“Recipe for an atomic salad: protons, neutrons, electrons and croutons” is a joke that was printed in Boys’ Life in April 1987. “Crouton” sounds like it belongs with the other scientific names, but it does not.
 
“The atomic structure of salad is proton, neutron, crouton” was posted on Twitter by Mike Staresinic on November 8, 2017. “Which particle, when joined with a proton, neutron, and electron, makes the famous ‘Atomic Salad’? A crouton” was posted on Reddit—3amjokes on March 16, 2020.
 
       
Wikipedia: Crouton 
A crouton /ˈkruːtɒn/, colloquially referred to as a « crout », is a piece of sautéed or rebaked bread, often cubed and seasoned, that is used to add texture and flavor to salads—notably the Caesar salad—as an accompaniment to soups and stews, or eaten as a snack food.
 
Google Books
April 1987, Boys’ Life, “Think & Grin,” pg. 65, col. 2:
Recipe for an atomic salad: protons, neutrons, electrons and croutons.— Jamie Hardt, Shoreview, Minn. 
   
Google Books
Why do we gotta do this stuff, Mr. Nehring?:
Notes from a teacher’s day in school
By James Nehring
New York, NY: M. Evans
1989
Pg. 55:
First he taught about the proton by using a diagram of an atom highlighting the proton. Students listened and wrote. He did likewise with electron and neutron. Finally he showed a diagram with the “crouton,” explaining “tasty with salads and soups.” Students listened and wrote.
 
Google Groups: rec.music.hip-hop
Black Knights of North Star
Big E 2224
7/14/99
The Wu-Fam group Black Knights of the North Star is the best thing to come along since AZ’s last album.  For all those who are sleeping on this crew, WAKE UP!  They have the most spellbinding lyrics ever written.  Holocaust and Dr. Doom illuminate on the dope ass Bobby Digital track “Holocaust” and Holocaust is going to be dropping a solo album on us in Y2K.  I can’t wait, it’s going to be off the meter!  Don’t forget about the other members too!  Peep “Fatal Sting” on The Swarm CD, this crazy cat, I don’t know his name, but he comes out with some to infinity and beyond shit, sayin “protons neutrons… salad with the croutons.”  I was amazed enough by that, but just to read that line in print does not do it justice…you have to hear this guy’s delivery…it’s so tight!
   
Google Books
In the Secret Place of the Most High:
Expanded Second Edition
By Ephrain Tristan Ortiz
Bloomington, IN: iUniverse
2009
Pg. 71:
Well they magnified the atom all right, and found it to be made up of electrons going around a nucleus made up of protons and neutrons. (No, croutons go in salads.)
   
Twitter
K.O. Beatz Prime
@KOBeatzPrime
“supernova yoga”.. just terrible..but a classic smh. RT @sintexera @KOBeatz “the proton neutron…salad with a crouton, dip it in the Yukon”
1:26 PM · Sep 6, 2010·Twitter Web Client
   
Twitter
Casey
@happy_go_casey
“proton, neutron, electron and crouton” “what does that mean?”-mom “first 3 are science terms.. last one is the thing you put in a salad”
3:43 PM · Sep 28, 2014·Twitter for iPhone
   
Twitter
Kaleb
@DBeetees
Proton, neutron, electron, and crouton. My salad would be unstable without them.
7:01 PM · Apr 16, 2015·Twitter for iPhone
   
Twitter
Spencer Blair
@2much_su_spence
Only thing I learned in chemistry this year:
a crouton is not an ion like a proton, neutron, electron
it goes in salad
9:14 PM · Aug 26, 2015·Twitter for iPhone
 
Twitter
Mike Staresinic
@MikeStaresinic
The atomic structure of salad is proton, neutron, crouton.
12:51 PM · Nov 8, 2017·Twitter for iPhone
     
Twitter
Mike Staresinic
@MikeStaresinic
The salad atom is composed of the proton, the neutron, and the crouton.
2:21 PM · Feb 25, 2018·Twitter Web Client
 
Reddit—3amjokes
Posted by u/ohthatLSAT March 16, 2020
Which particle, when joined with a proton, neutron, and electron, makes the famous “Atomic Salad”?
A crouton.