IPA: //ˈbjuːtɪfəl.nəs//
KK: /ˈbjuːtɪfəlnəs/
The quality of being attractive or pleasing to the senses.
The beautifulness of the sunset took my breath away.
Beautifulness → It is formed from "beauty" (from Old French *beaute*, meaning physical attractiveness or goodness) and "-ful" (meaning full of) and "-ness" (meaning the state or quality of). The word "beautifulness" means the state of being full of beauty or possessing beauty.
Think of 'beauty' as the core idea, and remember that 'beautifulness' refers to the quality of being full of beauty.