The former result from the overlap of an sp3 orbital of carbon with the 1s orbital of a hydrogen; the latter by the overlap of two sp3 orbitals on adjacent carbon atoms.
ðə fˈɔːɹmɚ ɹɪzˈʌlt fɹʌmðɪ ˌoʊvɚlˈæp əvən ˌɛspˈiː θɹˈiː ˈɔːɹbɪɾəl ʌv kˈɑːɹbən wɪððə wˈʌn z ˈɔːɹbɪɾəl əvə hˈaɪdɹədʒən; ðə lˈæɾɚ baɪ ðɪ ˌoʊvɚlˈæp ʌv tˈuː ˌɛspˈiː θɹˈiː ˈɔːɹbɪɾəlz ˌɔn ɐdʒˈeɪsənt kˈɑːɹbən ˈæɾəmz.