The word "á" doesn't exist and "à" is a contraction of the words "a" feminine article and "a" preposition and "ao" is the masculine form. The "a" preposition is usually used when you want to arrive in a place, for example "I want to go to the beach" is "Eu quero ir à praia" and " I'm two steps away from being there." "Estou a dois passos de estar lá". As Brazil is a masculine noun, you use "Bem-Vindo ao Brasil.", if it was Argentina (feminine noun) for example, you would use "Bem-Vindo à Argentina", if it was Portugal (neutral noun), you would use "Bem-Vindo a Portugal", and if the United States (plural masculine noun), you would use "Bem-Vindo aos Estados Unidos" Hope I've helped.

I think there is no neutral noun in portuguese....... There is just masculines and feminines nouns. There is also the case where you don't use a definite article (a/o), but a indefinite article (uma/um). As in the exemple : I'm going to a theater → Estou indo a um teatro There is no contraction, but just the preposition "a" followed by the indefinite article "um" (since "teatro" is a masculine noun)

'A' is a preposition. depending on the sentence, this preposition must come together with the portuguese articles: A and O. For exemplo: O brasil (the Brazil); A noruega (The Norway). When we use this articles with that prepositon "A", so one thing that we call "contração" happens. With the contraction, the article and the preposition be joined.: - A + A = à - A + O = ao So, Bem-vindo ao Brasil; Bem-vindo à Noruega. there are many words that don't accept an article (a or o). Many cities names do this. Eu fui a São Paulo (and not Eu fui à São Paulo). I hope this help you!