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化合物是由兩種或兩種以上不同元素按固定比例結合形成的純凈物，其化學性質與組成元素不同，組成可通過化學式（如NaCl、H?O）表示。根據元素組成分為有機化合物（含碳，如甲烷）和無機化合物（如硫酸）；按化學鍵類型分為離子化合物（如NaCl）、共價化合物（如CO?）及配位化合物（如[Co(NH?)?]Cl?）。
化合物的分類體系隨化學研究進展不斷完善，早期側重元素組成，現代結合分子結構與化學鍵特性。已知化合物數量超過300萬種，每年新增約30萬種，其中90%為有機化合物。非等比化合物（如合金、過渡金屬氧化物）的發現突破了傳統定組成概念，純度鑒定采用色譜分析、熔程測定及光譜技術，支撐化學研究與工業應用。

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Now that you've learned about atomic structure and elements let's look at chemical compounds
現在,既然你已經了解了原子結構和元素,接下來我們來看看化學化合物·
A chemical compound is a substance made of two or more elements that are chemically bonded together in fixed proportions
化學化合物是由兩種或更多種元素通過化學鍵結合在一起,且以固定比例組成的物質·
Common examples are water and table salt
常見的例子有水和食鹽·
You've probably heard of H2O That's a chemical formula for water
你可能聽說過H2O·這是水的化學式·
In the chemical formula for water the little number slightly below the letter H for hydrogen is called a subscript
在水的化學式中,稍微低于氫元素符號H的那個小數字叫做下標·
A subscript in the chemical formula tells you how many atoms of that element are in one unit of the compound
化學式中的下標告訴你在化合物的一個單位中該元素有多少個原子·
There is no subscript next to the O for oxygen
氧元素符號O旁邊沒有下標·
That means there's only one oxygen atom
這意味著只有一個氧原子·
We don't write the one
我們不寫1·
When there is no subscript it's understood that we mean one atom
當沒有下標時,通常意味著是一個原子·
So H2O is the formula for one unit of water
因此,H2O是一個水分子的化學式·
The formula shows that a unit of water contains a fixed proportion of two atoms of hydrogen to one atom of oxygen
這個化學式顯示,一個水分子包含兩個氫原子和一個氧原子的固定比例·
It's important to know that compounds usually have very different physical and chemical properties than the individual elements they contain
重要的是要知道,化合物通常具有與其組成的單個元素非常不同的物理和化學性質·
For example although water is liquid at room temperature the elements hydrogen and oxygen are gasses
例如,雖然水在常溫下是液體,但氫和氧這兩種元素是氣體·
The next compound we'll look at is sodium chloride commonly known as table salt
接下來我們來看一個化合物:氯化鈉,通常叫做食鹽·
Sodium chloride consists of sodium and chloride ions
氯化鈉由鈉離子和氯離子組成·
Ions are charged particles because they have gained or lost electrons
離子是帶電的粒子,因為它們獲得或失去了電子·
How many ions of sodium are there in one unit of the chemical formula for sodium chloride
氯化鈉的化學式中,一個單位有多少個鈉離子呢?
No subscript means there's only one sodium ion
沒有下標意味著只有一個鈉離子·
And how many chloride ions
那么有多少個氯離子呢?
Again there's only one
同樣,只有一個·
So sodium chloride has a one to one ratio of sodium to chloride ions
所以,氯化鈉的鈉離子和氯離子的比例是1:1·
Okay The last example of a compound is glucose A simple sugar your body cells use for energy
好的·最后一個化合物的例子是葡萄糖,一種你的身體細胞用作能量的簡單糖·
The chemical formula for one unit of glucose is C6H12O6
葡萄糖一個單位的化學式是C6H12O6·
Here we can see that one unit of glucose has six carbon atoms and 12 hydrogen atoms
這里我們可以看到,葡萄糖一個單位含有6個碳原子和12個氫原子·
How many oxygen atoms does one unit of glucose have
葡萄糖一個單位有多少個氧原子?
If you said six oxygen atoms you'd be right
如果你說是六個氧原子,那你是對的·
In review a compound is a substance containing the chemically bonded atoms of two or more elements
總結一下,化合物是包含兩個或更多元素的化學鍵結合原子的物質·
A chemical formula gives the proportional number of atoms or ions of each element in a compound
化學式給出了每種元素在化合物中的原子或離子的比例數·
And the compound usually has different physical and chemical properties than the elements it contains
而且,化合物通常具有與其組成元素不同的物理和化學性質·
We'll explore how elements chemically bond together in the next video
我們將在下一個視頻中探索元素如何通過化學鍵結合在一起·