What is the atomic number of iron?

Hydrogen (H) and helium (He) are the exceptions to the octet rule. These two elements are perfectly happy with only two electrons in their outer energy levels.

For example, the element lithium has three protons in its nucleus. These positively charged protons are separated and stabilized by either three or four neutrons. Depending on whether the atom has three or four neutrons, it will weigh different amounts. These two atoms—with either three or four neutrons—are isotopes of lithium.

Atoms like to be stable. Stability requires less energy. As with everything in the nature, atoms try to do the most work with the least amount of effort. A stable atom has a complete outer energy level. The electrons in the atom’s outer energy level are called valence electrons. The number of valence electrons determines the properties of the element. The octet rule in chemistry says that atoms are most stable when they have full outer energy levels, complete with eight electrons. Sometimes, an atom will have more or less than eight valence electrons. They will try to gain or lose valence electrons in order to reach their goal of eight. They can gain or lose electrons by bonding with other atoms, either of the same element or with different elements. KEY QUESTIONS

• What are the main parts of an atom? What role does each part have? • Which elements do you come in contact with on a daily basis? What do you use them for? • Why is the periodic table useful? Would you group the elements in different ways?