The fuel used in many disposable lighters is liquid butane, c4h10. butane has a molecular weight of 58.1 grams in one mole. how many carbon atoms are in 4.00 g of butane?

Matter is made of very tiny particles. One mole of a substance contains a given amount of particles. This number is constant for all particles- 6.022×10^23 , and is called Avogadro's constant. To get the number of particles in 4g of butane, we need to calculate the number of moles represented by 4 g, then multiply by the Avogadro's constant (Since each mole has particles equivalent to the Avogadro's constant) 
Therefore, if 1 mole = 58.1g
                how many moles will be equal to 4g?
                we cross multiply  (4x1) divide by 58.1
                equals 0.068847moles
multiply by Avogadro constant = 4.1466 x 10^molecules
But remember the question is not about the number of molecules in butane, its about the number of carbon atoms. 
Since there are four carbons in butane, and each has  4.1466 x 10^22 atoms
to get the number of atoms, we multiply by four
equals 1.6586 x 10^atoms

Determine the  wavelength of light emitted when an electron  in an li2+ ion  makes a transition from an orbital with n  =  6 to an orbital with n  = 5.

Outermost (valence) electrons