HELIUM, THE FIRST ATOM WITH A FILLED ELECTRON SHELL

HELIUM, THE FIRST ATOM WITH A FILLED ELECTRON SHELL 

In Section 3.4, it was mentioned that the electron configurations of atoms determine their chemical behavior. Electrons in atoms occupy distinct energy levels. 

A helium atom has a filled electron shell containing 2 electrons, 

At this point, it is useful to introduce the concept of the electron shell to help explain electron energy levels and their influence on chemical behavior. Each electron shell can hold a maximum number of electrons. An atom with a filled electron shell is especially content in a chemical sense, with little or no tendency to lose, gain, or share electrons. Elements with these characteristics exist as gas-phase atoms and are called noble gases. The high stability of the noble gas electron configuration is explained in more detail in Section 3.9. 

Examined in order of increasing atomic number, the first element consisting of atoms with filled electron shells is helium. He, atomic number 2. All helium atoms contain 2 protons and 2 electrons. Virtually all helium atoms are He, containing 2 neutrons in their nuclei the jHe isotope containing only 1 neutron in its nucleus occurs to a very small extent. The atomic mass of helium is 4.00260. 

---

Section 3.7. Helium, the First Atom with a Filled Electron Shell... 

---