The International School of Panama

IB Chemistry HL Notes - Bonding

•         A chemical bond is a strong attracting force between atoms or ions in a compound. It’s a strong electronic interaction.

•         Why do atoms form bonds?

•         Because the formation of the bond results in a decrease of their potential energy making the system more stable.

•         What electrons are involved in the formation of bonds?

•         The electrons in the valence shell, that is valence electrons.

The type of bond formed depends on the attraction for electrons the atoms involved have.  This attraction is called Electronegativity.

Why is it important to know the type of bond?

•         Because the properties of substances depend mainly on the types of bonds they have…

•         Ionic Bond: A chemical bond formed by electrostatic attraction between a cation (positive ion) and an anion (negative ion).

•         How does the ionic bond form?

Example

Na = 1s22s22p63s1          By level (or shell): 2  8  1

Cl = 1s22s22p63s23p5   By level (or shell): 2  8  7

Na has a single electron in the outer level, so it needs to get rid of the outer electron.

Cl has 7e in the outer level, so it needs to get an extra electron to complete the octect.

Chlorine has high electronegativity (3.5), Na has low electronegativity (0.9)…Therefore when they are close to each other one electron is transferred from Na to Cl, and both acquire a stable configuration with the outer level complete..both get more stable, both are happier…Let´s see the process

http://207.10.97.102/chemzone/lessons/03bonding/mleebonding/ionic_bonds.htm

Covalent bond

 •         Covalent: Bond formed when electrons are shared by two atoms.

•         How does the covalent bond form?

Example

Cl = 1s22s22p63s23p5   By level (or shell): 2  8  7

Cl = 1s22s22p63s23p5   By level (or shell): 2  8  7

Cl has 7e in the outer level, so it needs to get an extra electron to complete the octet.

Chlorine has high electronegativity (3.5). When two chlorine atoms approach each other both nuclei  attract the outer electrons equally so they share a pair of electrons acquiring both a configuration with the outer level complete. Both get more stable, both are happier…Let’s see the process

      http://www.chemguide.co.uk/atoms/bonding/covalent.html

http://207.10.97.102/chemzone/lessons/03bonding/mleebonding/covalent_bonds.htm

Chlorine and other halogens diatomic molecules:

                        http://www.chem.ufl.edu/~chm2040/Notes/Chapter_11/covalent.html

 Metallic Bond

•         Metallic Bond : All the positive nuclei of the metals attract the delocalized outer electrons forming a lattice of positive ions surrounded by a sea of electrons. (electron sea model)

•         Why does a metallic bond form?

Example

Na= 1s22s22p63s1   By level (or shell): 2  8  1

Fe= 1s22s22p63s23p64s23d6   By level (or shell): 2  14  2

Metals have always few electrons in the outer level.

Metals have low electronegativity, but when several atoms of the metal are together all nuclei attract the outer electrons. This non directional attraction between all nuclei and the outer electrons holds the atoms strongly attached to each other but at the same time allows these outer electrons move freely. This is known as the electron sea model. 

http://207.10.97.102/chemzone/lessons/03bonding/mleebonding/metallicbonding.htm

The Lewis electron dot structure of molecules and ions

1. Sum  the valence electrons of all the atoms in the molecule or ion. If it is an ion, add one electron per each negative charge and take out one per each positive charge.
2. Draw the skeleton

o       Usually the element that appears only once in the molecule is the central atom.

o       Fluorine and Hydrogen are always terminal elements (never central)

o       Also remember the following tendency in the number of bonds formed by the atoms of the following elements:

http://www.miramar.sdccd.cc.ca.us/faculty/fgarces/ChemComon/Tutorial/Lewis/LewisTutorial.pdf

3. Put  one pair of electrons between the atoms connected to each other.
4. Put electrons around each atom trying to comply with the octet rule, including the bonding pairs and the lone pairs

·        Octet rule: most atoms form stable compounds when they have 8 electrons around (the octet is complete) attaining a noble gas configuration.

·        Hydrogen follows the duet rule, that is it forms stable compounds when it has 2 electrons around (attaining He configuration).

5. If you have more electrons that the sum of all valence electrons, use double bonds between atoms. Each double bond will allow you to eliminate one pair of electrons.
6. Take into account the following:

• The second row elements C, N, O and F should always follow the octet rule.
• The second row elements Be and B often have less than 8 electrons.
• Second row elements never exceed the octet rule (never have more than 8 electrons)
• Third row elements and heavier elements often satisfy the octet rule but can exceed the octet rule occupying their empty d orbitals.
• If, after following all the steps you must add more electrons to use all the valence electrons, place them around an atom from the third row on.