Answer
The decreasing order of the atomic radius is,
Li$\gt$ Be$\gt$ B$\gt$C$\gt$N$\gt$O$\gt$F$\gt$ Ne.
Work Step by Step
The second-period elements are Li, Be, B, C, N, O, F, Ne with atomic numbers 3, 4, 5, 6, 7, 8, 9, 10 respectively. As the atomic number increases nuclear charge also increases. Here the electrons are added to the same shell, which is the second shell. The atomic radius decreases as the atomic number increases. As the nuclear charge increases electrons are firmly held by the nucleus and hence atomic radius decreases.
The decreasing order of atomic radius is,
Li$\gt$ Be$\gt$ B$\gt$C$\gt$N$\gt$O$\gt$F$\gt$ Ne.
