SME
Grade 10 Science · Chemistry
The periodic table and periodic trends
Use this for “Which atom is larger, sodium or chlorine, and by how many picometers?” and why radius changes across a period.
Diagram
Atomic radius of Sodium: 155 pm.
Chlorine: 99 pm, so Na has the larger radius.
Atomic radius decreases across a period and increases down a group; the stronger the shade, the larger the value.
Difference (first − second): 56 pm
This is an example. Type your own values to start a new problem.
Δx (pm)
=
x₁ (pm)
−
x₂ (pm)
Z₁
Atomic number of the first element
example
Z₂
Atomic number of the second element
example
Formulas
x₁
=
atomic
radius
of
element
Z₁
155 = atomic radius of element 11
x₂
=
atomic
radius
of
element
Z₂
99 = atomic radius of element 17
Δx
=
x₁
−
x₂
56 = 155 − 99
Assumptions
•
Radii are covalent radii, in picometers (1 pm = 10⁻¹² m).
•
Across a period, more protons pull the same shell closer, so atoms shrink.
•
Down a group, each new shell makes atoms larger.
Step-by-step
WE KNOW
Z₁ = 11, Z₂ = 17
FIND
atomic radius of the first element (x₁), atomic radius of the second element (x₂), difference (first − second) (Δx)
Step 1 · Find atomic radius of the first element (x₁)
Use x₁ = atomic radius of element Z₁
Read the element’s atomic radius from the table.
x₁
=
atomic
radius
of
element
11
Z = 11 (sodium): period 3, group 1
x₁
=
155
pm
Step 2 · Find atomic radius of the second element (x₂)
Use x₂ = atomic radius of element Z₂
Read the element’s atomic radius from the table.
x₂
=
atomic
radius
of
element
17
Z = 17 (chlorine): period 3, group 17
x₂
=
99
pm
Step 3 · Find difference (first − second): Δx
Use Δx = x₁ − x₂
Subtract to see which is larger, and by how much.
Δx = 155 − 99
Same period: Z = 17 is farther right, so the atomic radius of Z = 11 is the larger
Δx = 56 pm
Check
Put the numbers back in. Both sides should match.
155 = atomic radius of element 11 ✓
99 = atomic radius of element 17 ✓
56 = 155 − 99 ✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.