Two members of the boron family owe their names to bright lines in their emission spectra. indium has a bright indigo blue line (451.1nm) and thallium has a bright green line (535.0 nm).
what are the energies of these two spectra lines?

E Indium =     4.407 x 10⁻¹⁹ J

E Thallium =  3.716 x 10⁻¹⁹ J

Explanation:

From Planck´s equation for a given wavelength, the energy is given by.

E = h ( c/ λ )

h: Planck´s constant, 6.626 x 10⁻³⁴ J·s

c: light speed, 3 x 10⁸ m/s

λ: wavelength in m

We will need first to convert the given wavelengths to m:

451.1  nm x ( 1 m/10⁹  nm ) = 4.511 x 10⁻⁷  m

535.0 nm x ( 1 m/10⁹ nm ) = 5.350 x 10⁻⁷ m

E Indium =    6.626 x 10⁻³⁴  J·s x 3x 10 ⁸ m/s/ 4.511 x 10⁻⁷  m = 4.407 x 10⁻¹⁹ J

E Thallium =  6.626 x 10⁻³⁴ J·s x 3x 10 ⁸ m/s/ 5.350 x 10⁻⁷ m = 3.716 x 10⁻¹⁹ J

2fe + 3h2so4 -> fe2(so4)3 + 3h2

it is necessary to balance equations in order to get the proper amount of reactants as well as products in order not to make a miscalculation.

you cannot change the subscripts as that would change the substance itself. for instance, diatomic elements such as h2 only have the subscript two when a gas. another example would be so4, if you were to change the subscript, the element would no longer be sulfate.

we change the coefficients instead as that only changes the number of molecules rather than the substance itself.

hope this !