At stp, an element that is a brittle solid and a poor conductor of heat and electricity could have an atomic number of
(1) 12 (3) 16
(2) 13 (4) 17

Option (2) is the correct answer.

Explanation:

It is known that non-metals do not conduct electricity.

Carbon has atomic number 12, sulfur has atomic number 16, and chlorine has an atomic number 17. All these elements are non-metals.

Whereas aluminium has atomic number 13 and it is a poor metal.

So, aluminium being a poor metal is a brittle solid and a poor conductor of heat and electricity.

answer: enthalpy

explanation:

for a exothermic reaction, the energy is released i.e. the energy of the products is lower than the energy of the reactants.

= enthaply

= energy of products

= energy of reactants

as , thus comes out to be negative. negative value of enthalpy shows that the reaction is exothermic.

in scientific notation, the value of the  speed of light in a vacuum

  will be written as 2.995 * 10^8. when this value is represented in the form of

  a * 10^b, then, a = 2.995 and b = 8.

the value of the speed of light in a vacuum given above can also be rounded up to approximately 300000000. in this case, the scientific notation will be written as 3 *10^8.