元素周期表简介
视频：
元素周期表
元素介绍
视频：
元素

化学元素周期表（Periodic table of elements）是根据元素原子核电荷数从小至大排序的化学元素列表。列表大体呈长方形，某些元素周期中留有空格，使特性相近的元素归在同一族中，如碱金属元素、碱土金属、卤族元素、稀有气体、非金属元素、过渡元素等。这使周期表中形成元素分区且分有七主族、七副族、Ⅷ族、18族。由于周期表能够准确地预测各种元素的特性及其之间的关系，因此它在化学及其他科学范畴中被广泛使用，作为分析化学行为时十分有用的框架。
俄国化学家德米特里·伊万诺维奇·门捷列夫（Dmitri Mendeleev）于1869年总结发表此周期表（第一代元素周期表），此后不断有人提出各种类型周期表不下170余种，归纳起来主要有：短式表（以门捷列夫为代表）、长式表（以维尔纳式为代表）、特长表（以波尔塔式为代表）；平面螺线表和圆形表（以达姆开夫式为代表）；立体周期表（以莱西的圆锥柱立体表为代表）等众多类型表。
注：中国教学上长期使用的是长式周期表，即维尔纳式为代表。

In this video we'll discuss the periodic table of elements.
在本视频中,我们将讨论元素周期表·
A chart called the periodic table of the elements organizes all of the known elements.
一张名为元素周期表的图表组织了所有已知的元素·
Rows in the periodic table are called periods and columns are called groups or families.
周期表中的行称为周期,
and columns are called groups or families.
列称为族或家族·
A section from each of the two bottom periods has been pulled out and placed below the table to avoid making the table too wide.
每个底部两个周期的部分被拉出并放置在表格下方,以避免使表格过宽·
The elements are organized left to right and top to bottom by their atomic number meaning the number of protons in one atom of the element.
元素按照它们的原子序数从左到右、从上到下组织,原子序数表示一个元素原子中的质子数·
Each box shows an element represented by its unique symbol.
每个框表示一个元素,元素用其独特的符号表示·
The smaller number next to each element's symbol represents the atomic number.
每个元素符号旁边的较小数字代表原子序数·
The atomic number increases by one as you go from left to right across each period.
原子序数从左到右依次递增·
The larger number represents the atomic mass.
较大的数字表示原子质量·
Notice that the atomic mass of many elements is a decimal number rather than a whole number.
注意,许多元素的原子质量是小数而非整数·
This is because the atomic mass is a weighted average of the mass numbers for the isotopes of an element.
这是因为原子质量是该元素同位素质量数的加权平均值·
A weighted average takes into account how common each isotope of an element is in nature.
加权平均考虑了元素各同位素在自然界中的常见程度·
The most common isotope counts for much more in the average than less common isotopes
最常见的同位素在平均值中的权重远大于不太常见的同位素,
just like a final exam may count more than quizzes towards your grade in a class.
就像期末考试可能比小测验在课堂成绩中占更大比重一样·
Notice that the atomic mass isn't the same as mass number
注意,原子质量不同于质量数,
which is simply the total number of protons and neutrons in the nucleus of a particular isotope.
质量数是指特定同位素原子核中质子和中子的总数·
However you can determine the mass number of an element's most common isotope by rounding its atomic mass up or down to the nearest whole number.
然而,你可以通过将元素的原子质量四舍五入到最接近的整数来确定该元素最常见同位素的质量数·
You can do this because the most common isotope has the most influence on the atomic mass.
你可以这样做,因为最常见的同位素对原子质量的影响最大·
Round up if an element's atomic mass ends in .5 or greater round down if an element's atomic mass ends in less than .5.
如果元素的原子质量末尾是0.5或更大,则四舍五入到上一个整数;如果末尾小于0.5,则四舍五入到下一个整数·
Let's look at some examples from the periodic table.
让我们看一些周期表中的例子·
Helium has an atomic mass of 4.003.
氦的原子质量为4.003·
We can easily round that down to get a mass number of 4.
我们可以轻松地将其四舍五入得到质量数4·
We can also see that helium's atomic number is 2 which means it has 2 protons.
我们还可以看到氦的原子序数是2,这意味着它有2个质子·
Now we can subtract the atomic number from the mass number to see that the most common isotope of helium has 2 neutrons.
现在,我们可以从质量数中减去原子序数,看到氦的最常见同位素有2个中子·
In the case of oxygen we can round its atomic mass up to get a mass number of 16.
对于氧气,我们可以将其原子质量四舍五入得到质量数16·
Since its atomic number is 8 we know oxygen has 8 protons.
由于它的原子序数是8,我们知道氧气有8个质子·
And by simple subtraction we can determine oxygen also has 8 neutrons.
通过简单的减法,我们可以确定氧气也有8个中子·
How does this work in a less common isotope of an element such as hydrogen 3
这种方法在元素的不太常见同位素中如何运作呢?比如氢-3?
The most common hydrogen isotope is hydrogen 1
氢的最常见同位素是氢-1,
as you can see from rounding the atomic mass listed in the periodic table.
正如你可以从周期表中列出的原子质量四舍五入看到的·
Recall that isotopes are identified by their mass number.
记住,同位素是通过它们的质量数来识别的·
So we know hydrogen 3's mass number is 3.
因此,我们知道氢-3的质量数是3·
So we can subtract hydrogen's atomic number of 1 from its mass number and see that hydrogen 3 has 2 neutrons.
因此,我们可以从氢的原子序数1中减去质量数,看到氢-3有2个中子·
In summary the periodic table is an organization chart of all the known elements.
总结一下,周期表是所有已知元素的组织图表·
Each element is represented by its symbol atomic number and atomic mass.
每个元素由其符号、原子序数和原子质量表示·
Elements are arranged left to right and top to bottom by increasing atomic number.
元素按照原子序数从左到右、从上到下排列·
An element's atomic mass is a weighted average of its isotope's mass numbers.
元素的原子质量是其同位素质量数的加权平均值·
Round the element's atomic mass up or down to find the mass number of its most common isotope.
将元素的原子质量四舍五入,以找到其最常见同位素的质量数·
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