Potassium Breaks the Filling Rule for a Very Good Reason
Learn to find protons, neutrons and electrons from atomic number and mass number, apply the two-n-squared rule and the Bohr-Bury scheme to write electronic configurations up to atomic number twenty, and identify valence electrons.
Why is potassium written 2, 8, 8, 1 and not 2, 8, 9?
Because the outermost shell can never hold more than 8 electrons, whatever room the shell might seem to have.
Potassium has electrons. The M shell can in principle take , so after filling and you might expect the remaining to go in as . It does not happen. The M shell stops at while it is the outermost, and the nineteenth electron starts a brand-new N shell.
So the configuration is — and that single loosely held outer electron is why potassium behaves like sodium rather than like anything in between. This page covers the second part of the ICSE Class 8 Chemistry chapter on atomic structure: counting the particles in an atom, and arranging its electrons.
Potassium has electrons. The M shell can in principle take , so after filling and you might expect the remaining to go in as . It does not happen. The M shell stops at while it is the outermost, and the nineteenth electron starts a brand-new N shell.
So the configuration is — and that single loosely held outer electron is why potassium behaves like sodium rather than like anything in between. This page covers the second part of the ICSE Class 8 Chemistry chapter on atomic structure: counting the particles in an atom, and arranging its electrons.
Formula
How do you find the protons, neutrons and electrons in an atom?
Two numbers give you all three.
**Atomic number () is the number of protons** in the nucleus. In a neutral atom it also equals the number of electrons.
**Mass number () is the total number of protons and neutrons** — the nucleons.
So the neutrons follow by subtraction:
An atom is written as , with the mass number above and the atomic number below.
**Worked example 1 — sodium, .**
**Worked example 2 — chlorine, .**
**Worked example 3 — calcium, .**
Worked example 4 — working backwards. An atom has protons and neutrons. Then
and identifies it as oxygen, .
**The point that makes special. The atomic number alone fixes the identity of the element. Change the neutron count and you still have the same element — a heavier version of it, an isotope**. Change the proton count and it becomes a different element entirely. That is why the periodic table is ordered by and not by mass.
And the commonest slip. Neutrons are , never itself. Writing neutrons for sodium counts the protons twice.
**Atomic number () is the number of protons** in the nucleus. In a neutral atom it also equals the number of electrons.
**Mass number () is the total number of protons and neutrons** — the nucleons.
So the neutrons follow by subtraction:
An atom is written as , with the mass number above and the atomic number below.
**Worked example 1 — sodium, .**
**Worked example 2 — chlorine, .**
**Worked example 3 — calcium, .**
Worked example 4 — working backwards. An atom has protons and neutrons. Then
and identifies it as oxygen, .
**The point that makes special. The atomic number alone fixes the identity of the element. Change the neutron count and you still have the same element — a heavier version of it, an isotope**. Change the proton count and it becomes a different element entirely. That is why the periodic table is ordered by and not by mass.
And the commonest slip. Neutrons are , never itself. Writing neutrons for sodium counts the protons twice.
How many electrons fit in each shell?
The maximum a shell can hold is given by the ** rule**, where is the shell number.
Working it out shell by shell:
- K shell, :
- L shell, :
- M shell, :
- N shell, :
The Bohr-Bury scheme adds the rules for actually filling them:
- Shells are filled in order from the innermost outwards — K first, then L, then M.
- No shell may exceed its capacity.
- The outermost shell cannot hold more than 8 electrons.
- A new shell begins only once the previous outermost shell has reached .
Why the last two rules exist at all. They look like arbitrary extra conditions on top of , but they reflect a real difference: a shell's full capacity only becomes available once it is no longer the outermost one. Argon has with the M shell holding ; once a further shell exists outside it, the M shell can go on to take all .
**Worked example — potassium, .** Fill in order:
- K shell takes , leaving
- L shell takes , leaving
- M shell would take up to , but as the outermost shell it may take only , leaving
- The last electron starts the N shell
**Worked example — calcium, **, by the same route:
And a check that the total is right: , which equals . Always add your shells back up — it catches almost every configuration error.
