What elements (in the pure form) would harm a human, if eaten? What pure elements, if we eat it in a relatively small dose (around a piece of sugar), can be harmful/lethal for an average human? Is, for example, eating pure carbon bad for the organism?
Is, for example, eating pure carbon bad for the organism?


No, quite the contrary.

Activated charcoal (in the form of powder or tablets) was/is traditionally used to treat diarrea. It is listed on the WHO Model List of Essential Medicines on page 4.