The antimony and beryllium ions are so small because atomic number of antimony and beryllium is 51 and 4. Beryllium tends to lose 2 electrons to form beryllium ions (Be2+) so it forms 4 coordinated complex ions, it does not form simple ions. Antimony gains 3 electrons to become stable or form an ion (Sb+3) so these ions are small.
Both ions and atoms tend to grow larger as we go down the periodic table because protons are added and atomic number increases. So new shells are also added. The new energy shells provide shielding effect so atomic size increases going down the group in the periodic table.
1. For this question, the adjective small must be percepted in a relative sense. This is because it is not the smallest ion (that would be hydrogen). It could be that the antimony and beryllium ions are smaller compared to their neutral forms. This is because they donate electrons when ionized. As a result, the electrons are reduced, so does the electron cloud which makes the radius much smaller.
2. The periodic table is arranged in terms of increasing atomic number. For neutral atoms, the number of protons (atomic number) is equal to the number of electrons. So, the farther we go down the table, the higher the atomic number. The higher the atomic number, the bigger the electron cloud which makes the atomic radius bigger. Because by definition, atomic radius is the length from the nucleus to the farthest electron from the nucleus.
To evaluate the accuracy of a measurement, the measured value must be compared to the correct value. To evaluate the precision of a measurement, you must compare the values of two or more repeated measurements.
