Forming Chemical Bonds

Compounds

• Remember compounds are two or more different elements that combine chemically –Can be broken down by chemical means

–Have properties different from those of its component element

– Thousands of compounds exist

Chemical Bonds

• A chemical bond is a force that holds two atoms together

• Chemical bonds usually only involve valence electrons

Share electrons

transfer electrons

Valence Electrons• Valence electrons are electrons in the highest

occupied s and p energy level.

• The number of valence electrons largely determines the chemical properties of an element.

• To find the number of valence electrons in a representative (group A) element, simply look at its group number.

• Diagrams that show valence electrons as dots

Lewis dot diagram

Why do elements react?

• Noble Gases typically do not react and are chemically stable because they have eight valence electrons called an octet

• Elements tend to react in order to acquire a full octet or “eight is great” (gain stability)

• Octet rule –when forming compounds, atoms tend to achieve the stable electron configuration of a noble gas

Formation of Positive Ions-Cations• A cation is an atom or group of atoms with a

positive charge because of a loss of valence electrons. Ca “+” ion

• Metals only form cations• Reactivity of metals is based on the ease in

which they lose a valence electron to achieve a stable octet.

• Brainiac Alkali Metals Video

Electron Configuration of Positive Ions-Cations

• All of the above metal ions have electron configurations of the noble gas neon, Ne. 8 is great!

Na atom Na 1s22s22p63s1

Na ion (lost 1 electron) Na+ 1s22s22p6

Mg atom Mg 1s22s22p63s2

Mg ion (lost 2 electrons) Mg2+ 1s22s22p6

Al atom Al 1s22s22p63s23p1

Al ion (lost 3 electrons) Al3+ 1s22s22p6

Ne atom (noble gas) Ne 1s22s22p6

Cation Charges

• Group 1A lose 1 electron, making a 1+ ion • Group 2A loses 2 electrons, making a 2+ ion• Group 3A loses 3 electrons, making a 3+ ion

Formation of Negative Ions-Anions

• An anion is a negative atom or group of atoms that gain electrons

• Nonmetals tend to gain electrons

egative

Electron Configuration of Negative Ions-”Anions”

• Cl ion and S ion have the noble gas configuration of argon. 8 is great!

Cl atom Cl 1s22s22p63s23p5

Cl ion (gain 1 electron) Cl- 1s22s22p63s23p6

S atom S 1s22s22p63s23p4 S ion (gain 2 electrons) S2- 1s22s22p63s23p6

Ar atom Ar 1s22s22p63s23p6

Anion Charges• 5A tend to gain 3 electrons, making a 3- ion• 6A tend to gain 2 electrons, making a 2- ion• 7A tend to gain 1 electron, making a 1- ion

• The ion produced when atoms of halogens (group 7A) gain electrons are called halide ions