SME
Grade 10 Science · Chemistry
Electrons in atoms: energy levels, electron configuration and light
Use this for “Write the electron configuration of oxygen. How many valence and unpaired electrons does it have?”
Diagram
8 electrons (Oxygen): [He] 2s² 2p⁴.
Boxes fill from the lowest energy up; each box of a subshell takes one arrow before any takes two, and two arrows in a box point opposite ways.
Unpaired electrons (single arrows, lit): 2.
Valence electrons: 6
This is an example. Type your own values to start a new problem.
Z
Atomic number (electrons)
example
u
Unpaired electrons
worked out (not typed)
v
Valence electrons
worked out (not typed)
Formulas
u
=
unpaired
electrons
of
element
Z
2 = unpaired electrons of element 8
v
=
valence
electrons
of
element
Z
6 = valence electrons of element 8
Assumptions
•
Fill the lowest energy first: 4s fills before 3d.
•
One arrow in each box of a subshell before any pairs (Hund’s rule).
•
A pair points up and down (Pauli).
•
Chromium and copper move one 4s electron to 3d.
Step-by-step
WE KNOW
Z = 8
FIND
unpaired electrons (u), valence electrons (v)
Step 1 · Find unpaired electrons (u)
Use u = unpaired electrons of element Z
Fill the boxes in order, one arrow in each box of a subshell before pairing, then count the single arrows.
u
=
unpaired
electrons
of
element
8
Z
=
8
(oxygen):
1s²
2s²
2p⁴
u
=
2
Step 2 · Find valence electrons (v)
Use v = valence electrons of element Z
The electrons in the highest-numbered shell are the valence electrons.
v
=
valence
electrons
of
element
8
Shell 2 holds 2 + 4 = 6
v
=
6
Check
Put the numbers back in. Both sides should match.
2 = unpaired electrons of element 8 ✓
6 = valence electrons of element 8 ✓
Topic check
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