Describe the trends in the atomic size of elements within groups and across periods in the periodic table. Provide examples.
Answer: Atomic size increases with increasing atomic number within a group. For example, sodium atoms are larger than lithium atoms, and potassium atoms are larger than sodium atoms. Atomic size decreases with increasing atomic number across a period. For example, lithium atoms are larger than beryllium atoms, and beryllium atoms are larger than boron atoms.