Hello Jazakallah Khairyan.
Very nice to see you are trying to understand this most crucial verse inside Quran. I have done quite some research on this verse and I will share my understanding with you.
First, why is this verse so important? Well, let me apply Qurans logic...
If you are a believer this number should give you more certainty and remove doubt (give you proof of its divine origin). Furthermore, if you don't understand this verse or are still in doubt about its meaning, then by Qurans logic you are not a believer.
I've been truly flabbergasted by the lack of understanding of this verse in traditional tafsir. Muslim scholars don't understand the meaning of this verse and if you apply its logic you will find there are very few true believers around.
Now let's go to understanding the meaning of this verse.
The expression "over it is 19" is highly uncommon in the Quran. It expresses an amount, a number without specifying the subject. The number 19 is used as a subject itself. This is a first hint towards its meaning.
Let's look at context.
The preceding verses speak of hell. We find a description of hell. Quran describes hell as a place where people are continuously burned. Black pillars of smoke escaping from it. The malaika (I do not translate malaika as angels and will explain why later in this post) are the keepers of the fire.
We can compare this description of hell with the mundane example of a crematorium. When a human corpse is burned inside a crematorium, the carbon leaves as black smoke (oxidization process). Inside the human ashes the biggest remainder is potassium. The atomic index number of potassium is 19 because of its 19 protons/electrons.
Now, if this is true why would Quran use the atomic index number instead of its known name al-qalyah?
There is a good reason for this. Suppose the quran would say: "over it is kalium (al-qalyah)", would it still be a proof to believers and disturb the disbelievers? No it would not. In contrary it would be proof to the disbelievers and give doubt to the believers. Why? Because not until the 19th century was humankind able to distinct potassium from sodium. If the quran would state "kalium" or "al-qalya", sceptics would be able to say the Quran is wrong. Because "kalium" was until the 19th century used for both sodium and potassium.
Since there was no accurate word for potassium available at the time of writing, God used the atomic index number to describe the substance which is covering hell.
But wait a minute... Quran says this number is the number of Angels who are guarding hell.
This is true. But let's take a look at the translation of malaika to angels. Is this correct? The word angel literally means "messenger" from latin "angelus". The classical arabic word malaika is often used in the quran. Derivatives of its triliteral root "M-L-K" are also used to describe kings and rulers (those who control, are in a position of power).
I'm not here to give you a lesson in physics, but one has to be aware how this reality manifests itself. Chemical elements are defined by the electron/proton count. How matter reacts, what properties it has is defined by the amount of protons and electrons inside the atom. If you remove one electron from an atom, or add one to an atom, the chemical element is no longer the same, it becomes another chemical element with different properties. In short, electrons control and define material reality.
Taking you deeper down the rabbit hole...
Electrons when they were first described by Niels Bohr are very peculiar. They can pop in and out of existence. They can move faster than light. They can even be at two places at once. This is described in quantum mechanics. Niels Bohr believed this was unique, unheared of before but it was not. Some philosopher (Adler) talked about this with Niels Bohr and noticed these magical properties were described before: to be precise, by St. Thomas Aquinas, wrote about instantaneous movement of angels some 650 years before Bohr’s atomic theory.
I'll stop my theory about malaika here and bring it back to the topic of the verse you mentioned and test it's validity using the Quran itself.
Does this bring a trial for those who disbelieve? Certainly, it's quite disturbing for a disbeliever to find a 1400 year old book is showing knowledge of the electron count of potassium. No man, knew this proton/electron count until the 20th century. Leaving only divine explanations available.
Does this bring certainty to those who disbelieve? For the same argument as above, this confirms to the believer this can't possibly have been known by any man. Therefor confirming the divine origin of the Quranic text.
Does this convince the people of the book? No it does not. The people of the book didn't know the electron count of potassium.
Then how did it convince the people of the book?
Well, the answer is that the jews have 18 prayers (Shemoneh Esrei). There is a 19th prayer however, used to curse the disbelievers. As such, the people of the book would have recognized the 19 as confirmation this is coming from their God. Especially since it is used in context of hell.
Still not convinced by the 19 meaning potassium?
Then look at the evidence inside Quran in the descriptions of Hell. I'll give you a few, the rest of the homework is for you.
"And the companions of the left – what are the companions of the left? (They will be) in scorching fire and scalding water and a shade of black smoke, neither cool nor beneficial." (Quran 56:41-44)"
This describes the carbon leaving in an oxidization process.
"No food will there be for them except from a bitter, thorny plant which neither nourishes nor avails against hunger." (Quran 88:6-7)The taste of potassium is bitter. It's a salt. At the same time it's a fertilizer allowing things to grow on it.
"So no friend has he here this Day, nor has he any food except filth from the washing of wounds which none do eat but those in sin." (Quran 69:35-37)Washing of wounds with potassium soap. The oldest use of potash in history has been soap.
In some descriptions of hell, the destruction of sodom and gommorah is used as an example of hell. Sodom and gomorrah were most likely situated near the dead sea. Still to this day, there is a potassium plant there, extracting potassium from the dead sea.