Chemistry
Chemistry is the study of substances; their properties, structure, and the changes they undergo.
Elements and compounds
It has been known for at least a thousand years that some substances can be broken down by heating or chemical treatment into "simpler" ones, but there is always a limit; we eventually get substances known as elements that cannot be reduced to any simpler forms by ordinary chemical or physical means. What is our criterion for "simpler"? The most observable (and therefore macroscopic) property is the weight.
The idea of a minimal unit of chemical identity that we call an element developed from experimental observations of the relative weights of substances involved in chemical reactions. For example, the compound mercuric oxide can be broken down by heating into two other substances:
2 HgO → 2 Hg + O2
... but the two products, metallic mercury and dioxygen, cannot be decomposed into simpler substances, so they must be elements.
Macroscopic
Microscopic
Substances are defined at the macroscopic level by their formulas or compositions, and at the microscopic level by their structures.
The elements hydrogen and oxygen combine to form a compound whose composition is expressed by the formula H2O.
The molecule of water has the structure shown here.
Chemical substances that cannot be broken down into simpler ones are known as elements. The actual physical particles of which elements are composed are atoms or molecules.
Sulfur – the element in its orthorhombic crystalline form.
The S8 molecule is an octagonal ring of sulfur atoms. The crystal shown at the left is composed of an ordered array of these molecules.
(No, they don't actually move around like this, although they are in a constant state of vibrational motion.)
Molecules
A molecule is an assembly of atoms having a fixed composition, structure, and distinctive, measurable properties.
A molecule but not a compound - Ozone, O3, is not a compound because it contains only a single element.
This well-known molecule is a compound because it contains more than one element. [link]
Ordinary solid salt is a compound but not a molecule. It is built from interpenetrating lattices of sodium and chloride ions that extend indefinitely.
Chemical change
Elements and compounds
It has been known for at least a thousand years that some substances can be broken down by heating or chemical treatment into "simpler" ones, but there is always a limit; we eventually get substances known as elements that cannot be reduced to any simpler forms by ordinary chemical or physical means. What is our criterion for "simpler"? The most observable (and therefore macroscopic) property is the weight.
The idea of a minimal unit of chemical identity that we call an element developed from experimental observations of the relative weights of substances involved in chemical reactions. For example, the compound mercuric oxide can be broken down by heating into two other substances:
2 HgO → 2 Hg + O2
... but the two products, metallic mercury and dioxygen, cannot be decomposed into simpler substances, so they must be elements.
Macroscopic
Microscopic
Substances are defined at the macroscopic level by their formulas or compositions, and at the microscopic level by their structures.
The elements hydrogen and oxygen combine to form a compound whose composition is expressed by the formula H2O.
The molecule of water has the structure shown here.
Chemical substances that cannot be broken down into simpler ones are known as elements. The actual physical particles of which elements are composed are atoms or molecules.
Sulfur – the element in its orthorhombic crystalline form.
The S8 molecule is an octagonal ring of sulfur atoms. The crystal shown at the left is composed of an ordered array of these molecules.
(No, they don't actually move around like this, although they are in a constant state of vibrational motion.)
Molecules
A molecule is an assembly of atoms having a fixed composition, structure, and distinctive, measurable properties.
A molecule but not a compound - Ozone, O3, is not a compound because it contains only a single element.
This well-known molecule is a compound because it contains more than one element. [link]
Ordinary solid salt is a compound but not a molecule. It is built from interpenetrating lattices of sodium and chloride ions that extend indefinitely.
Chemical change
Chemical change is defined macroscopically as a process in which new substances are formed. On a microscopic basis it can be thought of as a re-arrangement of atoms. A given chemical change is commonly referred to as a chemical reaction and is described by a chemical equation that has the form
reactants → products
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