Bohr-Rutherford diagrams: how to draw them step by step (protons and neutrons in the nucleus, electrons in shells of 2, 8, 8), the first 20 elements drawn out as a table, the rules and the exceptions, how to draw an ion and an isotope, what the diagram shows about valence electrons and the periodic table, the Lewis dot diagram alongside, and a printable set with practice

"Bohr Rutherford diagram" is searched about 2,100 times a month in Canada, almost entirely by Ontario Grade 9 students, whose science course (SNC1W) asks them to draw the first 20 elements this way, and the same drawing appears in GCSE chemistry, NGSS middle-school science and CBSE Class 9 under other names (electronic structure, electron shell diagram, Bohr model). This page gives the five-step method, the first 20 elements as a table ready to copy, the rules and the one exception, ions and isotopes, what the diagrams show about the periodic table, the Lewis dot diagram beside it, the common mistakes and a printable set with practice. Our first 30 elements trick covers the order; this page covers the drawing.

Download the Bohr-Rutherford set with practice (PDF) (A4, the method, the rules and the table of 20 elements on page 1, ions, isotopes, the Lewis diagrams and the practice set with answers on page 2, free).

What the diagram shows

The Bohr-Rutherford model combines two discoveries. Rutherford (1911) found that an atom's positive charge and almost all its mass sit in a tiny central nucleus. Bohr (1913) proposed that electrons move around it only in certain fixed orbits, or shells, each holding a limited number. The diagram draws exactly that:

• A central circle, the nucleus, labelled with the number of protons (p⁺) and neutrons (n⁰).
• Concentric circles, the shells (energy levels), with electrons (e⁻) drawn as dots or crosses on them.
• Shell capacities for the first 20 elements: 2, 8, 8.

Modern physics replaces the circular orbits with probability clouds (orbitals), but the shell counts are right, and the diagram is the fastest way to see why sodium reacts the way it does and neon does not.

How to draw one in five steps

Take sodium, Na, atomic number 11, mass number 23.

1. Protons = atomic number = 11. Write 11p⁺ in the nucleus.
2. Neutrons = mass number − atomic number = 23 − 11 = 12. Write 12n⁰ in the nucleus. (Use the rounded atomic mass from the periodic table as the mass number: 22.99 rounds to 23.)
3. Electrons = protons in a neutral atom = 11.
4. Fill the shells from the inside: first shell 2, second shell 8, which uses 10; the eleventh electron goes on the third shell. Arrangement 2, 8, 1.
5. Place the electrons on each shell evenly: put one at the top, bottom, left and right before pairing any (some teachers accept any even spread; Ontario marking usually wants the four-position rule). Draw the dots on the circles, not inside or outside them.

Check: the number of dots equals the atomic number, and no shell is over its capacity.

The first 20 elements

The mass numbers are the most common isotope's (chlorine's periodic-table mass of 35.45 rounds to 35; argon's 39.95 rounds to 40). Notice that potassium (39) has a smaller mass number than argon (40) although it comes after it: the periodic table is ordered by protons, not mass.

The rules, and the one exception

• Fill from the inside. The first shell takes 2 before the second gets any; the second takes 8 before the third gets any.
• The outer shell is the valence shell. Its electrons are the valence electrons, the ones that take part in bonding. The number of valence electrons equals the group number for groups 1 and 2 and the group number minus 10 for groups 13 to 18.
• A full outer shell is stable. Helium (2), neon (2, 8) and argon (2, 8, 8) react with almost nothing: the noble gases. Every other atom bonds in a way that gets it to a full outer shell.
• The number of shells equals the period. Lithium to neon have two shells: period 2. Sodium to argon have three: period 3.
• The exception that ends the simple diagram: the third shell can hold 18 in total, but the fourth shell starts filling once the third has 8. So potassium is 2, 8, 8, 1 and calcium 2, 8, 8, 2, not 2, 8, 9 and 2, 8, 10. From scandium (21) onward the third shell fills up to 18 while the fourth already has 2, and the plain shell diagram is replaced by sub-shell notation (1s² 2s² 2p⁶ ...) in senior chemistry. That is why the school set stops at 20.

Drawing ions

An ion is an atom that has lost or gained electrons. The nucleus does not change.

