The statement that a helium nucleus contains no neutrons at all is incorrect. A helium nucleus, also known as an alpha particle, consists of two protons and two neutrons, giving it a mass number of four and an atomic number of two.
The atomic number of an element represents the number of protons in the nucleus. Since helium has an atomic number of two, it means it has two protons. The mass number, on the other hand, represents the total number of protons and neutrons in the nucleus. In the case of helium, the mass number is four, indicating that it contains two protons and two neutrons.
Neutrons are electrically neutral particles found in the nucleus of an atom. They contribute to the mass of the nucleus but do not affect the atomic number, as they have no charge. Helium, like other elements, can have different isotopes with varying numbers of neutrons, but the most common isotope, helium-4, has two neutrons.
In summary, a helium nucleus does contain neutrons, specifically two neutrons along with two protons, giving it a mass number of four and an atomic number of two.