SME
Grade 10 Science · Problem type
Ionization energy
Use this to compare how much energy it takes to pull an electron off two atoms.
Diagram
First ionization energy of Magnesium: 738 kJ/mol.
Calcium: 590 kJ/mol, so Mg has the higher value.
First ionization energy increases across a period and decreases down a group; the stronger the shade, the larger the value.
Difference (first − second): 148 kJ/mol
This is an example. Type your own values to start a new problem.
Δx (kJ/mol)
=
x₁ (kJ/mol)
−
x₂ (kJ/mol)
Z₁
Atomic number of the first element
example
Z₂
Atomic number of the second element
example
Formulas
x₁
=
ionization
energy
of
element
Z₁
738 = ionization energy of element 12
x₂
=
ionization
energy
of
element
Z₂
590 = ionization energy of element 20
Δx
=
x₁
−
x₂
148 = 738 − 590
Assumptions
•
First ionization energy removes one electron from a gas atom, in kJ/mol.
•
Down a group the outer electron sits in a higher shell, farther from the nucleus, so it comes off more easily.
•
Across a period the growing nuclear charge holds the electrons tighter.
Step-by-step
WE KNOW
Z₁ = 12, Z₂ = 20
FIND
first ionization energy of the first element (x₁), first ionization energy of the second element (x₂), difference (first − second) (Δx)
Step 1 · Find first ionization energy of the first element (x₁)
Use x₁ = ionization energy of element Z₁
Read the element’s ionization energy from the table.
x₁
=
ionization
energy
of
element
12
Z = 12 (magnesium): period 3, group 2
x₁
=
738
kJ/mol
Step 2 · Find first ionization energy of the second element (x₂)
Use x₂ = ionization energy of element Z₂
Read the element’s ionization energy from the table.
x₂
=
ionization
energy
of
element
20
Z = 20 (calcium): period 4, group 2
x₂
=
590
kJ/mol
Step 3 · Find difference (first − second): Δx
Use Δx = x₁ − x₂
Subtract to see which is larger, and by how much.
Δx = 738 − 590
Same group: Z = 20 is lower down, so the ionization energy of Z = 12 is the larger
Δx = 148 kJ/mol
Check
Put the numbers back in. Both sides should match.
738 = ionization energy of element 12 ✓
590 = ionization energy of element 20 ✓
148 = 738 − 590 ✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.