The #sigma# shortcut is do by a **head-on overlap** in between two compatible atom orbitals that room symmetric around the internuclear axis. Two typical examples are #s"/"s# and #s"/"p_z# combinations, whereby let"s speak the #z# axis is the horizontal, internuclear axis.

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The #pi# bonds room made through the **sidelong overlap** between two compatible atomic orbitals, which room specifically the #p_x"/"p_x# and also #p_y"/"p_y# combinations. Let"s say the #x# is vertical and #y# is towards or far from you.

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

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

The MO chart is:

From bottom to top, wherein again, #z# is horizontal, #x# is vertical, and also #y# is in or out of the screen/paper, we have:

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

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

**head-on**combination

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

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

That offers a shortcut order that #(<2+6> - 2)/2 = 3#, together expected.

For a **diatomic molecule** whose bond order is #3#, we deserve to conclude the there is one #sigma# bond and also two #pi# bonds.

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*CHALLENGE: If #NO^(+)# ion is isoelectronic v #CO#, what around neutral #NO#; what is the link order on that, and also from that, how countless #pi# binding on mean does that have? Hint: the still has one #sigma# bond.*