Working it out shell by shell:
- K shell, :
- L shell, :
- M shell, :
- N shell, :
The Bohr-Bury scheme adds the rules for actually filling them:
- Shells are filled in order from the innermost outwards — K first, then L, then M.
- No shell may exceed its capacity.
- The outermost shell cannot hold more than 8 electrons.
- A new shell begins only once the previous outermost shell has reached .
Why the last two rules exist at all. They look like arbitrary extra conditions on top of , but they reflect a real difference: a shell's full capacity only becomes available once it is no longer the outermost one. Argon has with the M shell holding ; once a further shell exists outside it, the M shell can go on to take all .
**Worked example — potassium, .** Fill in order:
- K shell takes , leaving
- L shell takes , leaving
- M shell would take up to , but as the outermost shell it may take only , leaving
- The last electron starts the N shell
**Worked example — calcium, **, by the same route:
And a check that the total is right: , which equals . Always add your shells back up — it catches almost every configuration error.
What is the electronic configuration of the first twenty elements?
Applying the rules in order gives the whole list, and it is worth being able to produce it rather than recall it.
- Hydrogen, —
- Helium, —
- Lithium, —
- Beryllium, —
- Boron, —
- Carbon, —
- Nitrogen, —
- Oxygen, —
- Fluorine, —
- Neon, —
- Sodium, —
- Magnesium, —
- Aluminium, —
- Silicon, —
- Phosphorus, —
- Sulphur, —
- Chlorine, —
- Argon, —
- Potassium, —
- Calcium, —
The pattern in the columns. Read down the outermost numbers and a repetition appears. Lithium, sodium and potassium all end in **. Beryllium, magnesium and calcium all end in . Fluorine and chlorine both end in . Neon and argon both end in .
That repetition is the whole reason a periodic table is possible. Elements in the same column of the table have the same number of outermost electrons, which is why they behave chemically alike — and why sodium and potassium both react violently with water while neon and argon react with nothing.
Two configurations worth marking specially.** Helium with and neon with are complete — their outermost shells are full. So are argon's and, in effect, every noble gas. A complete outer shell means the atom has nothing to gain, lose or share, which is why these elements are inert.
**The boundary case at .** The jump from argon's to potassium's is where the outermost-shell rule becomes visible. Nothing else in the first twenty elements tests it, which is exactly why it is asked.
- Hydrogen, —
- Helium, —
- Lithium, —
- Beryllium, —
- Boron, —
- Carbon, —
- Nitrogen, —
- Oxygen, —
- Fluorine, —
- Neon, —
- Sodium, —
- Magnesium, —
- Aluminium, —
- Silicon, —
- Phosphorus, —
- Sulphur, —
- Chlorine, —
- Argon, —
- Potassium, —
- Calcium, —
The pattern in the columns. Read down the outermost numbers and a repetition appears. Lithium, sodium and potassium all end in **. Beryllium, magnesium and calcium all end in . Fluorine and chlorine both end in . Neon and argon both end in .
That repetition is the whole reason a periodic table is possible. Elements in the same column of the table have the same number of outermost electrons, which is why they behave chemically alike — and why sodium and potassium both react violently with water while neon and argon react with nothing.
Two configurations worth marking specially.** Helium with and neon with are complete — their outermost shells are full. So are argon's and, in effect, every noble gas. A complete outer shell means the atom has nothing to gain, lose or share, which is why these elements are inert.
**The boundary case at .** The jump from argon's to potassium's is where the outermost-shell rule becomes visible. Nothing else in the first twenty elements tests it, which is exactly why it is asked.
How do you identify the valence shell and valence electrons?
The valence shell is the outermost shell of an atom, and the valence electrons are the electrons in it.
Read them straight off the configuration — the valence shell is the last group and the valence electrons are its count.
