(a) What is a period in a modern periodic table? How do atomic structures (electronic arrangements) change in a period with increase in atomic numbers from left to right? (b) How do the following change on going from left to right in a period of the periodic table? i.Chemical reactivity of elements. ii.Nature of oxides of elements. Give examples in support of your answer.
Hint: Modern periodic law is defined as the physical and the chemical trait of each element are the periodic functions of its atomic number. In the modern periodic table, the elements are arranged in a well-defined manner. The horizontal rows are termed as periods and vertical columns are termed as groups.
Complete answer: (a) Period: In the modern periodic table, the horizontal rows are called periods. All elements present in a period will have the same number of electron shells. In a period, each next element exceeds by one proton and therefore has less metallic character than its precursor. On moving left to right in a period, atomic number increases that indicates the increase in number of electrons and protons in an atom. As in a period, elements have the same electron shells, so an increase in the number of electrons causes an increase in the effective nuclear charge. Therefore, the atomic size of the elements decreases in a period. (b) (i) For metals, the chemical reactivity decreases when you move left to right in a period. For example, sodium, magnesium and aluminium metals are present in the third group. All the three metals are electropositive in nature and are in need of donated electrons to gain stability. Because, sodium metals require to donate a single electron while magnesium and aluminium donates two and three electrons respectively. Therefore, the sodium metal is the most reactive metal in a third period. Hence it concludes that reactivity of metals decreases on moving left to right in a period. For non-metals, the chemical reactivity increases when you move left to right in a period. For example: In the third period, phosphorus, sulphur and chlorine are non-metals and do not have a tendency to lose electrons but they can accept electrons. Because, chlorine needs one electron to reach noble gas configuration whereas phosphorus and sulphur needs three and two electrons respectively. Therefore, the chlorine atom is the most reactive nonmetal in the third period. Thus, it concludes that reactivity of non-metals increases on moving left to right in a period. (ii) As we move from left to right in a period, the acidic nature of oxides increases whereas the basic nature of oxides decreases. For example: Consider elements from the third period of the modern periodic table. Sodium metal is the most electropositive element in the third period therefore forms the most basic oxide while aluminium and silicon forms amphoteric oxides and sulphur and chlorine atoms form acidic oxide.
Note: Amphoteric oxides: It is an oxide that can show the properties of both acidic as well as basic oxides. It depends on the oxidation state of a chemical molecule. Some examples of amphoteric oxides are aluminium oxide and zinc oxide.