Elements and Compounds元素、单质与化合物

A hundred-odd elements, millions of substances

The 26 letters of the alphabet spell hundreds of thousands of words. In the same way, just over a hundred elements make up the more than 100 million substances known today. This lesson introduces the element, the most basic idea in chemistry, and uses it to sort pure substances further.

Elements

Water molecules contain oxygen atoms. So do oxygen gas molecules, and so do carbon dioxide molecules. All of these oxygen atoms are the same kind of atom, and chemists say they all belong to the element oxygen.

An element元素 is a kind of atom. (What makes atoms the “same kind” is the number of protons in the nucleus, which you’ll meet in lesson 1.3.3.)

When you talk about the elements in a substance, you name which kinds are present, not how many:

• Correct: water is made of the elements hydrogen and oxygen.
• Wrong: water is made of two hydrogen elements and one oxygen element.
• To count, talk about atoms: one water molecule contains two hydrogen atoms and one oxygen atom.

The four most abundant elements in Earth’s crust are, in order, oxygen, silicon, aluminum, and iron.

Elements and compounds as substances

Pure substances can be sorted by how many elements they contain:

• An element单质, as a substance, is a pure substance made of only one element. Examples: oxygen gas $\ce{O2}$, iron $\ce{Fe}$, helium $\ce{He}$, ozone $\ce{O3}$.
• A compound化合物 is a pure substance made of two or more different elements. Examples: water $\ce{H2O}$, carbon dioxide $\ce{CO2}$, sodium chloride $\ce{NaCl}$.

A note on words: English uses “element” both for a kind of atom and for a substance made of one kind of atom. Chinese uses two different words, 元素 for the kind of atom and 单质 for the substance. When it matters, say “the element oxygen” for the kind of atom and “oxygen gas” for the substance.

One very common family of compounds is the oxides氧化物: compounds made of exactly two elements, one of which is oxygen. Examples: $\ce{H2O}$, $\ce{CO2}$, $\ce{Fe2O3}$, $\ce{MnO2}$. $\ce{KClO3}$ contains oxygen, but it has three elements, so it isn’t an oxide.

Putting the last few lessons together gives a classification tree for matter:

• Matter
  • Mixtures: two or more substances (air, seawater)
  • Pure substances: one substance only
    • Elements: one kind of atom ($\ce{O2}$, $\ce{Fe}$, $\ce{C}$)
    • Compounds: two or more kinds of atoms
      • Oxides: two elements, one of them oxygen ($\ce{H2O}$, $\ce{CO2}$)
      • Other compounds ($\ce{NaCl}$, $\ce{KMnO4}$, $\ce{H2SO4}$)

The order of the questions matters: first ask whether it’s a pure substance, then ask how many elements it contains.

At the particle level

In a particle diagram, different colors stand for atoms of different elements:

• One kind of molecule, made of one color of atom: an element.
• One kind of molecule, made of different colors of atoms: a compound.
• More than one kind of molecule: a mixture.

Example 1: Sorting substances

Classify each: (1) liquid oxygen; (2) iron; (3) water; (4) air; (5) potassium permanganate, $\ce{KMnO4}$; (6) carbon dioxide; (7) salt water.

Example 2: Is anything made of one element automatically an element?

Common mistakes

Quick check

Summary

• An element is a kind of atom. Elements describe which kinds are present, not how many.
• Pure substances are either elements (one kind of atom) or compounds (two or more kinds of atoms).
• An oxide is a compound of exactly two elements, one of which is oxygen.
• To classify, first decide whether it’s a pure substance, then count the elements.