- Sodium, — valence shell M, valence electrons 1
- Magnesium, — valence shell M, valence electrons 2
- Carbon, — valence shell L, valence electrons 4
- Oxygen, — valence shell L, valence electrons 6
- Chlorine, — valence shell M, valence electrons 7
- Neon, — valence shell L, valence electrons 8
- Potassium, — valence shell N, valence electrons 1
Why only these electrons matter. Valence electrons are the loosely held ones furthest from the nucleus, and they are the only ones an atom can lose, gain or share. Every chemical property of an element — how it reacts, what ion it forms, which compounds it makes — follows from this one number.
This is why sodium () and potassium () behave so similarly despite potassium having eight more electrons. Both have exactly one valence electron, and the rest of the atom is chemically irrelevant.
Two full shells and one exception. An outer shell holding 8 electrons is called a complete octet; helium's outer shell holding 2 is a complete duplet. Atoms with either are stable and unreactive.
The distinction to keep straight. The valence shell is named by a letter — K, L, M, N — while the valence electrons are a number. A question asking for one and answered with the other gets no mark, and both are asked.
Where this leads next. The count of valence electrons is what decides valency, whether the atom forms a cation or an anion, and therefore every formula it appears in. That is the subject of the third part of this chapter, and none of it can be done without being able to write a configuration first.
Read them straight off the configuration — the valence shell is the last group and the valence electrons are its count.
- Sodium, — valence shell M, valence electrons 1
- Magnesium, — valence shell M, valence electrons 2
- Carbon, — valence shell L, valence electrons 4
- Oxygen, — valence shell L, valence electrons 6
- Chlorine, — valence shell M, valence electrons 7
- Neon, — valence shell L, valence electrons 8
- Potassium, — valence shell N, valence electrons 1
Why only these electrons matter. Valence electrons are the loosely held ones furthest from the nucleus, and they are the only ones an atom can lose, gain or share. Every chemical property of an element — how it reacts, what ion it forms, which compounds it makes — follows from this one number.
This is why sodium () and potassium () behave so similarly despite potassium having eight more electrons. Both have exactly one valence electron, and the rest of the atom is chemically irrelevant.
Two full shells and one exception. An outer shell holding 8 electrons is called a complete octet; helium's outer shell holding 2 is a complete duplet. Atoms with either are stable and unreactive.
The distinction to keep straight. The valence shell is named by a letter — K, L, M, N — while the valence electrons are a number. A question asking for one and answered with the other gets no mark, and both are asked.
Where this leads next. The count of valence electrons is what decides valency, whether the atom forms a cation or an anion, and therefore every formula it appears in. That is the subject of the third part of this chapter, and none of it can be done without being able to write a configuration first.
Exam tip
Exam tip: add the shells back up to check
After writing any electronic configuration, add the numbers and check the total equals . Calcium as gives , so it is right. This one check catches almost every mistake in this chapter.
Remember the outermost shell never exceeds 8. Writing potassium as is the classic error, and the M shell reaching only happens once it is no longer outermost.
Compute neutrons as , never as . Show the subtraction: .
State the **number of electrons equals ** for a neutral atom, and say neutral when you use it — for an ion the electron count differs.
Quote with the substitution, not just the answer: the M shell is .
When asked for the valence shell, answer with a letter (K, L, M, N). When asked for valence electrons, answer with a number. Both appear, and they are different questions.
Write configurations with commas between shells — — so the shells are unambiguous.
And learn the noble-gas configurations as complete shells: helium is a duplet, neon and argon are octets. Questions about stability and inertness are answered from these.
Remember the outermost shell never exceeds 8. Writing potassium as is the classic error, and the M shell reaching only happens once it is no longer outermost.
Compute neutrons as , never as . Show the subtraction: .
State the **number of electrons equals ** for a neutral atom, and say neutral when you use it — for an ion the electron count differs.
Quote with the substitution, not just the answer: the M shell is .