• Sodium ion, Na⁺. Sodium 2, 8, 1 loses its one valence electron to leave a full second shell: 2, 8. Eleven protons, ten electrons, net charge +1. Draw the atom with two shells and write [ ]⁺ around it or Na⁺ beside it.
• Magnesium ion, Mg²⁺. 2, 8, 2 loses two: 2, 8, charge +2.
• Chloride ion, Cl⁻. Chlorine 2, 8, 7 gains one to fill its third shell: 2, 8, 8. Seventeen protons, eighteen electrons, charge −1.
• Oxide ion, O²⁻. 2, 6 gains two: 2, 8, charge −2.

Metals (left of the table, 1 to 3 valence electrons) lose electrons and form positive ions; non-metals (right, 5 to 7 valence electrons) gain and form negative ions. Na⁺ and Cl⁻ attracting each other is sodium chloride, and the two diagrams side by side are the Grade 9 and GCSE picture of an ionic bond.

Drawing isotopes

Isotopes are atoms of the same element (same protons) with different numbers of neutrons. Only the nucleus label changes.

• Carbon-12: 6p⁺, 6n⁰, electrons 2, 4. Carbon-14: 6p⁺, 8n⁰, electrons 2, 4. Same chemistry, different mass; carbon-14 is radioactive and used for dating.
• Hydrogen-1 (protium): 1p⁺, 0n⁰. Hydrogen-2 (deuterium): 1p⁺, 1n⁰. Hydrogen-3 (tritium): 1p⁺, 2n⁰.
• Chlorine-35 (17p⁺, 18n⁰) and chlorine-37 (17p⁺, 20n⁰), about three to one in nature, which is why the periodic-table mass is 35.45.

What the diagrams show about the periodic table

Line the 20 diagrams up as the periodic table does and two patterns appear. Down a group, each element has the same number of valence electrons (lithium 2, 1; sodium 2, 8, 1; potassium 2, 8, 8, 1), which is why a group shares its chemistry. Across a period, each element adds one electron to the same outer shell (sodium 1, magnesium 2, aluminium 3 ... argon 8), with the number of shells fixed. The diagram turns the periodic table from a list to be memorised into a picture of electrons filling shells.

The Lewis dot diagram beside it

Once the valence electrons are what matter, the Lewis dot diagram (electron dot diagram) drops everything else: the element's symbol stands for the nucleus and the inner shells, and dots around it show the valence electrons only, one on each side before pairing. Sodium: Na with one dot. Chlorine: Cl with seven dots (three pairs and a single). Carbon: C with four single dots. Lewis diagrams are the tool for drawing covalent bonds (shared pairs) from Grade 10 onward; the Bohr-Rutherford diagram is where the counts come from.

The mistakes that cost marks

• Putting electrons in the nucleus or between the circles. Electrons sit on the shell lines.
• Starting a new shell before the inner one is full, or putting 9 or 10 on the third shell (potassium and calcium start the fourth).
• Using the atomic number as the mass number (sodium is 11 protons and 12 neutrons, not 11 and 11).
• Forgetting to round the atomic mass (35.45 → 35 for chlorine).
• Changing the protons when drawing an ion. Only electrons move.
• Drawing shells for an ion as if it were the neutral atom (Na⁺ has two shells, not three).
• Pairing electrons before each of the four positions has one (where the teacher requires the spread).

Where it sits in each curriculum

Practice (answers in the PDF)

1. Draw the Bohr-Rutherford diagram for phosphorus (atomic number 15, mass number 31). State its number of valence electrons and its group.
2. How many neutrons are in an atom of potassium-39? Write its shell arrangement.
3. Draw the diagrams for a magnesium atom and a magnesium ion. What changes and what stays the same?
4. Oxygen and sulfur are both in group 16. Use their shell arrangements to explain why they form ions with the same charge.
5. Carbon-12 and carbon-13 are isotopes. Which part of the diagram differs?
6. Why does the simple 2, 8, 8 shell diagram stop at calcium?

Download the set and the practice sheet (PDF)

Where to get help

A 1-on-1 online chemistry class with a tutor who teaches Ontario Grade 9 and 10 science, GCSE and A-level chemistry, CBSE Class 9 and 10 and the Australian Years 9 and 10 every week: atomic structure taught by drawing the 20 atoms once, then the ions, then the bonds, so the periodic table becomes a picture rather than a list. The chemistry page lists the courses; the free 30-minute first class is a real lesson on this week's topic. Related pages: the trick for the first 30 elements and the 118 elements with symbols and atomic masses.

Questions parents ask

See how we teach this, 1-on-1 online →