C. Chemistry: The Nature of Matter · Atoms, Elements and Compounds

Drawing Bohr-Rutherford and Lewis Dot Diagrams

C2.3 C2.4 A1.4About 70 minutes

Learning goals

I can draw a Bohr-Rutherford diagram for any of the first 20 elements.
I can state how many electrons fit in each of the first energy levels (2, 8, 8, then 2 for potassium and calcium).
I can identify valence electrons and draw Lewis dot diagrams.
I can read a short program that models how electrons fill energy levels.

Words to know

Bohr-Rutherford diagram
A drawing of an atom showing the numbers of protons and neutrons in the nucleus and the electrons in each energy level.

energy level (shell)
A region around the nucleus where electrons are found. For the first 20 elements, the levels hold up to 2, 8, 8 and 2 electrons (in this course).

valence shell
The outermost energy level that contains electrons.

valence electrons
The electrons in the valence shell. They decide how an atom reacts.

Lewis dot diagram
A diagram that shows only the element's symbol and its valence electrons as dots.

stable octet
A full valence shell of 8 electrons (2 for helium), which makes an atom very stable.

Drawing what you can't see

Scientists, engineers and students all over the world use the same simple drawing to show what an atom looks like inside: the Bohr-Rutherford diagram. Once you can draw one, you can explain why sodium explodes in water, why neon doesn't react with anything, and why table salt is NaCl and not NaCl₂. All of it comes down to the electrons in the outer shell.

Rules for the first 20 elements

Electrons fill energy levels from the inside out. In this course, use these maximums:

1st energy level: up to 2 electrons
2nd energy level: up to 8 electrons
3rd energy level: up to 8 electrons (for the first 20 elements)
4th energy level: the last 2 electrons (potassium and calcium)

(The 3rd level can actually hold more than 8, but for the first 20 elements it is filled with 8 before the 4th level starts. You will learn the full rules in Grade 11 chemistry.)

Steps to draw a Bohr-Rutherford diagram:

Find the atomic number (= protons = electrons in a neutral atom).
Find the number of neutrons: mass number − atomic number. If you aren't given a mass number, round the atomic mass on the periodic table.
Draw a small circle for the nucleus and write the numbers of protons and neutrons inside (for example, 8p and 8n).
Draw circles for the energy levels you need.
Place the electrons, filling each level before starting the next. A common way: put the first electrons of a level one at a time at the top, right, bottom and left, then start pairing them up.

Element (Z)	Electron arrangement	Element (Z)	Electron arrangement
H (1)	1	Na (11)	2, 8, 1
He (2)	2	Mg (12)	2, 8, 2
Li (3)	2, 1	Al (13)	2, 8, 3
Be (4)	2, 2	Si (14)	2, 8, 4
B (5)	2, 3	P (15)	2, 8, 5
C (6)	2, 4	S (16)	2, 8, 6
N (7)	2, 5	Cl (17)	2, 8, 7
O (8)	2, 6	Ar (18)	2, 8, 8
F (9)	2, 7	K (19)	2, 8, 8, 1
Ne (10)	2, 8	Ca (20)	2, 8, 8, 2

Electron arrangements of the first 20 elements. Notice the repeating pattern in the last number.

Worked example

Example 1: Draw magnesium-24

Draw the Bohr-Rutherford diagram for magnesium-24. Magnesium's atomic number is 12.

Protons = 12. Electrons = 12 (neutral atom). Neutrons = 24 − 12 = 12.
Draw the nucleus and write 12p and 12n inside.
First level: 2 electrons. 12 − 2 = 10 left.
Second level: 8 electrons. 10 − 8 = 2 left.
Third level: the last 2 electrons. Arrangement: 2, 8, 2.

Magnesium-24: nucleus 12p, 12n; electrons 2, 8, 2. It has 2 valence electrons in its third (valence) shell.

Building magnesium step by step: nucleus first, then fill each level from the inside out.

Bohr-Rutherford diagrams for oxygen, sodium and chlorine. Each arrangement (below the name) adds up to the atomic number: 2 + 6 = 8, 2 + 8 + 1 = 11, and 2 + 8 + 7 = 17.

Valence electrons and Lewis dot diagrams

The electrons in the outermost occupied shell are the valence electrons. They are the ones that interact with other atoms, so they decide how an element reacts.

Sodium (2, 8, 1) has 1 valence electron.
Chlorine (2, 8, 7) has 7 valence electrons.
Neon (2, 8) has a full valence shell of 8, called a stable octet. Helium's single shell is full with 2. Atoms with full valence shells are very stable and almost never react.

A Lewis dot diagram is a shortcut that shows only the symbol and the valence electrons as dots. Place dots one at a time on the four sides of the symbol (top, right, bottom, left) before pairing any of them.

Lewis dot diagrams across period 2. Single dots come first on each side; pairs only start after all four sides have one dot (nitrogen onward).

