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In the diagram of ATP below this paragraph there are no lines between the atoms in the $\ce{NH2}$ molecule or in the $\ce{OH}$ molecules; however, there are lines between all the other atoms. Does the omission of those lines represent some facts about $\ce{NH2}$ and $\ce{OH}$ in the ATP molecule, or did the illustrator omit them in order to emphasize the rest of the molecule?
Your example is one of a variety of shorthands that exist because every chemist after the third year of studies knows what is meant so they are too lazy to draw.
In a nutshell, the $\ce{NH2}$ group will always be a nitrogen with a lone pair, two single bonds (one to each hydrogen) and one bond to the molecular skeleton. Much like $\ce{-OH}$ will always be $\ce{-O-H}$. It is also common practice to omit any insignificant hydrogen atoms in organic molecules; in the image above, a total of eight hydrogens has been omitted because every chemist will know where they are (and that they are there).
In principle, the phosphate chain could also be shortened to $\ce{HO3P-PO2H-PO2H-{}}$ rather than writing out all of it. Any chemist will mentally ‘expand’ the molecule in the same (correct) way.
As you continue to study, you will notice when which groups are generally shortened, which rules to observe while doing so and which hydrogens are so superfluous that they are not drawn — and, most importantly maybe, how atoms are labelled that actually are missing hydrogens. it is beyond the scope of a single answer to note them all.
Essentially scientists have better things to do than draw (an look at) every single atom. This is a skeletal diagram of the molecule. It is meant as a quick and efficient way to convey the structure of the molecule. In fact if you look closely you'll notice that hydrogens bonded to carbons are not even shown. This is done because chemists know they are there, but showing them would take time, clutter the molecule and obscure the functional groups that are more important to the properties of the molecule. The $\ce{OH}$ and $\ce{NH2}$ groups lack lines because they are functional groups and again scientists know but have better things to do. Occasionally functional groups get even more short-handed. the phenyl group which is a bonded benzene ring is often abbreviated $\ce{Ph}$ to conserve space on a paper and save time. Hydrogen atoms will often be shown if they are bonded to a chiral center, but even they may be omitted and assumes to be opposite a show stereo group.
