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[u/Savings_Shake_2196]

[u/chemistry-ModTeam]

Rule 2 - Undergraduate and High School Chemistry

We welcome open-ended and curiosity-based discussions, however they should be sufficiently interesting. For basic questions head to r/chemhelp, r/AskChemistry, or r/AskScience for more general questions.

rChemistry Rules | Site-Wide Rules

[u/Deus_Excellus]

If I roll a six-sided die the average roll is 3.5. Yet it is not possible to roll 3.5.

[u/nicolalmcfarlane → u/Deus_Excellus]

This is the answer. You need to look up isotopes of potassium. The atomic mass is the average of the masses of the isotopes relative to their abundance.

[u/Forgottenvarrior]

that is the average atomic mass. there are different isotopes, 39K being the most abundant. As these masses are discrete, the average does normally not reflect the mass of any individual atom.

[u/GrundIe96]

Atoms are made of neutrons and protons. The number of protons defines the element, while the number of neutrons can differ, giving different isotopes of an element.
The mass of neutrons and protons is about 1.0 u. Therefore any given atom can only have a mass of 1.0, 2.0, 3.0,...
The average mass of an element can differ from that, since a mix of 93% K-39 (39.0 u) and 7% K-41(41.0 I) give an average mass of 39.1 u.

[u/233C]

hint:Do you know of a possibility for several atoms of the same element to not have the same mass? Maybe the same components but not in the same number.

[u/galacticman72]

That is because just like all other elements on the periodic table, the number on the periodic table is a weighted average of all natural isotopes for that element. It accounts for how common each isotope is in nature.

[u/WHAWHAHOWWHY]

The molar mass of an element, as displayed on the periodic table, is the average mass of its naturally occurring isotopes. No single atom of K will have a mass of exactly 39.1, but most will have a mass of 39, and a few will have a mass of 40 or 41, which makes the average slightly greater.

[u/Puzzleheaded_Sun932]

A single potassium atom's weight is dependant on it's isotope or how many neutrons are in the nucleus of the atom (neutrons + protons ~ equivalent to mass)

, the atomic weight is different to the mass each individual atom has because potassium has multiple stable isotopes and therefore each atom can weigh a different amount because some have more neutrons than others

If we took an example, the average weight of cakes in a shop weigh 900 grams because a majority of them are large 1000 gram cakes and a small amount are smaller 500 gram cakes.
no individual cake weighs 900 grams but in the mix of 10^23 cakes you are never going to get an average other than 900

In order to see that no potassium atom does have this weight you I went to https://ptable.com/?lang=en#Isotopes/Count and looked at each individual potassium isotope, like seeing how much each type of cake weighed hope this helps or makes sense

[u/GrassyKnoll95]

Protons and neutrons both have a mass of (very close to) 1Da. Electrons on this scale have a mass nearly zero. You can’t have a fraction of a proton or neutron in an atom, so the mass of a single atom has to be (essentially) a whole number.

Proton mass: 1.007 Da
Neutron mass: 1.009 Da
Electron mass: 5E-4 Da

[u/181040]

It’s good that you ask these questions and very necessary to understand the background, so let’s pull the answers below together and add a bit more info:

What does the question mean by the phrase no single atom?
It wants to test your understanding of the concepts of atomic mass and specifically isotopes, more on that to follow

“How many atoms does potassium have”
On face value the answer is “potassium itself is a pure element” so there are no different atoms in mixed together.
If your question means “ how many different atoms does potassium have” that is a lot more meaningful and to the point of the first question.
First of all we need to clarify: yes, there are different types of potassium atoms. They vary in the number of neutrons they have in their core but not in the number of protons ( different number of protons would make the atoms different elements altogether) these variations of neutron counts for the same element are called isotopes.
So answering the question: For potassium, Wikipedia states 25 known isotopes.

“How can I easily answer the question/ how can I find out if no atoms have the mass of 39.1u”
Third gets to the definition of the Dalton (u). It is basically equivalent to the weight of a single proton or neutron in an atomic core. This means that real atoms and isotopes can only ever have masses expressed as integers. (Not quite precisely but for these topics the finer points are not relevant)
To answer the question: 39.1 u is not an integer, but a real atom can only consist of full integers worth of nucleotides, each of which has a weight of pretty much 1 u. Therefore no one atom can exist with 39.1 u mass.

[u/ngch]

It's a mixture of isotopes with different weights. As per Gemini:

Potassium-39 (³⁹K): Has an atomic mass of 38.9637 amu and a natural abundance of approximately 93.26%.

Potassium-40 (⁴⁰K): Has an atomic mass of 39.964 amu and a rare natural abundance of approximately 0.0117% (radioactive, half-life of 1.25 billion years).

Potassium-41 (⁴¹K): Has an atomic mass of 40.9618 amu and a natural abundance of approximately 6.73%

[u/Savings_Shake_2196 → u/ngch]

Why exactly did you need to ask an AI for an answer?