OBJECTIVE: To understand the definitions of elements and compounds.
As we examine the chemical changes of matter, we encounter a series of fundamental substances called elements. Elements cannot be broken down into other substances by chemical means. Element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus.
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Conservation of Mass
Atoms is the smallest basic unit of matter.
Protons
- Matter cannot be created or destroyed but it can be converted into a different form.
Atoms is the smallest basic unit of matter.
Protons
- Positive charge
- No charge
- Negative charge
As we will discuss in more detail in Chapter 4, each element is made up of a particular kind of atom: a pure sample of the element aluminum contains only aluminum atoms, elemental copper contains only copper atoms, and so on. Thus an element contains only one kind of atom; a sample of iron contains many atoms, but they are all iron atoms. However, any pure sample of an element contains only atoms of that element, never any atoms of any other element.
A compound always contains atoms of different elements. For example, water contains hydrogen atoms and oxygen atoms, and there are always exactly twice as many hydrogen atoms as oxygen atoms because water consists of HOOOH molecules. A different compound, carbon dioxide, consists of CO2 molecules and so contains carbon atoms and oxygen atoms (always in the ratio 1:2).
A compound, although it contains more than one type of atom, always has the same composition—that is, the same combination of atoms.
A compound always contains atoms of different elements. For example, water contains hydrogen atoms and oxygen atoms, and there are always exactly twice as many hydrogen atoms as oxygen atoms because water consists of HOOOH molecules. A different compound, carbon dioxide, consists of CO2 molecules and so contains carbon atoms and oxygen atoms (always in the ratio 1:2).
A compound, although it contains more than one type of atom, always has the same composition—that is, the same combination of atoms.
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