What are sexually transmitted diseases? List two examples of each diseases caused due to (i) bacterial infection and (ii) viral infection. Which device may be used to prevent the spread of such diseases?

From the following elements:

₄Be; ₉F; ₁₉K ₂₀Ca

(i) Select the element having one electron in the outermost shell.

(ii) two elements of the same group.

Write the formula of and mention the nature of the compound formed by the union of ₁₉K and element X(2,8,7).

What is meant by isomers? Draw the structures of two isomers of butane, C₄H₁₀ Explain why we cannot have isomers of first three members of alkane series.

The image of a candle flame placed at a distance of 30 cm from a spherical lens is formed on a screen placed on the other side of the lens at a distance of 60 cm from the optical centre of the lens. Identify the type of lens and calculate its focal length. If the height of the flame is 3 cm, find the height of its image.

Write the name and formula of the 2nd member of homologous series having general formula C_nH2_n-2