1. Group
Ans: The eighteen vertical columns in the modem periodic table are called as groups.
2. Period
Ans: The seven horizontal rows in the modem periodic table are called as periods.
3. Atomic radius
Ans: For an isolated atom, the distance between the centre of the nucleus of the atom and the outermost shell is called its atomic radius.
4. Electropositivity
Ans: The tendency of an atom of an element to form cation by losing its valence electrons is called electropositivity of that element.
5. Electronegativity
Ans: The tendency of an element to form an anion by gaining electron in valence shell is called electronegativity
6. Balanced equations
Ans: A chemical equation in which the number of atoms of the elements in the reactants is same as the number of atoms of those elements in the products is called a balanced chemical equation.
7. Endothermic reaction
Ans: The chemical reactions which are accompanied by absorption of heat are called endothermic reactions.
8. Critical angle
Ans: For a particular value of i (angle of incidence) for which, the value of r (angle of refraction) becomes equal to 90°, is called the critical angle.
9. Centre of curvature of the lens
Ans: The centre of sphere whose part forms the surface of the lens is called centre of curvature of the lens.
10. The optical centre of the lens
Ans: The point inside a lens on the principal axis, through which light rays pass without changing their paths is called the optical centre of a lens.
11. Principal focus
Ans: Rays travelling parallel to the principal axis of a lens, after refraction, converge to a point on the principal axis or diverge in such a way that they appear to be coming out of a point on the principal axis. This point is called the principal focus of the lens.
12. Focal length
Ans: The distance between the optical centre and principal focus of a lens is called its focal length.
13. Principal axis
Ans: The imaginary line passing through both centres of curvature is called the principal axis of the lens.
14. Minimum distance of distinct vision
Ans: The minimum distance of an object from a normal human eye, at which it is clearly visible without stress on the eye is called as minimum distance of distinct vision.
15. Farthest distance of distinct vision
Ans: The farthest distance of an object from a normal human eye, at which it is clearly visible without stress on the eye is called as farthest distance of distinct vision.
16. Magnification
Ans: Magnification due to a lens is the ratio of the height of the image to the height of the object.
17. Power of accommodation
Ans: The capacity of the eye lens to change its focal length as per need is called as its power of accommodation.
18. Persistence of vision
Ans: The phenomenon that allows the image formed on retina to remain imprinted there for — th of a second after the object is removed from the front of the eyes is called as persistence of vision.
19. Alkane
Ans: The saturated hydrocarbons containing only single bonds between carbon atoms are called alkanes.
20. Alkene
Ans: The unsaturated hydrocarbons containing a carbon-carbon double bond are called alkenes.
21. Alkyne
Ans: The unsaturated hydrocarbons whose structures contain a carbon-carbon triple bond are called alkynes.