秒（钟），分钟，小时 are all measure words, 一分钟，二分钟，几分钟，一小时，三秒钟..... 一小时=60分钟=3600秒 一会儿means a moment, a while...it might be a few minutes as well as a few hours, it is a pretty subjective thing. 你等多久了？How long have you been waiting? 嗯，20分钟了。Umm...20 minutes 嗯， 有一会了。Umm...for a while. (maybe 10 minutes, 1 hour, 2hours... ) Sometimes we also use "一会儿...一会儿...." pattern to express the idea that someone does something for a while and then does another thing for a while. for example, 他一会儿哭，一会笑，疯了吗？ He cries now and then laughs. Is he crazy? 这只小鸟多可爱，一会儿喝水，一会儿梳理。 What a lovely little bird, sometimes she drinks, sometimes she preens. hannah 喜欢语言，一会儿学英文，一会儿学汉语。 hannah loves learning languages, sometimes she learns English, sometimes she learns Chinese. However sometimes this pattern is used to express the idea that someone can't concentrate on anything, or someone has short attention span. of course it shows disapproval here. for example, Hannah 总是不专心，一会儿学英语，一会儿又学汉语。 Hannah has the attention span of a bolt of lightning. sometimes she learns English, sometimes she learns Chinese. Hannah, Is your attention span only 45 Minutes long? 汉娜，你的注意力就只有45分钟吗？（I used “分钟”here）

一会儿：a little while, 分钟--- it's the word which representatives the unit of time, for example, 1分钟 is more detailed than 一会儿.

一会儿：for a little while 一分钟：two meanings 1) 60 seconds 2) for a little while

我一会儿到。i will reach in a little white. 我一分钟内到。 i'll arrive within 1 minute.

一会儿 emphasizes the fact that something will soon be done or ready, but you don't know exactly when. 一分钟 emphasizes the fact that something will soon be done or ready, and you expect it to happen within that time frame.