chemical bonding

• why do atoms bond?

  - obtain the electron structure of a noble gas for stability--> by obtaining full orbits and energy levels it increases stability- to obtain a lower state of energy--> since energy is needed to break chemical bonds and energy is released when chemical bonds are formed--> the formation of chemical bonds allows the atoms involved to achieve lower energy state
• chemical bonding


  chemical bonding is the process by which atoms bond with each other and in this way achieve noble gas configuration, with low potential energy
• molecule


  a molecule is a group of two or more atoms which are bonded together strongly enough to behave as a single unit in a chemical reaction
• what forces are between atoms

  - there are forces between atoms of a molecule to make it possible to form such a unit

  - this mutual bonding force is known as a chemical bond

  - both attractive and repulsive force that approach each other to form a chemical bond
• attractive forces are found between

  - the nucleus of an atom (+) and its own electrons (-)

  - the nucleus of an atom (+) and the electrons of another atom (-)
• repulsive forces are found between

  - the nucleus of an atom (+) and the nucleus of another atom (+)

  - the electrons of an atom and the electrons of another atoms (-)
• how do chemical bonds form?

  Chemical bonds form when the atoms reach a point where the forces of attraction equal the forces of repulsion
• what happens when two atoms approach each other

  - the attractive force between the nucleus of one atom and the electron cloud of the other increases, decreasing the potential energy of the system

  - at the same time the repulsive forces between two positive nuclei and two negative electron clouds increases, increasing the potential energy of the system
• energy pathway of atoms

  
• bond strength

  bond strength is the amount of energy needed to break a chemical bond

  - (more work needed to be done= stronger bonds)
• bond length trend

  - bonds between larger atoms are longer

  - bonds between smaller atoms are smaller
• relationship between bond length and bond strength

  - inversely proportional

  - short bond is generally stronger

  - long bonds are generally weaker
• difference between weak and strong bonds

  - weak bonds= easy to break, typically longer fairly reactive

  - strong bonds= difficult to break, typically shorter, give rise to stable molecules
• what are valence electrons?

  the electrons that occupy the outermost energy level of an atom. They are involved in bonding
• how can you determine the number of valence electrons of an element?

  the group number
• what is valency?

  valency is a measure of the number of chemical bonds formed by the atoms of a given elements

  - valency of an electron= a measure of its ability to combine with other elements
• valency of zinc

  2+
• valency of silver


  1+
• lewis dot method

  
• how do you represent the valence electron structure using the lewis dot method?

  1. dots represent orbitals

  2. follow the Aufbau structure in filling of orbitals except the group 4 elements, filled 1 orbital on each side of nucleus
• how can elements gain noble gas electron configuration and therefore stability

  - share electrons with another atom

  - donate one or more electrons to another atom

  - gain one or more electrons from another atom
• what are the types of chemical bonding?

  covalent, ionic and metallic
• what influences certain properties of compounds

  the strength of the intermolecular forces influences the properties of the compound eg: shape, volume, melting & boiling points
• the stronger the intermolecular force is

  the more energy needed to move molecules apart again
• what are the van der Waals forces?

  • london
  • dipole-dipole
  • hydrogen
• what type of molecules have london forces

  All interactions between molecules involve London forces.
• how are london forces formed

  London forces are a temporary attractive force that result when the electrons in two adjacent atoms or molecules occupy positions that make the atoms or molecules form temporary dipoles.
• how is the strength of london forces determined

  how easily a given electron cloud can be polarised.
• The strength of the London forces is influenced by the size of the induced dipoles, which is

  influenced by the:

  • number of electrons

  • interacting surface (size) of the molecule
• when do london forces get stronger

  as a molecular size + mass increases, the intermolecular forces get stronger
• As you move down group 17:

  • the molecules get bigger therefore they have more electrons

  • The strength of the induced dipoles increases as you move down the group

  • The strength of the London forces increases

  therefore more energy is required to overcome the stronger London forces

  therefore the boiling point increases
• dipole-dipole forces are found between

  polar molecules
• when does a hydrogen bond form

  A hydrogen bond forms when the δ+ H of one such molecule attracts the lone pair on the N, O, or F of another molecule.
• Hydrogen bonding is stronger than dipole – dipole forces due to:

  • the high polarity of the covalent bod formed between H and either N, O or F

  • the fact that the molecules can get close together (because N, O and F are small atoms)
• what is the strongest van der waal force

  hydrogen bonding
• The strength of each hydrogen bond is determined by the strength of the dipole.


  A greater difference in electronegativity results in a covalent bond that is more polar

  therefore the molecule is a stronger dipole.
• NUMBER of hydrogen bonds in H2O


  On average each H2O molecule forms FOUR hydrogen bonds to other H2O molecules. Each molecule has two δ+ hydrogen atoms and two lone pairs of electrons so that every one of them can be involved in hydrogen bonding.
• NUMBER of hydrogen bonds in NH3


  On average each NH3 molecule forms TWO hydrogen bonds to other NH3 molecules. Each ammonia molecule can form one hydrogen bond using its lone pair and one involving one of its δ+ hydrogens. (The other 2 hydrogen atoms are “wasted”.)
• NUMBER of hydrogen bonds in HF


  On average each HF molecule forms TWO hydrogen bonds to other HF molecules. Each molecule can only form one hydrogen bond using its δ+ hydrogen and one involving one of its lone pairs. (The other 2 lone pairs are essentially “wasted”.)
• the boiling points in H2O HF NH3 in C are

  H2O - 100

  HF - 19.5

  NH3 - 33.34