Can't be gravitational, due to the fact that atoms are way too small to have a gravitational pull for something of similar size.
The strong nuclear force is the answer you're looking for.
A 'red' apple does not emit red light. Rather, it simply absorbs all the frequencies of visible light shining on it EXCEPT for a group of frequencies that is perceived as red, which are REFLECTED. An apple looks red to us because it REFLECTS infrared waves.
The nucleus is an organelle found in eukaryotic cells. Inside its fully enclosed nuclear membrane, it contains the majority of the cell's genetic material. This material is organized as DNA molecules, along with a variety of proteins, to form chromosomes.
