原子核是由____ 和中子组成的，它们统称为核子．具有相同的质子数和不同的中子数的原子互称为____．

    Every day when I enter the classroom, I will take a look at the wall beside my seat. You will find nothing special about this old wall if you just look at it. But for the students in my class, it is a special wall. Take a good look at it, and you will get to know the real feelings and thoughts of us, the 9^th graders.
    In the middle of the wall, there is a big “VICTORY”. It was written in pencil. I guess it must have been written by someone who got a good mark in an exam.
    A little higher above the formulas, there is a poem. It only has two sentences. It reads: All those sweet memories have disappeared. Like tears dropping in the heavy rain.
    Oh! It must have been written at the end of the last semester in middle school. Classmates had to leave school and good friends had to part. What a sad poem!
    If you look at the wall more carefully, you will find many other interesting things, like a crying face, or a happy face, and other patterns(图案). There are still some patterns and letters that I can't understand, but they all show the feeling of the students who drew them.
    For years, the wall has witnessed(目睹) all the things that have happened in the classroom. I don’t know how it will be next year, two years from now, or even ten years from now. But I hope more smiling faces will be drawn on it.