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Atoms are building materials of all matter, because by putting together atoms in different ways, all elements and compounds can be formed．
An element is a substance or a kind of thing that has only one type of atom． Pure oxygen has only oxygen atoms． Elements are substances which cannot be broken down chemically into any simpler substances．
All the atoms of an element have the same atomic number． You will remember that the atomic number is the number of protons (= very very small pieces of matter that carry positive or Å electricity) in the nucleus or centre of an atom, and this number determines or decides the nature of the element． Each oxygen atom has eight protons in its nucleus, while each aluminum has thirteen． We call these substances elements because all their atoms are of the same type;they all have the same number of protons in the nucleus．
The element is made up of chemical units which have two or more atoms held together by chemical bonds or connections． A chemical unit is the smallest unit of a substance which shows all the chemical nature of the substance． Chemical units of elements are called molecules． The number of atoms in a molecule depends on the element．
At room temperature, some elements are gases (in a gas, there are larger spaces between the (molecules )． In the Noble Gases, each molecule has only one atom, because the atoms of these elements do not form chemical bonds with each other． In other gases, however, the molecules each have two atoms held together by chemical bonds． For example, nitrogen, chlorine, fluorine, oxygen, and hydrogen are all gases whose molecules have two atoms each．
The chemical symbol or sign of an element stands for one atom of the element． The formula of the element, however, stands for one molecule of the element．
Most elements are solid at room temperature． Some of these elements are made up of single atoms placed closely together within the solid, e．g． copper, iron, and nickel．
Other solid elements are made up of molecules with two or more atoms each． Iodine is a solid whose molecules have two atoms each(I2 ), but phosphorus has molecules made up of four atoms (P4 ), while the molecules of sulphur may have up to eight atoms each ( S8 )． In each of these elements, the molecules are put so closely together that the result is a solid substance．
Bromine and mercury are two elements which are liquids at room temperature． Bromine, like its neighbours chlorine and iodine in the periodic ( = of period ) table, has molecules made up of two atoms each． In these liquid elements, the molecules are close together, but not so crowded;they can pass lightly by one another．
1．All elements are made up of ．
A．atoms B． matter C． building materials D． molecules
2Pure elements can ______．
A．never be broken away in any way
B．not be broken into smaller parts in any simpler way
C．seldom be made up of only one atom
D．have only oxygen atoms
3．Which of the following statements is TRUE?
A．The nature of atoms decides the nature of an element they are in．
B．The number of atoms decides the nature of an element they are in．
C．The nature of protons inside an atom decides the nature of the atom．
D．The number of protons inside an atom decides the number of the atom．
4． The symbol of aluminum is ______ and its atom number is ______
A．Al, 8 B．Alu, 9 C．Al, 13 D．Alu, 14
5． The molecule is ________of a substance．
A．a small chemical unit B．the smallest unit
C．the chemical bonds D．a chemical unit
6． A noble gas molecule has ________．
A．something to do with room temperature
B．no chemical bonds with each other
C．only one atom
D．has no chemical bonds with other elements
7．The formula of fluoride , sulphur , iodine and phosphorus are __________．
A 2, 4, 6, 8 B．2, 2, 4, 8
C．2, 4, 2, 8 D．2, 8, 2, 4
8． Molecules in a solid are those in a gas．
A．as closely placed as B．more closely put than
C．less closely placed than D．smaller in space than
9． It seems that elements in an ordinary lab are solids．
A．mostly B．sometimes C．seldom D．never
10． Which of the following statements is TRUE?
A．Bromine and mercury are always solids．
B．Molecules of liquids are less closely put than those of gases．
C．Each bromine element has the same number of atoms as that of nitrogen．
D．Liquid molecules are always more closely put than those of solids．
查看习题详情和答案>>单词拼写（共10小题，每小题1分，满分10分）
【小题1】He c_________ that we should come back before 9 o’clock.
【小题2】 An e___________ is a kind of machine for raising and lowering something.
【小题3】If you want to read quickly and well, you should enlarge your v__________.
【小题4】People aged from 13 to 19 are called t__________.
【小题5】 His friend has _________(遭受) from lung cancer for some years.
【小题6】They carried out a s_________ of experiments to test the new drug.
【小题7】The mother________(使平静) the little boy by telling him a story.
【小题8】I’m rather _________(挂念的) about your health.
【小题9】He made a long j_________ from Beijing to London.
【小题10】 The doctor p___________ him to give up smoking finally.
单词拼写（共10小题，每小题1分，满分10分）
1.He c_________ that we should come back before 9 o’clock.
2. An e___________ is a kind of machine for raising and lowering something.
3.If you want to read quickly and well, you should enlarge your v__________.
4.People aged from 13 to 19 are called t__________.
5. His friend has _________(遭受) from lung cancer for some years.
6.They carried out a s_________ of experiments to test the new drug.
7.The mother________(使平静) the little boy by telling him a story.
8.I’m rather _________(挂念的) about your health.
9.He made a long j_________ from Beijing to London.
10. The doctor p___________ him to give up smoking finally.
查看习题详情和答案>>
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单词拼写（共10小题，每小题1分，满分10分）
He c_________ that we should come back before 9 o’clock.
An e___________ is a kind of machine for raising and lowering something.
If you want to read quickly and well, you should enlarge your v__________.
People aged from 13 to 19 are called t__________.
His friend has _________(遭受) from lung cancer for some years.
They carried out a s_________ of experiments to test the new drug.
The mother________(使平静) the little boy by telling him a story.
I’m rather _________(挂念的) about your health.
He made a long j_________ from Beijing to London.
The doctor p___________ him to give up smoking finally.
查看习题详情和答案>>