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  • General

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  • 质量数量数量

    A digital balance used for weighing chemicals in a laboratory setting.

    How can you determine the mass of a chemical?
    ::如何确定化学物质的质量?

    Often a student will need to weigh out a chemical for an .  If he or she uses a watch glass (a small, round piece that will hold the solid chemical), the weight of the watch glass must be determined first.  Then the solid is added to the glass and the weight of the glass plus the solid is measured.  The balance reading will be the total of the glass plus the chemical.
    ::如果学生使用手表玻璃(一个小的、圆的、能保住固体化学剂的圆块),必须首先确定手表玻璃的重量。然后将固体添加到玻璃上,再测量玻璃的重量加上固体。平衡读数将是玻璃加化学剂的总数。

    History of Atomic Weight Determinations
    ::原子重量确定历史

    As a part of his on atoms, John Dalton determined a number of atomic weights of in the early 1800s.  Atomic weights were the basis for the periodic table that Mendeleev developed.  Originally all atomic weights were based on a comparison to hydrogen, which has an atomic weight of one.  After the discovery of the , scientists assumed that the weight of an was essentially that of the protons – electrons were known to contribute almost nothing to the atomic weight of the element.
    ::作为原子的一部分,约翰·道尔顿在1800年代初期确定了若干原子权重。 原子权重是门捷列夫开发的周期表的基础。 最初所有原子权重都基于对氢的比较,氢的原子权重为1。 在发现一个质子后,科学家假设一个质子的权重基本上是质子的权重 — — 已知电子几乎对元素的原子权重毫无贡献。

    This approach worked until we learned how to determine the number of protons in an element.  We then saw that the atomic weight for an element was often twice the number of protons (or more).  The discovery of the provided the missing part of the picture.  The atomic mass now known to be the sum of the protons and neutrons in the nucleus .
    ::这种方法一直有效,直到我们学会了如何确定元素中的质子数量。 然后我们看到元素的原子重量往往是质子数量的两倍(或更多 ) 。 发现提供的图像缺失的部分。 现在已知的原子质量是核中的质子和中子的总和。

    Mass Number
    ::质量数量数量

    Rutherford showed that the vast majority of the mass of an atom is concentrated in its nucleus, which is composed of protons and neutrons. The mass number is defined as the total number of protons and neutrons in an atom. It can be calculated by adding the number of  neutrons and the number of protons  (atomic number) together.
    ::卢瑟福表明,原子的绝大多数质量集中在由质子和中子组成的核核中,质量数被定义为原子中质子和中子的总数,可以通过将中子和质子(原子数)加在一起来计算。

    Mass number =  + number of neutrons
    ::质量数量 =+ 中子数量

    Consider Table below which shows data from the first six elements of the periodic table.
    ::下面的表格显示定期表格前六个要素的数据。

    Atoms of the First Six Elements
    Name Symbol Atomic Number Protons Neutrons Electrons Mass Number
    Hydrogen H 1 1 0 1 1
    Helium He 2 2 2 2 4
    Lithium Li 3 3 4 3 7
    Beryllium Be 4 4 5 4 9
    Boron B 5 5 6

    5

    		11
    Carbon C 6 6

    6

    		6

    12

    Consider the element helium. Its atomic number is 2, so it has two protons in its nucleus. Its nucleus also contains two neutrons. Since 2 + 2 = 4, we know that the mass number of the helium atom is 4. Finally, the helium atom also contains two electrons since the number of electrons must equal the number of protons. This example may lead you to believe that atoms have the same number of protons and neutrons, but further examination of  Table above will show that this is not the case. Lithium, for example has three protons and four neutrons, leaving it with a mass number of 7.
    ::其原子数是 2 。 它的原子数是 2 。 它的核核中有两个质子。 它的核中也包含两个中子。 由于 2 + 2 = 4, 我们知道, 原子的质量数是 4 。 最后, 原子也包含两个电子, 因为电子数量必须和质子数量相等 。 这个示例可能导致您相信原子的质子和中子数量相同, 但是对上表的进一步检查将显示情况并非如此 。 例如, 锂有3 个质子和4 个中子, 留下一个质数为 7 。

    Knowing the mass number and the atomic number of an atom allows you to determine the number of neutrons present in that atom by subtraction.
    ::了解原子的质数和原子数,就能够以减法确定原子中的中子数量。

    Number of neutrons = mass number - atomic number
    ::中子数 = 质量数 - 原子数

    Atoms of the element chromium (Cr) have an atomic number of 24 and a mass number of 52. How many neutrons are in the nucleus of a chromium atom? To determine this, you would subtract as shown:
    ::元素铬原子的原子数为24,质数为52,在铬原子核心中有多少中子?要确定这一点,您将按所示减去:

    52 - 24 = 28 neutrons in a chromium atom
    ::52 - 24 = 28 中子在铬原子中

    The composition of any atom can be illustrated with a shorthand notation using the atomic number and the mass number. Both are written before the chemical symbol , with the mass number written as a superscript and the atomic number written as a subscript. The chromium atom discussed above would be written as:
    ::任何原子的构成都可以用原子编号和质数的缩写符号来说明。两者都写在化学符号之前,质量编号写成上标,原子编号写成下标。上面讨论的铬原子将写成:

    24 52 Cr
    ::2452Crr 2452Cr

    Another way to refer to a specific atom is to write the mass number of the atom after the name, separated by a hyphen. The above atom would be written as chromium-52.
    ::提及特定原子的另一种方法是在名称之后写出原子的质数,用连字符分隔。上述原子将写成铬-52。

     

     

    Summary
    ::摘要

    • The mass number is defined as the total number of protons and neutrons in an atom.
      ::质量数的定义是原子中的质子和中子总数。
    • The  mass number =  number of neutrons +  atomic number.
      ::质量数 = 中子数 + 原子数 。

    Review
    ::回顾

    1. Who first determined atomic weights for elements?
      ::谁先确定元素的原子权重?
    2. What were the original atomic weights based on?
      ::原原子权重是基于什么?
    3. Why were calculations based on numbers of protons not valid for determining atomic weights?
      ::为什么基于质子数的计算对确定原子权重无效?
    4. A tin atom has an atomic number of 50 and a mass number of 118. How many neutrons are present in this atom?
      ::锡原子原子的原子数为50,质量数为118,这个原子中有多少中子?
    5. What is the mass number of a cobalt atom that has 27 protons and 30 neutrons?
      ::有27个质子和30个中子的钴原子的质数是多少?