黄土高原世界上最大的黄土堆积区，下列说法正确的是（　　）

 元素周期表是学习和研究化学的重要工具．回答下列问题：

（1）元素周期表中不同元素间最本质的区别是____ （填字母）．

A．质子数不同   B．中子数不同   C．相对原子质量不同

（2）1869年，____ （填字母）发现了元素周期律并编制出元素周期表．

A．张青莲   B．门捷列夫   C．拉瓦锡

（3）元素周期表中氟元素的有关信息如图所示，下列说法正确的是____ （填字母）．

A．氟属于金属元素   B．氟的原子序数是9   C．氟的相对原子质量是19.00g

（4）氟原子结构示意图为，氟原子在化学反应中易____ （填“得到”或“失去”）电子，由钠元素和氟元素组成的化合物氟化钠的化学式为____ 　．

阅读与回答问题
    Do you feel lonely when your best friend isn't around? Of course, yes. Here's our advice to you—and to all kids who feel lonely sometimes. 
You may feel lonely at break or lunchtime when a best friend isn't around—maybe because he or she moved to a different school or a different class. You want to have new friends, but how do you make them? Maybe it seems everybody else already has his or her friends. But remember, there's always room for more friends.
    Start by looking around your classroom—think about which kids you'd like to play with at break. Look for chances to say hi to them, smile, and be friendly. Offer to share something or express your appreciation (欣赏) to them. Invite someone to play with you or say, “Do you want to sit here?” in the lunchroom. When you're at break, walk over to kids you want to play with, act friendly, and say “Hi, can I play, too?” or just join in. 
     If you have trouble doing this or if you're feeling shy, ask your teacher to help you make new friends. Teachers are usually pretty good at matching up friends. The best way to make friends is to be a friend. Be kind, be friendly, share, say nice things, offer to help ---and pretty soon, you'll have one, or two, or even more new friends.
    You might still miss that special best friend. But when you see each other, you can share something you didn't have before he or she left. You can introduce him or her to your new friends!