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What is the largest atom (in terms of volume not atomic mass or weight)? 9/21/99 Vocabulary atomic radius effective nuclear charge lanthanide contraction pm valence electron valence shell Cesium (Cs), tucked in the lower left hand corner of the table, has the largest known atoms.The atomic radius of Cs is given variously as 273.1 pm [1], 265 pm [2], 265.5 pm [3] or 260 pm [4].ubidium also has large atoms, but its atomic radius is almost 30 pm less than cesium's. Why are cesium atoms so big? Cesium has a large valence shell and a relatively low effective nuclear charge. The valence shell (the outer 'peel' of the atom) is largest in atoms at the bottom of the periodic table.
Notice how the elements are arranged on the periodic table in increasing order of atomic number. There is, first of all, hydrogen--then to its right is helium. Below hydrogen is lithium--then to its right is beryllium, boron, and so forth on across with increasing atomic number. The elements on the periodic table are arranged so that their atomic numbers increase in order from left to right. There is a special name given to the horizontal rows on the periodic table. They are called periods. The first row, the one containing hydrogen and helium, is called the first period.