An isotope refers to elements with the same atomic number but different numbers of neutrons.

Hydrogen is the most abundant element in the universe.

Transition elements play an important role in chemical reactions.

Rare earth elements are essential materials for the manufacture of electronic devices.

Oxygen, which is one of the chemical elements, is necessary for us to breathe.

The atomic number of helium is 2.

Our opinions differ.

He pointed at the location on the map.

Radioactive elements are produced in nature and scientific experiments.

A neutron is part of an atomic nucleus.