O setor de ciências da biblioteca fica no segundo andar.

Here, setor means a section, department, or area within a larger place. In Brazilian Portuguese, it is commonly used for organized parts of places like libraries, hospitals, stores, and offices.

So in this sentence, o setor de ciências is the part of the library devoted to science.

Yes, often you could.

• setor sounds a little broader or more institutional
• seção emphasizes a section or subdivision

In a library, both can make sense, but setor de ciências sounds very natural. A library sign might use either one depending on style.

De + subject/field is a very common pattern in Portuguese:

• livro de história
• professor de matemática
• setor de ciências

So de ciências means for science / related to the subject of science.

Why not científico? Because científico means scientific, not science as a school or library subject.
So setor científico would sound more like scientific sector, which is a different idea.

Why ciências in the plural? In Portuguese, school subjects and knowledge areas are often named in the plural, so ciências is very normal.

Because da is the contraction of de + a.

• de = of
• a = the

So:

• da biblioteca = of the library

This shows that the science section belongs to or is part of the library.

Also, biblioteca is a feminine singular noun, which is why the article is a.

Portuguese uses definite articles very naturally and often more regularly than English does.

So o setor means the section. In this sentence, it refers to a specific, identifiable section of the library, so the article is expected.

In English, learners sometimes want to drop articles, but in Portuguese that usually sounds incomplete or unnatural.

In Brazilian Portuguese, ficar is very commonly used to talk about location.

So fica here means something like:

• is located
• is situated

That is why O setor de ciências da biblioteca fica no segundo andar sounds very natural.

Compare:

• é is usually for identity or classification
  
  • A biblioteca é grande
• está is for states or temporary conditions, though it can also be used for location in some contexts
  
  • A biblioteca está fechada

For the location of a place, room, section, or building, ficar is often the most idiomatic choice.

Fica is the third person singular, present indicative form of ficar.

The subject is o setor, which is singular, so the verb is singular too:

• o setor fica
• os setores ficam

Portuguese usually does not need a subject pronoun here, so you just say fica, not ele fica.

Because no is the contraction of em + o.

• em = in / on / at
• o = the

So:

• no segundo andar = on the second floor

Portuguese normally uses em for location, and with masculine singular nouns it contracts:

• em + o = no
• em + a = na

Examples:

• no térreo
• no primeiro andar
• na biblioteca

Because segundo has to agree with andar, and andar is a masculine noun.

So:

• o segundo andar
• o primeiro andar

If the noun were feminine, the ordinal would change:

• a segunda sala
• a primeira seção

This is normal adjective agreement in Portuguese.

Not always, because floor numbering can differ.

In Brazil, the usual system is:

• térreo = ground floor
• primeiro andar = the floor above the ground floor
• segundo andar = the next one above that

So for many American English speakers, segundo andar may correspond to what they would call the third floor.
For many British English speakers, it matches second floor more closely.

So it is best to think in terms of the local Brazilian system, not automatically translate the floor number directly from English.

Yes. The original sentence is the most neutral and straightforward:

• O setor de ciências da biblioteca fica no segundo andar.

But you could also say:

• No segundo andar fica o setor de ciências da biblioteca.

That version puts more focus on the location. It is grammatical, but a little more marked or descriptive in tone.

In Brazilian Portuguese, ciências is pronounced roughly like see-EN-see-as, with the stress on ÊN:

• ci-ÊN-ci-as

A few helpful points:

• the first ci sounds like see
• the stressed syllable is ên
• the word is plural, so it ends with -s

For an English speaker, the biggest thing is not to pronounce it like English science. Portuguese ciências has more syllables and a clearly stressed middle syllable.