# Helium Atomic Mass

1. The known isotopes of helium contain from one to six neutrons, so their mass numbers range from three to eight. Of these six isotopes, only those with mass numbers of three (helium-3, or 3 He) and four (helium-4, or 4 He) are stable; all the others are radioactive, decaying very rapidly into other substances.
2. Atomic Mass of Helium Atomic mass of Helium is 4.0026 u. Note that, each element may contain more isotopes, therefore this resulting atomic mass is calculated from naturally-occuring isotopes and their abundance. The unit of measure for mass is the atomic mass unit (amu).
3. Helium's first ionization energy is −36(25) eV. This value was derived by experiment. The theoretic value of Helium atom's second ionization energy is −1(2) eV. The total ground state energy of the helium atom is −42(40) eV, or −2.90338583(13) Atomic units a.u., which equals −5.80677166 (26) Ry.

Helium, the lightest of the noble gases, had actually been detected and helium is the only element in the periodic table that was discovered by an astronomer. Helium is the element which you can find on the upper right side of the periodic table with atomic number 2.

### What is the atomic number for helium?

The nucleus of the helium atom contains 2 massive, positively charged particles. Thus, $Z$, the atomic number$=$$2$.

#### Explanation:

$Z$ is simply the number of protons, massive, positively charged particles, present within the nucleus. $Z$ thus defines the identity of the element: $Z = 1$, hydrogen; $Z = 2$, helium; $Z = 3$, lithium;.........$Z = 80$, mercury.  You do not have to remember these numbers, because in every exam you ever sit you should be provided a copy of the Periodic Table, which lists all the elements in order of atomic number. Of course, you still have to be able to use the table. Imessage for pc.

## What Is Helium Atomic Mass

Given the examples, $Z = 1$, hydrogen; $Z = 2$, helium; $Z = 3$, lithium;.........$Z = 80$, mercury, how many electrons does each neutral element contain? Why?