Bohr Model Diagram: How to Draw an Atom's Electron Shells

What a Bohr diagram shows

According to the Royal Society of Chemistry (RSC), an electron configuration diagram is a model of where the electrons are around the nucleus. You draw the nucleus as a small circle with the element’s symbol inside. Around it go concentric circles, the shells, and each shell gets a dot or a cross for every electron it holds. The same drawing sometimes goes by the name Bohr model, or “solar system” model.

The name comes from Niels Bohr, who in 1913 was the first to propose electrons arranged in fixed shells. Why he got there, and how the idea accounted for the light hydrogen gives off, is covered in the Recap on the Bohr model of the atom and its spectral lines. This page is practical: pick an element and draw it.

A word on vocabulary. In school chemistry “electron shell” and “energy level” mean the same thing, and the RSC advises picking one and sticking with it inside a single explanation.

Step one: count the electrons from the atomic number

The atomic number is the number of protons in the nucleus. A neutral atom has as many electrons as protons, so the same number also tells you how many dots to draw in total: potassium, atomic number 19, gets 19.

An atom that gains or loses electrons becomes an ion, and the total changes. Every example below uses neutral atoms.

How to fill the shells

There is one basic rule. Each electron goes into the lowest shell that still has space, the one closest to the nucleus, and you only move outward once that shell is full. Energy is lowest in the first shell and rises the farther a shell sits from the nucleus.

How many electrons fit in a shell? In the version of the model the RSC uses with students aged 14 to 16, the first shell holds at most 2, the second 8 and the third 8. Calcium, element 20, puts its last two electrons in the fourth shell; potassium, just before it, puts only one there.

To avoid drawing every time, there is a shorthand: the number of electrons in each shell, innermost first, separated by commas or dots. Nitrogen is written 2,5 and calcium 2,8,8,2.

The RSC designed its method for the first 20 elements of the periodic table. Chemguide also recommends building up the electron structures of those 20 elements one energy level at a time, on a diagram with the levels drawn as stairs.

How to draw the Bohr model of carbon

Practical example: carbon has atomic number 6, so a neutral atom has 6 electrons. Draw a small circle with a C in it. The first shell takes 2 electrons and is full; the remaining 4 go on the second circle, which could hold as many as 8. In shorthand, carbon is 2,4.

You can check the count against the configuration PubChem lists for carbon, [He] 2s² 2p². The [He] stands for the 2 electrons of the first level, the ones helium has, and 2s² and 2p² are the 4 in the second.

Neon: a full second shell

Practical example: neon has atomic number 10. Two electrons go in the first shell and eight in the second: 2,8. The second circle is full, and nothing is left over for a third.

HyperPhysics calls this complete shell the “stable octet”, and neon is a noble gas. Helium shows the same thing one step earlier: its 2 electrons fill the first level, and it too is a chemically inert noble gas. Fluorine, one electron short of an octet, tends to pick one up. Sodium has one too many and tends to give it away, and both end up with neon’s arrangement.

How to draw the Bohr model of aluminum

Practical example: aluminum has atomic number 13. The first shell takes 2 electrons, the second takes 8, and 3 are left for the third: 2,8,3. The drawing has three circles, with 3 dots on the outermost one.

PubChem gives aluminum the configuration [Ne] 3s² 3p¹. The [Ne] block is neon’s 10 electrons, the third level adds 3 more, and the total agrees with the count made using the rule.

The same procedure works for every element up to calcium. In the table below, the configuration comes from PubChem; the electrons-per-shell column was worked out here by applying the rule, and for nitrogen and calcium it matches the figures printed by the RSC.

