Chemistry

Matter

Matter occupies space and has weight.

It can exist as a solid, liquid, or gas.

It may be possible to break some kinds of matter down into other kinds of matter with different properties. For example, water (H2O) can be broken down into hydrogen and oxygen.

Hydrogen and oxygen in the above example cannot be broken down any further because they are elements.

Elements

Elements cannot be broken down into substances with different properties by chemical reactions. For example, water (H2O) is not an element because it can be broken down into hydrogen (H) and oxygen (O).

Substances that are composed of two or more different elements are called compounds. For example, water is a compound because it is composed of hydrogen and oxygen.

The smallest particles of an element that have the characteristics of that element are atoms.

Elements are substances made up of only one kind of atom.

There are 92 naturally occurring elements. Matter is therefore composed of 92 different kinds of elements.

The following elements make up 96% of the body weight of organisms: Oxygen, Carbon, Hydrogen, Nitrogen.

Atoms

An atom is composed of subatomic particles. Three important kinds of subatomic particles are protons, neutrons, and electrons. Some atoms (ex: hydrogen) do not have neutrons.

Protons and neutrons are located in a central area called the nucleus.

Electrons move about the nucleus. The number of electrons is equal to the number of protons.

It is more accurate to represent the space occupied by electrons as a cloud. The electrons are likely to be located somewhere within the cloud.

Characteristics of Subatomic Particles

The mass of subatomic particles is measured in units called atomic mass units (amu) because grams are too large.

The mass of an electron is much less than that of a proton or a neutron. The total mass of an atom is due mostly to the mass of protons and neutrons.

 

mass

charge
protons

1 amu

+1
neutrons

1 amu

0
electrons

1/1836 amu

-1

Charge is a state in which particles are either attracted to each other or they repel each other. Two particles that are attracted to each other have opposite charges (positive and negative). Particles that repel each other have the same charge; they are both either positive or they are both negative.

Protons have a positive charge and electrons have a negative charge. Particles with positive charges are attracted to particles with negative charges. Two particles with the same charge (both positive or both negative) will repel each other.

Atoms are neutral. The number of electrons (negatively charged) is equal to the number of protons (positively charged), therefore the overall charge is zero.

Atomic Mass and Mass Number

The mass of electrons is small enough that we can generally disregard it in our calculations of mass. The mass number is the number of protons and neutrons.

Most of an atom is empty space.

Atomic Number

The atomic number is the number of protons.

All atoms of an element have the same atomic number.

The number of protons contributes to the physical properties of an element.

Atoms are neutral, therefore the number of electrons is equal to the number of protons.

Neutrons

Atoms of an element can vary in the number of neutrons that they contain. Three different forms of hydrogen are shown below.

Electron Shells

Electrons are distributed in spaces surrounding the nucleus called electron shells.

The maximum number of electrons in the first shell is 2. The maximum number in the 2nd shell is 8. Other shells may contain more electrons but the maximum number of electrons in the last (outer) shell is 8.

The Outer Shell

The inner shells of atoms are filled with the maximum number of electrons but the outer shells may or may not be filled with the maximum number.

An atom with only one shell requires two electrons to complete its outer shell. Atoms with more than one shell require 8 electrons to complete their outer shells. 

Periodic Table of the Elements

Letters are used to represent atoms. The letters are called atomic symbols. For example, the symbol for carbon is C. The symbol for sodium is Na. 

The periodic table (below) is a table showing the atomic symbol, atomic mass, and atomic number of all of the elements. The elements are arranged from left to right according to their atomic number. Elements in the first seven rows are also arranged by the number of electron shells. Elements in the first row have one shell, those in the second row have two shells etc.

1

H

1.008

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1

H

1.008

2

He

4.003

3

Li

6.939

4

Be

9.012

 

 

 

 

 

 

 

 

 

 

5

B

10.81

6

C

12.01

7

N

14.01

8

O

16.00

9

F

19.00

10

Ne

20.18

11

Na

22.99

12

Mg

24.31

 

 

 

 

 

 

 

 

 

 

13

Al

26.98

14

Si

28.09

15

P

30.97

16

S

32.06

17

Cl

35.45

18

Ar

39.95

19

K

39.10

20

Ca

40.08

21

Sc

44.96

22

Ti

47.90

23

V

50.94

24

Cr

52.00

25

Mn

54.94

26

Fe

55.85

27

Co

58.93

28

Ni

58.71

29

Cu

63.54

30

Zn

65.37

31

Ga

69.72

32

Ge

72.59

33

As

74.92

34

Se

78.96

35

Br

79.91

36

Kr

83.80

37

Rb

85.47

38

Sr

87.62

39

Y

88.91

40

Zr

91.22

41

Nb

92.91

42

Mo

95.94

43

Tc

(99)

44

Ru

101.1

45

Rh

102.9

46

Pd

106.4

47

Ag

107.9

48

Cd

112.4

49

In

114.8

50

Sn

118.7

51

Sb

121.8

52

Te

127.6

53

I

126.9

54

Xe

131.3

55

Cs

132.9

56

Ba

137.3

57

La

138.9

72

Hf

178.5

73

Ta

180.9

74

W

183.9

75

Re

186.2

76

Os

190.2

77

Ir

192.2

78

Pt

195.1

79

Au

197.0

80

Hg

200.6

81

Ti

204.4

82

Pb

207.2

83

Bi

209.0

84

Po

(209)

85

At

(210)

86

Rn

(222)

87

Fr

(223)

88

Ra

(226)

89

Ac

(227)

104

Rf

(261)

105

Ha

(262)

106

Sg

(263)

107

Ns

(261)

108

Hs

(265)

109

Mt

(266)

110

Uun

(269)

111

Uuu

(272)

112

Uub

(277)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

58

Ce

140.1

59

Pr

140.1

60

Nd

144.2

61

Pm

144.9

62

Sm

150.4