Teacher’s Notes
Electrostatic Attraction (aka Coulombic Attraction) Guided Inquiry
The positive charge of the proton and the negative charge of an electron create an electrostatic force of attraction
between the atom’s nucleus and its electrons. This force of attraction is also known as coulombic attraction. Two
variables affect the strength of this attraction:
The distance between the charged particles – The force of attraction has
an indirect relationship with the square of the distance. This means that
as the distance between the nucleus and the electron increases the
electrostatic attraction decreases. Because the relationship is with the
square of distance the force decreases rapidly as the distance increases.
For example, doubling the distance between the nucleus and electron
decreases the force of attraction by a factor of 4. Tripling the distance
between the nucleus and electron decreases the force of attraction by a
factor of 9.
The lithium atom is shown on the right. The force of attraction between
the nucleus and the 2 electrons in the n=1 energy level is stronger than
the force of attraction between the nucleus and the electron in the n=2
energy level, because the electrons in the n=1 energy level are closer to
the nucleus than the electron in the n=2 energy level.
The strength of the charged particles – The force of attraction has a
direct relationship with the strength of the charged particles. An electron
can only have a 1- charge; however, the strength of the nucleus’s charge
is equal to the number of protons in the nucleus. So as the number of
protons in the nucleus increases the strength of the electrostatic
attraction between the nucleus and the electron increases. If the
number of protons in the nucleus doubles, then the force of attraction
increases by a factor of 2. If the number of protons in the nucleus triples,
then the force of attraction increases by a factor of 3.
The boron atom is shown on the right. The force of attraction between
boron’s nucleus and its 3 electrons in the n=2 energy level is stronger
than the force of attraction between lithium’s nucleus and its electron in
the n=2 energy level, because boron’s nucleus has 5 protons compared to
only 3 protons for lithium’s nucleus.
+++
nucleus
n=1 n=2
lithium
+++++
nucleus
n=1 n=2
boron
What is the effect when both variables change?
The general relationship between the force of attraction, F, the distance between the electron and the nucleus, d,
𝑍
and the number of protons in the nucleus, Z, is 𝐹 ∝ 𝑑2 . Changing distance has a bigger effect on the attractive
force than changing the number of protons. So, if both the distance between the nucleus and electron, and the
number of protons in the nucleus doubled, the attractive force would decrease by a factor of 2. If the distance
between the nucleus and electron, and the number of protons in the nucleus tripled, the attractive force would
decrease by a factor of 3.
Use this information about the force of attraction between electrons and the nucleus and the periodic table to
solve the problems on the other side.
Complete each table below by writing down the number of protons each element has and the highest main
energy level of its electron cloud in the ground state. Then rank the strength of the electrostatic attraction
between their nucleus and the out most electron(s).
Element
# of
Protons
Highest Main
Energy Level
Relative Strength of
Coulombic Attraction
chlorine
17
3
2nd
magnesium
12
3
7th
argon
18
3
1st
silicon
14
3
5th
sodium
11
3
8th
sulfur
16
3
3rd
phosphorus
15
3
4th
aluminum
13
3
6th
# of
Protons
Highest Main
Energy Level
Relative Strength of
Coulombic Attraction
strontium
38
5
4th
barium
56
6
5th
magnesium
12
3
2nd
radium
88
7
6th
4
2
1st
calcium
20
4
3rd
Element
# of
Protons
Highest Main
Energy Level
Relative Strength of
Coulombic Attraction
cesium
55
6
5th
fluorine
9
2
1st
zirconium
40
5
4th
iron
26
4
3rd
silicon
14
3
2nd
Element
beryllium
All of these elements
have their outermost
electrons in the 3rd
main energy level, so
the element with the
most protons will have
the strongest
coulombic attraction.
All of these elements
are in Group 2, and
have their outermost
electrons in different
main energy levels.
The element with its
outermost electrons
closest to the nucleus
will have the strongest
coulombic attraction.
Once again, the
element with its
outermost electrons
closest to the nucleus
will have the strongest
coulombic attraction.