Question

In the modern periodic table, arrange the third-row elements Na, Mg, Al and Si in the increasing order of their atomic size.
(A) $Na>Mg>Al>Si$
(B) $Al>Si>Na>Mg$
(C) $Si>Al>Mg>Na$
(D) $Na>Al>Si>Mg$

Answer 1

The long form of the periodic table or the modern periodic table is the current form of the periodic table. It consists of a total of 118 elements. We can predict the properties of an element belonging to a particular group because of the periodicity of properties.

Complete step by step solution:
Before solving this question, we first need to know about the modern periodic table. In modern periodic tables, the elements are arranged in increasing order of their atomic numbers. The modern periodic table has 18 vertical columns and 7 horizontal rows.
Properties of the Groups in the Modern Periodic Table
They are the vertical columns in the long form of the periodic table and the total number of these vertical columns is 18, with the first group denoted as Group 1. For each group, the elements belonging to a particular group will have the same type of outer shell electronic configuration where for every element the principal quantum number will increase as we move down the group. As we go down a group there are certainly general trends such as the electronegativity will decrease, the atomic radii will increase, the electropositive character will increase, and ionization energy will decrease.
Properties of the Periods in the Modern Periodic Table
They are the horizontal rows in the long form of the periodic table and the total number of these horizontal rows is 7. The first period only has 2 elements (Hydrogen and Helium), the second and third period have 8 elements, the fourth and the fifth period have 18 elements while the sixth and the seventh period have 32 elements. With each successive period, the principal quantum number increases. As we move across a period there are certainly general trends such as the electronegativity will increase, the atomic radii will decrease, the electropositive character will decrease, and ionization energy will increase.
Now let us solve the question. Since, the atomic radii decrease as we move along a period due to an increase in the effective nuclear charge, the increasing order of the atomic size of Na, Mg, Al and Si will be: $Na>Mg>Al>Si$

Hence the correct answer is (A) $Na>Mg>Al>Si$.

Note: Due to the periodicity of the properties, the unknown properties of an element can be predicted but they are not always true. Certain exceptions within a group do exist. For example, the electron affinity trend of group 17 where the electron affinity of fluorine is less than that of Chlorine.