Atoms, elements and compounds
AdvancedAtoms are basic particles of matter; an element contains one type of atom, while a compound contains atoms of two or more elements joined in fixed proportions.
- An element contains one type of atom
- A compound is a fixed chemical combination of elements
- A mixture contains substances together in variable proportions
§Text Passage
1The substances around us change endlessly, yet the blueprint behind them is simple. Atoms are the basic building blocks of matter and the smallest units in a chemical change. A pure substance made of one kind of atom is an element — oxygen gas contains only oxygen atoms and a gold bar only gold atoms — while two or more elements joined in fixed proportions form a compound.
2Water is formed from hydrogen and oxygen in a fixed ratio, and table salt from sodium and chlorine; a rusty nail becomes a new substance, iron oxide. Air, however, is a mixture: its nitrogen and oxygen are merely mingled, not chemically combined. Salt water is the same — each part keeps its own properties, so evaporation, filtration and other physical methods can separate them.
3Remarkably, a compound can behave completely differently from the elements that make it. Hydrogen burns easily and oxygen feeds a flame, yet the water they form puts fires out; sodium is a metal that reacts violently with water and chlorine is a poisonous gas, but joined together they become the table salt on our dining table. Chemical bonding creates wholly new properties, so a substance must be judged as a whole.
4Element symbols and chemical formulae form an international language of chemistry: H for hydrogen, O for oxygen, C for carbon — and once H₂O is written, scientists anywhere know it means water. Mastering this language lets us describe the composition of matter precisely and understand how reactions create new materials, supporting the development of medicines, materials and electronic technology.
1Summary 重點摘要
Atoms are basic particles of matter; an element contains one type of atom, while a compound contains atoms of two or more elements joined in fixed proportions.
2Vocabulary cards Vocabulary
The basic particle of an element — the smallest unit in chemical change.
A pure substance made of only one type of atom.
A pure substance in which different elements are chemically joined in fixed proportions.
Several substances mixed without forming a new chemical substance; the proportions can vary.
A particle made of two or more atoms joined together; a water molecule contains two hydrogen atoms and one oxygen atom.
A shorthand using element symbols and numbers to show what a substance contains, e.g. H₂O for water.
A change that produces new substances, such as iron rusting into iron oxide.
A substance made of only one kind of matter with a fixed composition; elements and compounds are both pure substances.
3Step by step Steps
- Build the hierarchyThe ideas of this lesson are best remembered as a hierarchy from small to large: the atom is the smallest unit in a chemical change; atoms of one kind make up an element — oxygen gas contains only oxygen atoms, a gold bar only gold atoms; when different elements combine chemically in fixed proportions they form a compound — hydrogen and oxygen make water, iron and oxygen make rust. With the layers clear, classifying substances becomes far less confusing.
- Separate compounds from mixturesCompounds and mixtures must be told apart: water is a compound, its hydrogen and oxygen atoms locked in the fixed ratio 2:1, the same in every sea and river on Earth. The nitrogen and oxygen in air are merely mingled, their proportions shifting with place and weather — a mixture; salt water is the same, whether weak or strong. Chemical bonding in a fixed ratio is the hallmark of a compound.
- Properties can changeA compound can differ completely from the elements that make it: hydrogen burns easily and oxygen feeds flames, yet the water they form puts fires out — chemical bonding creates wholly new properties. Salt is built from sodium and chlorine: sodium is a metal that reacts violently with water and chlorine is a poisonous gas, but joined together they become an everyday seasoning. Always judge a substance as a whole, never by its ingredients alone.
- Formulae are a languageElement symbols and chemical formulae form an international language of chemistry: write H₂O and scientists anywhere know it means water — two hydrogen atoms joined to one oxygen atom; CO₂ stands for carbon dioxide. Mastering this language lets you describe composition and reactions precisely; like learning the alphabet before reading, it is the basic tool for all further study in chemistry.
- Learn the common symbolsMost element symbols take the first one or two letters of the English name: H for hydrogen, O for oxygen, C for carbon, N for nitrogen, Fe for iron and Cu for copper; a few come from Latin names, such as Na for sodium and K for potassium. The rule is a capital first letter and a lower-case second letter — get it wrong and the meaning changes completely: Co is the element cobalt, while CO is the gas carbon monoxide, so watch the capitalisation in exams.
- Separating mixturesMixtures can be separated by physical means: evaporating salt water leaves the salt behind, sand and water part by filtration, and iron powder hidden in sand can be drawn out with a magnet. These methods work precisely because nothing in a mixture is chemically bonded — every ingredient keeps its original properties, which is also a key clue for recognising a mixture. Physical separation is applied daily in drinking-water treatment and recycling, one of the most practical faces of chemistry.
4Diagrams Diagrams
5Worked example Example
Example: classify oxygen, seawater, carbon dioxide and gold.
- Oxygen contains one type of atom → element
- Seawater contains water, salts and more → mixture
- Carbon dioxide has carbon and oxygen in a fixed combination → compound
- Gold contains one type of atom → element
- Answer: Oxygen gas and gold each contain one kind of atom — elements. Carbon dioxide chemically bonds carbon and oxygen in a fixed ratio — a compound. Seawater holds water, salts and more in no fixed proportion — a mixture. The trick: count the substances present, then check for fixed-ratio chemical bonding; never blur the three, and in exams classify first, giving one reason each.
6Common mistakes Mistakes
7Quick check Quick check
Q1How many types of atom make up an element?
Q2Is water a compound or a mixture? Why?
Q3Why is air a mixture?
Q4Give the element symbols for oxygen, iron and sodium.
Q5Is salt water a mixture or a compound? How can it be separated?
8Recap Recap
- An element contains one type of atom
- A compound is a fixed chemical combination of elements
- A mixture contains substances together in variable proportions
- A compound can have completely different properties from its elements
- Chemical formulae are an international chemical language