Of the following elements, the one with the smallest atom is : Rb, K, Li, or F. Why?

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Of these elements, the atoms of lithium have the smallest mass but the atoms of fluorine are the smallest in diameter. This is due to a periodic trend called effective nuclear charge. Within a period, which is a row on the periodic table, atomic size decreases as the atomic number increases. The effective nuclear charge is the positive charge experienced by the outer electrons in an atom. It increases moving left to right within a period, resulting in the electrons being pulled in closer and the atom being smaller. Fluorine is in the same period as lithium but to the right of lithium so it's smaller. Potassium and rubidium are both larger than lithium because atomic size increases going down in a group.

The reason for the effective nuclear charge trend has to do with inner electrons that shield the outer electrons from the nuclear charge. Going left to right within a period each element adds one proton and one electron, but the amount of shielding by inner electrons stays the same. This means that more outer electrons are shielded by the same number of inner electrons as atomic number increases, so each electron experiences a smaller force of repulsion and is therefore more attracted to the nucleus.

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The smallest atom you have listed would be lithium (Li).  Lithium is the third smallest atom listed on the periodic table of the elements.  It has an atomic number of three, meaning it has three protons in it's nucleus.  It has a mass number of six point nine atomic mass units, meaning it also has three neutrons in it's nucleus.  Lithium has two electrons orbiting the nucleus in it's first energy level, and one third electron in the outer energy level.

The other elements, in order of increasing atomic number would be fluorine (F), atomic number nine, potassium (K), atomic number nineteen, and rubidium, atomic number thirty-seven.  With increasing atomic number, it is reasonable to expect increasing mass number as well.  The corresponding mass numbers for those elements, in increasing mass number order, are fluorine, nineteen, potassium, thirty-nine point one, and rubidium, eighty-five point five.

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