It is 간식이에요.

Don't worry, asking questions is a good thing^^ Well, it's not 이예요. That word doesn't exist. The word for snack is "간식". Since it ends with a consonant (it doesn't matter which one^^), you need to use 이에요. No, the 이 part of 이에요 is not a particle. It's just part of the word. Just like "theatre" and "theme" both have the letters "the" at the beginning of them, but aren't related. If the word for snack was "간식이", then adding "예요" would be correct and what you said would be right. However, it correctly should be "간식이에요." The rule is, a noun directly attaches to 이다 (conjugated, for example, as 이에요/예요 and 입니다). Therefore, there is no possible way a particle could be added. Plus, if you were to add the particle "이/가", well, both end in vowels and you would always use 예요. So then why would 이에요 even exist? So whenever you see something like 사과예요, 컵이에요, etc., know that there is no particle EVER between the noun and 이다. 이다 is like an equal sign. 이에요 and 예요 are the EXACT same in meaning. It's all about ease of pronunciation. Just like the English "a/an" (and originally, "the"). Let me repeat one more time: The 이 in 이에요 IS NOT a particle. It is just part of the word. Okay, let me know if you have more questions^^ To practice, you could try making a notebook entry here using a bunch of nouns and just correctly adding "이에요" or "예요" after them. If you need some ideas for words to use, let me know. Here's a dictionary too: endic.naver.com