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 too small to be conveniently measured in grams so atomic mass units (amu) are used instead. Atomic mass units are also called daltons. One amu (or dalton) is approximately 1.7 X 10-24g. 

Protons and neutrons have a mass of approximately 1 amu. The mass of an electron is much less. The total mass of an atom is due mostly to the mass of protons and neutrons.

 

mass

 charge 
protons

approx. 1 amu

+
neutrons

approx. 1 amu

0
electrons

  1/1836 amu 

-

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 atomic mass is calculated as the sum of the mass of protons, electrons, and neutrons.

The mass of electrons is small enough that we can generally disregard it in our calculations of atomic 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.

Atomic Symbols

Isotopes

Isotopes are atoms that have the same number of protons and differ only in the number of neutrons. Three different forms of hydrogen are shown below.

Most isotopes are stable but radioactive isotopes are unstable and break down into more stable forms by emitting particles and energy (radiation).

Radiation can be detected, so radioactive isotopes are useful as labels in scientific research and medical diagnostic procedures.

Energy

Energy is the capacity to produce change.  For example, energy is needed  to move matter.

Potential energy is energy that is stored in matter. For example, a ball located at the top of a hill will roll down the hill. Due to it's position, it has more potential than an identical ball located at the bottom of the hill.

Electrons at greater distances from the nucleus contain more potential (stored) energy.

Example - Many of the chemical reactions that are associated with energy (ex: photosynthesis) involve electrons moving to higher or lower energy levels.

Distribution of Electrons

Electron Shells

The different energy levels are also called electron shells.  The first shell is the K shell, the next is the L, followed by M, N, etc.

Electrons in the K shell have the least energy.  Electrons that are further from the nucleus have more potential energy.

The maximum number of electrons in each shell is given by the formula 2N2 where N is the shell number. From this formula, the maximum number in each shell is 2, 8, 18, 32, etc. The maximum number of electrons in the last (outer) shell is 8.

Orbitals

It is convenient to draw electrons as occupying a circular space around the nucleus. In reality, electrons do not move around the nucleus in circles.

Pairs of electrons occupy spaces called orbitals. An orbital can hold 2 electrons.

The first shell has one spherical orbital called the 1s orbital. The second shell has 4 orbitals. One is spherical (called 2s) and the other three are dumbbell-shaped and at right angles to each other. These are referred to as 2p orbitals. The arrangement of three dumbbell-shaped orbitals can be in the model shown below.

orbitals.jpg (318308 bytes)

The third and higher shells have s and p orbitals as well as other kinds of orbitals.

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..

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

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)