why does helium have 2 valence electrons

The octet rule states that atoms with eight valence electrons are most stable. The noble gases are the only non-transition-metal elements that have eight valence electrons in their neutral, ground state configuration, but atoms of other elements can produce a full octet -- an outer shell with eight electrons -- by gaining or losing electrons. Oxygen, for example, can gain two electrons to have a full octet, and potassium can lose one electron to have a full octet. These atoms with extra or fewer electrons, however, are known as ions, or charged particles. Because of this gain or loss of electrons, ions are more reactive than noble gases, which is why noble gases generally appear by themselves, rather than combined with other atoms to make chemical compounds.
Of course, things can t be that easy.

Because the electron configurations get more and more complicated as more electrons are added, the line between metal and nonmetal is not straight by the families. Near the carbon family, you will see a staircase written on some periodic tables. Elements to the left of the staircase are considered metals, those to the right are considered nonmetals.

Those that touch the staircase are the metalloids. As you read across the periodic table, metallic and nonmetallic properties become more obvious. The further left you are on the table, the more metallic the elements. The further right you are on the table, the more nonmetallic the elements. What about up and down? The most metallic family is the alkali metals. These elements most easily give up their one valence electron. Which would give up its electron easier, lithium or francium? Since the valence electron on francium is on the seventh energy level, it is a very long distance from the nucleus.

Also, there are 86 other electrons on the inner shells repelling the valence electron. Because of that, francium will lose its electron easiest. Since we have defined metals as those elements that lose electrons, francium is the most metallic element. As you go down the periodic table, elements become more metallic. Since nonmetallic traits are the opposite of metallic traits, as you go up the periodic table, elements become more nonmetallic. The most nonmetallic element is fluorine since it is furthest up and to the right (excluding the noble gases).

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