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If we replace one hydrogen in methane by a methyl group we arrive at ethane. Now we can say:

  1. Carbon is more electronegative, and so it withdraws electrons which leads to deshielding. This is why the chemical shift increases in ethane.

alternatively you might say:

  1. The methyl group is more electrondonating (+I) than hydrogen, so the shift should decrease

Why is the electronegativity more relevant here?

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    $\begingroup$ Methane has a higher H/C ratio. The molecule is symmetric, so each side has the same electron density as the other. The electronegativity of carbon gets split over three hydrogen in ethane, four in methane. $\endgroup$ – Karl Mar 1 at 16:46

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