The #sigma# shortcut is make by a **head-on overlap** between two compatible atomic orbitals that room symmetric around the internuclear axis. Two common examples are #s"/"s# and also #s"/"p_z# combinations, where let"s say the #z# axis is the horizontal, internuclear axis.

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The #pi# bonds space made by the **sidelong overlap** between two compatible atomic orbitals, which are specifically the #p_x"/"p_x# and #p_y"/"p_y# combinations. Let"s say that #x# is vertical and #y# is in the direction of or far from you.

Let us consider a diatomic molecule for simplicity. How about #"CO"#?

The lewis framework for #"CO"# is #""^((-)):"C"-="O":^((+))#

The MO chart is:

From bottom come top, whereby again, #z# is horizontal, #x# is vertical, and #y# is in or the end of the screen/paper, us have:

#sigma_(2s)# bonding MO, native the #2s(C)"/"2s(O)#**head-on**combination#sigma_(2s)^"*"# antibonding MO, native the #2s(C)"/"2s(O)#

**head-on**combination#pi_(2p_x)# and also #pi_(2p_y)# bonding MOs, native the #2p_x(C)"/"2p_x(O)# and also #2p_y(C)"/"2p_y(O)#

**sidelong**combination#sigma_(2p_z)# bonding MO, from the #2p_z(C)"/"2p_z(O)#

**head-on**combination. This is the

**HOMO**.#pi_(2p_x)^"*"# and also #pi_(2p_y)^"*"# antibonding MOs (empty), native the #2p_x(C)"/"2p_x(O)# and #2p_y(C)"/"2p_y(O)#

**sidelong**combination. This is the

**LUMO**.#sigma_(2p_z)^"*"# antibonding MO (empty), native the #2p_z(C)"/"2p_z(O)#

**head-on**combination

The #sigma_(2s)# and also #sigma_(2s)^"*"# cancel out and also contribute zero to the **bond order** because they contribute two *bonding*, but likewise two *antibonding* electrons. The is **2** bonding and also **2** antibonding.

The #sigma_(2p_z)# contributes 2 *bonding* electrons, and also the #pi_(2px)# and also #pi_(2p_y)# contribute two *bonding* electrons **each**. That is **6** bonding.

That offers a bond order the #(<2+6> - 2)/2 = 3#, as expected.

For a **diatomic molecule** whose bond bespeak is #3#, we have the right to conclude that there is one #sigma# bond and two #pi# bonds.

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*CHALLENGE: If #NO^(+)# ion is isoelectronic through #CO#, what around neutral #NO#; what is the link order top top that, and from that, how many #pi# bonds on median does the have? Hint: that still has one #sigma# bond.*