When asked for the valence shell, answer with a letter (K, L, M, N). When asked for valence electrons, answer with a number. Both appear, and they are different questions.
Write configurations with commas between shells — — so the shells are unambiguous.
And learn the noble-gas configurations as complete shells: helium is a duplet, neon and argon are octets. Questions about stability and inertness are answered from these.
Did you know
Why do sodium and potassium behave almost identically?
Sodium has electrons and potassium has — eight more, a whole extra shell, and roughly two-thirds more mass. By any obvious measure they are different atoms.
Yet both are soft enough to cut with a knife, both are less dense than water, both react violently with cold water to give a hydroxide and hydrogen, and both form compounds with the same formulae: and , and .
The reason sits entirely in the last digit of their configurations. Sodium is and potassium is . Each has a single electron in its outermost shell, held loosely and easily given away — and that is the only electron either atom offers to chemistry.
Everything else about them is buried under complete inner shells that no reaction ever reaches.
It also explains the difference that does exist: potassium's outer electron is further from the nucleus and more shielded, so it is held even more weakly and given up even more readily. Potassium is therefore the more reactive of the two — same behaviour, turned up.
Yet both are soft enough to cut with a knife, both are less dense than water, both react violently with cold water to give a hydroxide and hydrogen, and both form compounds with the same formulae: and , and .
The reason sits entirely in the last digit of their configurations. Sodium is and potassium is . Each has a single electron in its outermost shell, held loosely and easily given away — and that is the only electron either atom offers to chemistry.
Everything else about them is buried under complete inner shells that no reaction ever reaches.
It also explains the difference that does exist: potassium's outer electron is further from the nucleus and more shielded, so it is held even more weakly and given up even more readily. Potassium is therefore the more reactive of the two — same behaviour, turned up.
Key takeaways
Atomic number, configuration and valence electrons: quick revision
- **Atomic number = protons = electrons in a neutral atom. Mass number = protons + neutrons. So neutrons **.
- Written : has protons, electrons, neutrons. has neutrons; has .
- alone fixes the element's identity — changing neutrons gives an isotope, changing protons gives a different element.
- ** rule** for shell capacity: K , L , M , N .
- Bohr-Bury scheme: fill inwards to outwards; never exceed ; the outermost shell holds at most 8; a new shell starts only after the previous outermost reaches .
- Hence potassium is and not ; calcium is . Always check the shells add to .
- Configurations to : ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; .
- The repeating outermost numbers are why a periodic table exists — same outer count means similar chemistry.
- Valence shell = outermost shell, named by a letter. Valence electrons = the count in it, a number.
- A full outer shell of is an octet and of is a duplet; helium, neon and argon have these and are inert.
- Only valence electrons take part in reactions, which is why sodium () and potassium () behave alike — with potassium more reactive, its outer electron being further out and more shielded.
Practise writing all twenty configurations from the rules alone and then finding the valence electrons for each — deriving them beats memorising, and the check of adding back to makes it self-correcting.
- Written : has protons, electrons, neutrons. has neutrons; has .
- alone fixes the element's identity — changing neutrons gives an isotope, changing protons gives a different element.
- ** rule** for shell capacity: K , L , M , N .
- Bohr-Bury scheme: fill inwards to outwards; never exceed ; the outermost shell holds at most 8; a new shell starts only after the previous outermost reaches .
- Hence potassium is and not ; calcium is . Always check the shells add to .
- Configurations to : ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; .
- The repeating outermost numbers are why a periodic table exists — same outer count means similar chemistry.
- Valence shell = outermost shell, named by a letter. Valence electrons = the count in it, a number.
- A full outer shell of is an octet and of is a duplet; helium, neon and argon have these and are inert.
- Only valence electrons take part in reactions, which is why sodium () and potassium () behave alike — with potassium more reactive, its outer electron being further out and more shielded.
Practise writing all twenty configurations from the rules alone and then finding the valence electrons for each — deriving them beats memorising, and the check of adding back to makes it self-correcting.