An element.
A mineral
A substance that is composed of only one kind of matter is called an element. The smallest particle of an element is called an atom.
Usually, but not always.
If you mean what kind of element can exist as one atom, the answer is the noble gases in group 18. All elements are composed of one kind of atom, having a unique number of protons, or atomic number.
An element is composed of only one kind of atom. A compound is composed of atoms of more than one element.
An elemental molecule, like H2 or O2.
hydrogen
Water is a compound, composed of two atoms of hydrogen for every one atom of oxygen.
Yes. Diamond, for example, is composed entirely of carbon atoms.
An element is a substance that has one kind of atom.
Matter with only one particular kind of atom is known as an element. These atoms can either be located individually or combined into larger units. An example of this would be the notion that carbon is an element, because at it's pure substance state it's composed of only carbon atoms.
An atom has a nucleus composed of protons and usually neutrons as well (there is only one kind of atom which has no neutrons, which is the hydrogen 1 isotope) and it also has electrons surrounding the nucleus, with the same number of electrons as the number of protons in the neutrons.