An atom can be large or small.You can find the size on the periodic table of the elements.
Here’s Everything You Need to Know About Them.)
You may be slightly more specific about which atom you think.Because the masses are important.
However, even the heaviest atom does not weigh enough to escape evaporation as a black hole.
Black holes that are so small we call a minutest black hole and they must be smaller than the Planck mass (about 22μg micrograms).
The largest atoms we observed, Organesson (UUO with number 118) has a molar mass of 492.05722 ± 0.00036 g/mol.
A mole of dust has exactly 6, 02214076e23 particles (constant of Avogadro).
So 492, 05722g/6,02214076 = 8, 17080224e-22 grams is the weight of 1 atom of the heaviest element we currently know.This is 22μg/8, 17080224e-22 grams = 2,692,514,070,904,000 times smaller than the Planck mass. Either 3, 714th-17 Planck Mass.
What happened is that this black hole actually evaporates immediately in radiation.You may be experiencing some of this radiation.
Suppose this doesn’t happen, the black hole persists and you try to hold it.What then?
There are several calculators online where you can calculate characteristics of a black hole.One of them we find here: Hawking radiation Calculator
If we fill in the previous data with the mass set in Planck mass we get for gravity: 3, 74317e67 m/s2 If we give it in gram, a similar high value of 3, 70316e67 m/s2 will look like.
Very small black holes are very hot (see also here: What are ‘ mini ‘ black holes?).There are several calculators online where you can calculate this. One of them we find here: Black Hole Temperature Calculator.
So answer your question “What would happen if I kept it in my hand”, fill in 1 with Unit U (1 Dalton- atomic mass Unit-Wikipedia).The temperature of a black hole with the mass of 1 carbon atom is: 73.925.588.686.361.734.578.865.477.491.718.104.22.1684.354.657.053 degrees Celcius.
How fireproof are your hands?