The chemical element with eight protons and nine neutrons is oxygen, which is denoted by the symbol 17O. It is the least naturally occurring isotope of oxygen.
The atom is the smallest unit that is able to retain all the properties of a chemical element. It contains three primary subatomic particles, including protons, neutrons and electrons. In the periodic table, an element is represented by its atomic symbol, atomic number and mass number. The atoms of different chemical elements possess a unique atomic number, which is equivalent to the number of protons present in the atomic nucleus. In a neutral atom, the atomic number is also equal to the number of electrons. The mass number, meanwhile, indicates the sum of the protons and neutrons in the nucleus. However, the mass number may vary between atoms of an element due to the existence of isotopes, which are different forms of the same element.
Oxygen has the chemical symbol O and atomic number eight. The element's most abundant isotope has a mass number of 16, containing eight protons and eight neutrons in its nucleus and denoted by the symbol 16O. Another isotope of oxygen, represented by 18O, has eight protons and 10 neutrons. The isotope 17O is the least abundant form of oxygen, which has a percent abundance of approximately 0.037 percent.