There is a certain number of electrons that is optimal for atoms to have in their energy shells. That number is 8 and is called an octet. When an atom has eight electrons in an energy shell, the atom obtains a lower state of energy and is stable.
Ionic bonds are achieved through the transfer of valence electrons (outermost energy shell electrons). Metals that have three or less electrons, in their valence energy shells, tend to transfer their electrons to nonmetal elements that possess five, six or seven electrons in their valence energy shells.
The transfer of valence electrons forms positively and negatively charged “ions”. These oppositely charged ions are attracted to each other. The attractions hold the ions together and formula units are formed. However, the number of ionically bonded compounds is quite small compared with the total number of compounds formed in the universe.
The vast majority of compounds are formed from sharing valence electrons. These bonds are called covalent bonds and the compound that is formed is called a molecule. In a molecule, the electrons are considered to be part of the valence shell of both atoms.
Hydrogen and oxygen are both _____________. If they bond together, would we expect an ionic bond or covalent bond to form? How many hydrogen atoms would be needed to bond to one oxygen atom?
Let’s consider water, H 2 O, as an example. Hydrogen has one electron in its only energy shell. ________ electrons can fit in this inner energy shell. Oxygen has six electrons in its outermost energy shell (valence energy shell); it wants __________.
By sharing their valence electrons, both hydrogen and oxygen acquire an octet completing their outermost energy shells (valence shells). Possessing an octet in the valence energy shell causes atoms to be more __________.
The graphic we saw of H 2 O shows all of the electrons that belong to hydrogen and oxygen. Lewis Dot Diagrams are more convenient to use. They only show the electrons we are interested in, the valence electrons. The shared electrons are shown as dashes and the remaining electrons (not being shared) are paired up and called “lone pairs”.
A water molecule has ________shared electrons (two shared pairs), shown here as dashes, and _______ lone pairs of electrons.
How many valence electrons does carbon have? How many valence electrons does carbon need to complete its octet? How many valence electrons does hydrogen have? How many valence electrons does hydrogen need to complete its valence?
Draw what you think a molecule between hydrogen and carbon would look like. ( HINT: Start with carbon as the center of the molecule )
When sulfur and hydrogen bond, _______electrons are shared (each dash represents ______shared electrons). There are _____lone pairs of electrons in H 2 S molecule.
Electronegativity describes electron affinity. In other words, it is a measure of the tendency of an atom to accept an electron. Bonding is often not clearly ionic or covalent.
One of the periodic table trends we observed was that electronegativity _____________ as you move from left to right across the periodic table. To find the difference in the electronegativity between two elements, you ___________their electronegativity values.
If the electronegativity difference between two bonding elements is very large, the element with the larger electronegative value will “grab” the valence electrons. This transfer of electrons leads to an ionic bond. If the difference is small the electrons will be shared. This sharing of electrons leads to a covalent bond.
Ionic ( transfer ) ∆ EN 3.3 Mostly Ionic ∆ EN > 1.7 Mostly Covalent ( Polar ) Unequal sharing of electrons ∆ EN 0.4 – 1.7 Covalent ( Non-polar ) Equal sharing of electrons ∆ EN = 0.0
The bonding between hydrogen and chlorine: Cl EN value 3.16 H EN value -2.20.96 The electronegativity difference is.96 which means the bond formed between chlorine and hydrogen is a covalent bond and a HCl molecule is the result.
The bonding between sodium and chlorine: Cl EN value 3.16 Na EN value -0.93 2.23 The electronegativity difference is 2.23 which means the bond formed between chlorine and sodium is an ionic bond and a NaCl formula unit is the result.