5. Periodic Classification of Elements Science class 10 in English Medium ncert book solutions Chapter Review

Chapter Review:

• Groups having three elements each. So he called these groups called triads.
• According to Dobereiner's Traids "The order of increasing atomic masses; the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements".
• According to Newlands’ Law of Octaves "Every eighth element repeat the properties of the first element".
• Newlands’ Law of Octaves worked well with lighter elements only.
• When Mendeleev started work on Periodic Table, oniy 63 elements were known.
• The Modern Periodic Table has 18 vertical columns known as ‘groups’
  and 7 horizontal rows known as ‘periods’.
• The Modern Periodic Law state that ‘Properties of elements are a periodic function of their atomic number.’
• The valency of an element is determined by the number of valence electrons present in the outermost shell of its atom.
• The atomic size may be visualised as the distance between the centre of the nucleus and the outermost shell of an isolated atom.
• The atomic radius decreases in moving from left to right along a period. This is due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.
• Boron, silicon, germanium, arsenic, antimony, tellurium and polonium are intermediate in properties and are called metalloids or semi   metals.