Periodic Classification of Elements

18

Step 1: The modern periodic table has 18 vertical columns called groups. Step 2: Groups are numbered from 1 to 18 using Arabic numerals. Step 3: Elements in the same group have the same number of valence electrons. Step 4: This is different from Mendeleev's table which had 8 groups with subgroups A and B.

First period

Step 1: The first period contains only hydrogen (H) and helium (He). Step 2: This is the shortest period in the periodic table. Step 3: Second and third periods have 8 elements each (short periods). Step 4: Fourth and fifth periods have 18 elements each (long periods).

Decreases

Step 1: As we move left to right in a period, nuclear charge increases. Step 2: More protons in the nucleus attract electrons more strongly. Step 3: Electrons are pulled closer to the nucleus, reducing atomic size. Step 4: This trend is observed in all periods, for example, Li > Be > B > C in period 2.

Group 1

Step 1: Group 1 elements (except hydrogen) are called alkali metals. Step 2: These include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). Step 3: They all have one valence electron and are highly reactive. Step 4: Group 2 has alkaline earth metals, Group 17 has halogens, and Group 18 has noble gases.