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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.
The elements in a group of the periodic table have the same number of valence electrons in the same shapes of orbitals. In general, the number of valence electrons of an atom equals its group number. The valence electron periodic table is given below. For elements in Groups IIIA through VIIIA the outermost s and p electrons in the highest energy shell are valence electrons. 2  For transition elements the valence electrons are those in the outermost s sub-shell and in the d sub-shell of the next to outermost energy shell.