the ion (two single bonds and one double bond), therefore, the bond order is:
4/3 =1.33
What are the two types of covalent bonds
Non-polar and polar
Electronegativity difference for non-polar and polar bonds
Polar covalent= 0.5-1.7
Non polar= 0.1-0.4
What are pure covalent bonds?
Pure covalent bonds occur between atoms that have no difference in electronegativity, such as those found between the diatomic molecules.
What are polar bonds
Polar covalent bonds occur between atoms that have a difference in electronegativity of between 0.5 and 1.7 units.
the bonding electrons are not equally shared between the atoms, creating a bond dipole.
A bond dipole will result in one atom in the bond having a partial positive charge and the other having a partial negative charge.
These partial positive and negative charges are shown by the δ+ and δ- signs respectively.
What is a bond dipole caused by
the unequal sharing of electrons in a covalent bond.
What is the octet rule
The octet rule states that the most stable arrangement for an atom is to have eight electrons in its outermost energy level with the electron configuration of a noble gas.
What are the exceptions to the octet rule
Hydrogen is stable with only two electrons in its outer shell.
Atoms such as boron, beryllium and aluminium (in compounds) are stable with fewer than eight electrons in their outer shell.
Atoms in period three and higher, such as sulfur, can form expanded octets with up to twelve electrons in their valence shell.
What do you call atoms with less than eight electrons in their outer shells
Incomplete octet or electron deficient
What are Lewis structures
Lewis structures (also known as electron dot diagrams) represent the bonding in a molecule; they show both the bonding electrons and the non-bonding electrons (or lone pairs).
What are resonance structures?
When molecules contain multiple bonds (double or triple bonds), there is more than one possible Lewis structure that can be drawn. The different Lewis structures that can be drawn for the same molecule, or polyatomic ion, are called resonance structures.
What is a resonance hybrid structure
The actual structure of compounds with resonance structures. In the resonance hybrid structure all bonds are of equal lengths and strengths which is not the case in resonance structures.
It is important to realise that the molecule for which we depict several different resonance structures is not flipping between these forms. The actual structure is an intermediate between the forms known as the resonance hybrid structure.
What is VSEPR
The valence shell electron pair repulsion theory
What is the order of repulsion between non-bonding and bonding domains?
Non-bonding domain > bonding domain
What is the electron domain geometry
the total number of electron domains (both bonding and non-bonding) around the central atom.
Electron domain geometry of molecules with two electron domains
- linear
-180 degrees
Ex. CO2
Electron domain geometry with three electron domains
- trigonal planer
- molecular geometry can be either bent or trigonal planar, depending on lone pairs on central atom. No lone pair= trigonal planar, lone pair= bent
- if both geometries trigonal planar then 120 degrees, or else if bent then a little less than 120.
Electron domain geometry with four electron domains
- tetrahedral
- molecular geometry: tetrahedral, trigonal pyramidal or bent
- no lone pairs = tetrahedral, 109.5 degrees
- one lone pair= trigonal pyramidal, 107.8 degrees
- two lone pair= bent, 104.5
What are electron domains
Both bonding electrons and non-bonding electrons are collectively known as electron domains.
What is the net dipole moment of a molecule
The net (or overall) dipole moment of a molecule is a measure of its overall polarity. It is the sum of all the bond dipoles in a molecule.
Two things to consider when evaluating the polarity of a molecule
- the presence of polar bonds
- the molecular geometry
What molecular geometry do non-polar molecules have
Symmetrical so the net dipole moment cancels out, even if the bonds in the molecules themselves are polar.
What molecular geometry do polar molecules have
Usually not symmetrical molecules are polar
What structure to covalent substances form other than discrete molecules, like H2O or CO2
Giant covalent structures, ex diamond, silicon and silicon dioxide
Properties of silicon and silicon dioxide
the strong covalent bonds between the atoms are responsible for the properties of silicon dioxide: SiO2 is a very hard substance, a poor conductor of electricity at low temperature and has a high melting and boiling point. Sand is an impure form of silicon dioxide with a yellow colour due to the presence of iron(III) oxide.
What are allotropes
different forms of the same element in the same physical state.
Name all three allotropes of carbon
Diamond, graphite and fullerenes
Properties of diamond
The strong covalent bonds between the carbon atoms mean that diamond is a very hard substance with a high melting and boiling point. It is a poor electrical conductor as it has no delocalised electrons within its structure.
Properties of graphite and structure
In graphite, each carbon atom is covalently bonded to three other carbon atoms. Graphite has a layered structure consisting of carbon atoms arranged in fused hexagonal rings. A graphite crystal is composed of many such planar sheets of hexagonally arranged carbon atoms, stacked on top of one another. The layers are held together by relatively weak London dispersion forces. Each carbon atom has an electron which becomes delocalised across the plane.
- soft, can conduct electricity
Properties and structure of fullerene C60
Simple covalent structure
The structure is made up of carbon atoms bonded together in 20 hexagons (six-carbon rings) and 12 pentagons (five-carbon rings), known as a truncated icosahedron. Like graphite, each carbon atom is covalently bonded to 3 other carbon atoms. C60 does have delocalised electrons but the electrons cannot jump between individual fullerenes, therefore, it is a poorer electrical conductor than graphite.
What is graphene
A single layer of graphite which
very high tensile strength, and high electrical and thermal conductivity
Very reactive due to unoccupied bonds in the edges of the structure
It is composed of single layers of graphite with a bond angle of 120o between the carbon atoms in a trigonal planar arrangement.