Worked example

Example 2: From diagram to Lewis dots

Draw the Lewis dot diagram for sulfur (Z = 16).

Electron arrangement: 2, 8, 6. Sulfur has 6 valence electrons.
Write the symbol S.
Place 4 single dots: top, right, bottom, left.
Pair up the remaining 2: add a second dot on the top and on the right.

S with 6 dots: two pairs and two single dots. (It looks just like oxygen's, because both have 6 valence electrons.)

Coding connection (A1.4): a program that fills shells

Scientists use code to model patterns. This short Python function follows the filling rule for the first 20 elements. It fills each level up to its limit (2, 8, 8, 2) until all electrons are placed.

def shells(electrons):

limits = [2, 8, 8, 2]

result = []

for limit in limits:

if electrons == 0:

break

placed = min(electrons, limit)

result.append(placed)

electrons = electrons - placed

return result

for name, z in [("carbon", 6), ("sodium", 11), ("argon", 18), ("calcium", 20)]:

print(name, shells(z))

# Output:

# carbon [2, 4]

# sodium [2, 8, 1]

# argon [2, 8, 8]

# calcium [2, 8, 8, 2]

min(electrons, limit) places whichever is smaller: all the remaining electrons, or as many as the level can hold.

Hands-on activity

Build a Bohr-Rutherford model

You need: a paper plate or a large sheet of paper, a pencil, a cup and a bowl to trace circles, and two kinds of small items for the nucleus and electrons (for example, dried beans and buttons, or coloured paper dots). A periodic table (printed or online).

Safety: keep small items away from young children.

Choose an element from 11 to 20 (your birthday date works if it is in that range!).
Work out its protons, neutrons and electron arrangement.
Trace 3 or 4 circles around the centre for the energy levels. Write the numbers of protons and neutrons in the centre.
Place the electrons using your items, filling from the inside out and using the top-right-bottom-left order.
Circle the valence shell and write the number of valence electrons. Then draw the Lewis dot diagram beside your model.
Coding extension: if you have a computer, type the program above into a free online Python editor and add your own element to the list. Does the output match your model?

What to look for: the electrons on the model add up to the atomic number, each inner shell is full before the next starts (2, then 8, then 8), and the Lewis diagram has the same number of dots as the valence shell.

Just for fun

Why are noble gases so calm at parties?

They already have a full shell, so they don't need to bond with anyone.

Your turn

Give the electron arrangement for nitrogen (Z = 7).

Give the electron arrangement for argon (Z = 18). Is its valence shell full?

How many valence electrons does aluminium (Z = 13) have?

Describe the Bohr-Rutherford diagram for fluorine-19 (Z = 9).

Describe the Lewis dot diagram for carbon.

Which element has the arrangement 2, 8, 6?

Using the program, what would shells(15) print?

More practice: warm-up

How many electrons can the first energy level hold?

Give the electron arrangement for lithium (Z = 3).

How many valence electrons does oxygen have?

What is the outermost occupied shell called?

Give the electron arrangement for calcium (Z = 20).

More practice: on your own

Describe the full Bohr-Rutherford diagram for potassium-39 (Z = 19).

Which two elements in the first 20 have 5 valence electrons?

A student draws chlorine as 2, 7, 8. What mistake did she make?

Draw (describe) the Lewis dot diagrams for Na and Mg. How are they different?

Why do neon and argon have similar Lewis dot diagrams?

Challenge

An atom has 3 energy levels and 6 valence electrons. Identify it and give its electron arrangement.

What would happen if you ran shells(21) with this program? Is the answer correct for scandium (Z = 21)? Explain.

Explain why the Lewis dot diagram is more useful than the full Bohr-Rutherford diagram when you study how atoms bond.

Check your understanding

Quiz

What is the electron arrangement of silicon (Z = 14)?

How many valence electrons does chlorine (2, 8, 7) have?

In a Bohr-Rutherford diagram of oxygen-16, what is written in the nucleus?

Which element has a full valence shell?

A Lewis dot diagram shows...

What does shells(9) return with the program in this lesson?

For parents and teachers

Big idea: C2.3 asks students to show the location, relative mass and charge of subatomic particles using the Bohr-Rutherford model, and C2.4 connects these diagrams to the periodic table. Drawing B-R diagrams for elements 1 to 20 quickly and accurately is a key skill for the rest of the unit. A1.4 (coding) is touched by reading and predicting the output of a short program.

Common difficulties:

Putting more than 2 electrons in the first shell.
Writing the atomic mass instead of the number of neutrons in the nucleus.
Pairing dots too early in Lewis diagrams. One dot on each side first, then pairs.
Expecting the 2-8-8-2 rule to work past calcium. It is a simplified rule for the first 20 elements.

How to help: have your teen draw one diagram a day for a week, choosing different elements. Ask, "How many valence electrons? What other element has the same number?" That last question sets up the next lesson on the periodic table.