CBSE Class 10 Science 2017 · Paper 31/3 · Question 9

Periodicity: when the elements are arranged in order of increasing atomic number, as in the Modern Periodic Table, elements with similar properties recur at regular intervals. This repetition of properties is called periodicity. It happens because the outer-shell electronic configuration repeats; the Modern Periodic Law states that the properties of elements are a periodic function of their atomic number. Similar properties in a group: the elements of a group have the same number of valence electrons (among the first twenty elements, helium in group 18 is the exception, with a full shell of 2). Chemical properties depend on the valence electrons, so the elements of a group have the same valency and similar properties. Tendency to gain electrons: it increases from left to right across a period (up to group 17). Reason: across a period the nuclear charge increases while the electrons are added to the same shell, so the atomic size decreases and the nucleus attracts electrons more strongly; atoms therefore gain electrons more readily.