Element	Atomic number	Configuration (PubChem)	Electrons per shell
Hydrogen (H)	1	1s¹	1
Helium (He)	2	1s²	2
Carbon (C)	6	[He] 2s² 2p²	2,4
Nitrogen (N)	7	[He] 2s² 2p³	2,5
Oxygen (O)	8	[He] 2s² 2p⁴	2,6
Neon (Ne)	10	[He] 2s² 2p⁶	2,8
Sodium (Na)	11	[Ne] 3s¹	2,8,1
Magnesium (Mg)	12	[Ne] 3s²	2,8,2
Aluminum (Al)	13	[Ne] 3s² 3p¹	2,8,3
Chlorine (Cl)	17	[Ne] 3s² 3p⁵	2,8,7
Argon (Ar)	18	[Ne] 3s² 3p⁶	2,8,8
Potassium (K)	19	[Ar] 4s¹	2,8,8,1
Calcium (Ca)	20	[Ar] 4s²	2,8,8,2

What a finished diagram tells you

A finished diagram also shows where the element sits. According to the RSC, the number of shells gives the period, meaning the row of the periodic table, while the electrons in the outer shell point to the group. Aluminum, with its three shells, belongs to period 3. Group 1 elements all have a single electron in the outer shell; in the table above, that applies to hydrogen, sodium and potassium.

The electrons in the outer shell are called valence electrons. The LibreTexts module notes that those in full inner shells don’t contribute to an element’s chemical properties. Sodium, for example, has one electron outside a closed shell, and HyperPhysics puts the energy needed to remove it at 5.14 electron volts; chlorine is one electron short of a full shell and releases 3.62 electron volts when it gains one. That is how the Na+ and Cl- ions form, and their mutual attraction is the ionic bond.

From Bohr diagram to Lewis symbol

Keep only the outer-shell electrons of a Bohr diagram and what remains is the Lewis symbol, which LibreTexts describes as a simplified version of it. The complete inner shells vanish into the element’s symbol, and the valence dots are arranged around it in four groups of one or two. Eight dots mean a closed shell, the configuration of a noble gas.

Practical example: the aluminum diagram has three circles and 13 dots. Its Lewis symbol is Al with 3 dots around it, the electrons from the third shell.

	Bohr diagram	Lewis symbol
Nucleus	Small circle with the symbol	Just the symbol
Inner shells	Drawn, with all their dots	Absorbed into the symbol
Outer shell	A circle with its dots	Dots in four groups of one or two
What it’s for	Seeing how the atom is built up	Keeping track of valence electrons

What the circles stand for

Chemguide flatly rejects the planetary atom of popular illustrations, the one with electrons wheeling around the nucleus. The circle diagram stays useful as long as you understand that each circle is an energy level and that electrons do not move along it. The same sodium diagram can be redrawn with its levels straightened out into a row of steps, and the sense of motion disappears.

How electrons actually move inside an atom is unknown, and the theory Chemguide relies on says it cannot be known. Put simply, Heisenberg’s uncertainty principle means nobody can pin down an electron’s position and its next move with certainty at once, which leaves no orbit to draw. The RSC warns that students may picture exactly this miniature solar system, or imagine shells as rigid structures that electrons sit in.

HyperPhysics accepts that the “planetary” model describes electrons poorly, yet still finds it a good aid for following how heavier atoms are built up. According to the official Nobel biography, published in 1965, the elements’ physical and chemical properties could still be explained with Bohr’s picture, after improvements that came mainly from Heisenberg’s ideas in 1925.

Where the 2, 8, 8 rule stops

In the more advanced model each shell splits into subshells, which in turn are made of orbitals. An orbital is a region of space where an electron is likely to be found (Chemguide sets the cutoff at a 95% probability), and it holds at most two electrons, with opposite spins.

The first level has 1 orbital, the second 4 and the third 9: one 3s, three 3p and five 3d. Doing the arithmetic, nine orbitals of two electrons each give the third shell room for 18, far more than the 8 allowed by the school rule. Potassium and calcium still put electrons in the fourth shell because, in these two elements, the 3d orbitals are higher in energy than 4s, which therefore fills first. Scandium, atomic number 21, is where the two swap places in energy, and one of its electrons ends up in a 3d orbital. Its structure is 2,8,9,2, where the school rule would have predicted 2,8,8,3. In period 4, HyperPhysics explains, ten elements known as transition elements slot in between 4s and 4p, because the d subshell takes 10 electrons.

For this reason the RSC recommends presenting the 2,8,8,2 scheme as exactly what it is, a model and a rule of thumb, useful but limited to the surface of what goes on inside the atom.