classification of matters

Below gives definitions of some basic keywords commonly encountered in chemistry. Since chemistry is the study of substances, we begin by looking at the basic unit of a substance, the atom:

What is an Atom ?

All substances are made up of matter and the fundamental unit of matter is the atom. The atom constitutes the smallest particle of an element. The atom is made of a central nucleus containing protons (positively-charged) and neutrons (with no charge). The electrons (negatively-charged with negligible mass) revolve around the nucleus in different imaginary paths called orbits or shells.

Atom - It is the smallest unit of a substance. The identity of a substance will be destroyed if its atom is further divided. Different substances will have different types of atoms. All atoms are made up of a number of protons, neutrons and electrons. As an example, diagram (left) below shows a schematic representation of the atom for helium.

The nucleus of an atom consists of protons (each with a positive charge +1) and neutrons (zero charge). The electrons orbit round the nucleus, each with a negative charge of -1. In fact the actual size of a nucleus is very small compare with the overall size (determined by the extension of electrons into space) of an atom. Diagram on the right shows the relative size of the nucleus (dot), compare to the electron orbital (circle), a schematic description where electrons are located.

All atoms have equal number of protons and electrons. In other words, they are always neutral. Charged atoms, or ions, contain different number of protons and electrons. For example, a +2 ion contains two less elctrons than the number of protons.

What is an Element ?

An element is a substance made up of atoms of one kind. There are about 82 naturally-occurring elements and about 31 artificially-made elements as listed in the Periodic Table

1A

2A

3B

4B

5B

6B

7B

8

8

8

1B

2B

3A

4A

5A

6A

7A

O

1

1 H

2 He

2

3 Li

4 Be

5 B

6 C

7 N

8 O

9 F

10 Ne

3

11 Na

12 Mg

13 Al

14 Si

15 P

16 S

17 Cl

18 Ar

4

19 K

20 Ca

21 Sc

22 Ti

23 V

24 Cr

25 Mn

26 Fe

27 Co

28 Ni

29 Cu

30 Zn

31 Ga

32 Ge

33 As

34 Se

35 Br

36 Kr

5

37 Rb

38 Sr

39 Y

40 Zr

41 Nb

42 Mo

43 Tc

44 Ru

45 Rh

46 Pd

47 Ag

48 Cd

49 In

50 Sn

51 Sb

52 Te

53 I

54 Xe

6

55 Cs

56 Ba

+

72 Hf

73 Ta

74 W

75 Re

76 Os

77 Ir

78 Pt

79 Au

80 Hg

81 Tl

82 Pb

83 Bi

84 Po

85 At

86 Rn

7

87 Fr

88 Ra

*

104 Rf

105 Db

106 Sg

107 Bh

108 Hs

109 Mt

110 Ds

+ Lanthanide:

57 La

58 Ce

59 Pr

60 Nd

61 Pm

62 Sm

63 Eu

64 Gd

65 Tb

66 Dy

67 Ho

68 Er

69 Tm

70 Yb

71 Lu

* Actinide:

89 Ac

90 Th

91 Pa

92 U

93 Np

94 Pu

95 Am

96 Cm

97 Bk

98 Cf

99 Es

100 Fm

101 Md

102 No

103 Lr

What is atomic number and atomic weight ?

• Atomic number of an element is the number of protons in the nucleus of an atom. Since atoms are electrically neutral, the number of protons equal the number of electrons in an atom.
• Atomic weight (or relative atomic mass) of an element is the number of times an atom of that element is heavier than an atom of hydrogen. The atomic weight of hydrogen is taken to be unity [1].
• Mass number of an element is the sum of the number of protons and neutrons in the nucleus of an atom.

The elements are arranged according to increasing atomic numbers (along with their atomic mass) in a table called the Periodic Table.

What is a molecule ?

A molecule is formed when atoms of the same or different elements combine. A molecule is the smallest particle of a substance that can normally exist independently.
Examples:

• Two atoms of oxygen combine to form a molecule of oxygen [O₂].
• One atom of carbon combines with two atoms of oxygen to form a molecule of carbon dioxide [CO₂].

What is a compound ?

A compound is formed when atoms or molecules of different elements combine. In a compound, elements are chemically combined in a fixed proportion.
Examples:

• Hydrogen and oxygen are combined in a fixed proportion of 2:1 to form the compound water [H₂O].
• Carbon and oxygen are combined in a fixed proportion of 1:2 to form the compound carbon dioxide [CO₂].

Examples :H₂O, CuSO₄, Salt "NaCl", Ammonia "NH₃", Iron III Oxide "Fe₂O₃", etc.

What is ion ?

Ion is the electrically charged atoms, called ions electrically charged cations and negatively charged ions are called anions. Cations and anions can be either a single ion consists of only one type of atom or can also be polyatomic ions containing two or more different atoms.

What is the difference between a solution and a mixture?

A solution is a mixture where one of the substances dissolves in the other. The substance that dissolves is called the solute. The substance that does not dissolve is called the solvent. An example of a solution is salt water.

What is a mixture and a compound ?

Compounds are chemicals made from atoms of different elements joined by chemical bonds. They can only be separated by a chemical reaction. A mixture is made from molecules of elements and compounds that are simply mixed together, without chemical bonds.

What is the difference between elements, compounds, and mixtures?

All matter can be classified as mixtures, elements and compounds.

Mixtures – two or more substances that are not chemically combined with each other and can be separated by physical means. The substances in a mixture retain their individual properties. Mixtures can be of homogeneous (existing in a single phase) and heterogeneous (existing in more than one phases) type. Solutions are a special kind of mixture where one substance dissolves in another.

Elements – simplest...

All matter can be classified as mixtures, elements and compounds.

Mixtures – two or more substances that are not chemically combined with each other and can be separated by physical means. The substances in a mixture retain their individual properties. Mixtures can be of homogeneous (existing in a single phase) and heterogeneous (existing in more than one phases) type. Solutions are a special kind of mixture where one substance dissolves in another.

Elements – simplest form of pure substance. They cannot be broken into anything else by physical or chemical means.

Compounds – pure substances that are the unions of two or more elements. They can be broken into simpler substances by chemical means. Compounds can be separated by chemical reactions into their elements and elements can be combined chemically to form compounds.

The difference among these three types of matter can be summarized in the following table: