Thursday, July 12, 2012

Atoms & Isotopes; Compounds & Minerals

Atoms are the smallest fraction of an element that can exist,and still show the characteristics of the element. Atoms themselves are composed essentially of electrons (1 negative charge), protons (1 positive charge), and neutrons (no charge). In a simplistic way we can visualize atoms as consisting of a nucleus with protons and neutrons that is surrounded by electrons for charge balance. Atoms of different elements are distinguished by the number of protons in the nucleus. Hydrogen has the simplest (and lightest) atom with just one proton and one electron, and then complexity gradually increases until we get to Uranium with 92 protons, 146 neutrons, and 92 electrons.

Normally the number of electrons equals the number of protons, however an atom may loose or pick up electrons and have a negative or positive charge, in which case we talk about ions rather than atoms.

Although the "planetary" model of the atom as shown above is easy to visualize, reality is a bit stranger. According to a quantum mechanic approach pioneered by Schrödinger, the behavior of electrons can be envisioned as probability functions of wave forms. If the probabilities are plotted in 3D, they look like the animated pictures above. All of the pictures describe electron distribution of a hydrogen atom in various energy states (quantum states, states if "excitation).		The number of neutrons (the "glue" between the protons)in the nucleus may also vary within a given element.These varieties of a given atom (same number ofprotons, different number of neutrons) are called isotopes. The illustration at the left shows three isotopes of hydrogen: "normal: hydrogen with 1 proton and 1 electron, deuterium, and tritium. Scientists in various laboratories are working to produce controlled fusion of deuterium into helium.If successful, this will be an almost inexhaustable source of energy. Although deuterium constitutes only a small fraction of the hydrogen on Earth (a few liters in every cubic kilometer of ocean water), there is a large volume of ocean water to draw on. Atoms can join together to form molecules. Most of the substances around us consist of molecules and are called compounds of various elements. Molecules are the smallest particle of a compound that has all thechemical properties of that compound. They are made up of two or more atoms, either of the same element orof two or more different elements. Molecules and compounds contain their constituent elements in specific proportions that are characteristic of the compound. For example, water (H₂O), a well known and simple compound,contains hydrogen and oxygen at a ratio of 2 to 1.The illustration below shows how Lithium and Fluorine combine to form the compound Lithiumfluoride. In essence, Lithium gives up an electron and becomes positively charged, whereas Fluorine picks up this electron and becomes negatively charged. The attraction between the oppositely charged ions causes them to join in a so called ionic bond.

Other types of bonds are the covalent bond (atoms share electrons) and the metallic bond (atoms are surrounded by an "electron soup"). Which bond is found in a given compound depends on the elements that are present and the chemical characteristics of the elements involved.
The materials that make up the Earth crust and mantle are called rocks (there is a great variety of them), and these rocks are composed of a mixture of pure elements (e.g. diamonds [pure carbon], gold) and various chemical compounds of silica, oxygen, iron, magnesium, aluminum, etc. As said above, each chemical compound shows very specific proportions of elements that it is composed of. These proportions depend on the electron configuration of the participant elements. One particular thing about the compounds that compose rocks is that they not only show compound-specific proportions of elements, but also that each has a compound specific internal arrangement of atoms. Because of this regular arrangement of atoms they are crystalline substances. We call this type of compounds minerals. Minerals are the main building blocks of all rocks.
By definition, minerals have the following characteristics:

1) they are natural (not artificial) substances
2) they are solid
3) they form by inorganic processes
4) they have a specific chemical composition
5) they have a characteristic crystal structure

The crystal structure of cooking salt, Sodium
Chloride (NaCl).The structure consists of
alternating Sodium ions (positive charge, small
black balls), and Chlorine ions (negative charge,
large yellow balls). In this case the ions are laid
out on rectangular grids, in three dimensions we
have a cubicstructure. Depending on the relative
size of the ions we get different structural angles
and structural types.

By above definition, all minerals have a crystalline structure. Not all crystalline substances, however, are necessarily minerals. Sugar, for example, forms very nice crystals, but it is not a mineral because it is an organic substance. Minerals are inorganic substances.
Minerals have specific physical properties that are used to distinguish and classify them. These properties are:

1) Internal structure (see above)/Crystal Morphology
2) Cleavage (calcite always breaks as rhombs[see below], cooking salt as cubes)
3) Color/Streak
4) Hardness (talc to diamond)
5) Density (PbS is denser than FeS)

These properties are a function of the bond strength, the internal structure, and the chemical composition of the mineral.

Cleavage in Calcite (CaCO₃). No matter how often the calcite
crystal is broken into smaller and smaller pieces, the resulting fragments always show rhombohedral cleavage. This is so because of the internal arrangement of atoms in the calcite
crystal. Cleavage angles are a fundamental property of any given
mineral.

A given compound may occur in more than one crystal structure.The picture on the left shows a diamond and graphite (used in pencils). Both consist of pure carbon (C), but differ markedly in their properties. Graphite is soft, shiny dark gray, and has a layered, sheetlike, internal structure. In contrast, diamond is denser, often transparent, and the hardest substance known to man. The difference lies in the 3D framework structure of diamond that gives it lots of strength and structural integrity (the reason is that diamond forms under the very high pressures of the upper mantle). When a compound can form more than one type of crystal structure it is called polymorphous. Diamond and graphite are polymorphs of carbon.

Scientific findings about the Atomic Structure

What is an atom? What are atoms made of?

Atoms are the basic building blocks of ordinary matter. Atoms can join together to form molecules, which in turn form most of the objects around you. Atoms are composed of particles called protons, electrons and neutrons. Protons carry a positive electrical charge, electrons carry a negative electrical charge and neutrons carry no electrical charge at all. The protons and neutrons cluster together in the central part of the atom, called the nucleus, and the electrons 'orbit' the nucleus. A particular atom will have the same number of protons and electrons and most atoms have at least as many neutrons as protons. Protons and neutrons are both composed of other particles called quarks and gluons. Protons contain two 'up' quarks and one 'down' quark while neutrons contain one 'up' quark and two 'down' quarks. The gluons are responsible for binding the quarks to one another.

Discovery of the electron

1898

Scientists worked with electricity long before they understood that current was made of electrons. The cathode tube was a prime example. By switching on some voltage, scientists could make fluorescent streams of electricity travel from the bottom part of a glass tube to the top -- but no one knew how it worked. Some thought the rays were a wave traveling through a mysterious "ether" which they thought permeated all space. Others thought the rays were streams of particles.
J.J. Thomson decided to find out for sure. Thomson was a physics professor at Cambridge University in the UK. He placed cathode tubes in electric and magnetic fields. He knew that these fields will move particles from side to side, but don't have much effect on how a wave moves. In his experiments, the cathode rays bent over to one side, so Thomson knew the cathode rays must be made of some small particle, which he dubbed a "corpuscle."
Thomson initially thought his corpuscles were much too small to be of interest to anyone outside a science lab. However, people quickly realized that electric current was in fact made of moving electrons. Since electricity is the lifeblood of everything from computers to phones to microwaves, the electron turned out to be interesting to just about everybody.

Discovery of the neutron

It is remarkable that the neutron was not discovered until 1932 when James Chadwick used scattering data to calculate the mass of this neutral particle. Since the time of Rutherford it had been known that the atomic mass number A of nuclei is a bit more than twice the atomic number Z for most atoms and that essentially all the mass of the atom is concentrated in the relatively tiny nucleus. As of about 1930 it was presumed that the fundamental particles were protons and electrons, but that required that somehow a number of electrons were bound in the nucleus to partially cancel the charge of A protons. But by this time it was known from the uncertainty principle and from "particle-in-a-box" type confinement calculations that there just wasn't enough energy available to contain electrons in the nucleus.

Atomic nucleus

The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The proton–neutron model of nucleus was proposed by Dmitry Ivanenko in 1932.^{[citation needed]} Almost all of the mass of an atom is located in the nucleus, with a very small contribution from the orbiting electrons.The diameter of the nucleus is in the range of 1.75 fm (femtometre) (1.75×10⁻¹⁵ m) for hydrogen (the diameter of a single proton)^[1] to about 15 fm for the heaviest atoms, such as uranium. These dimensions are much smaller than the diameter of the atom itself (nucleus + electron cloud), by a factor of about 23,000 (uranium) to about 145,000 (hydrogen).

The History of Atomic Structure

PROTONS

Positively charges sub atomic particles.
The existence of protons was first discovered by Eugene Goldstein in 1886.
He observed a cathode ray tube and found rays traveling in the direction opposite to that of the cathode rays. He called those canal rays and concluded that they were composed of positive particles. He called those canal rays and concluded that they were composed of positive particles.
Each proton has a mass about 1840 times that of an electron.

NEUTRONS

Neutrons are sub atomic particles with no charge but with a mass nearly equal to the proton's.
Sir James Chadwick confirmed the discovery of another atomic particle’s existence: the Neutron.
NOTE:
ü All atoms are made up of subatomic particles protons, neutrons and electron.
ü The electronic charge is measured in coulombs (C).

ATOMIC NUMBER AND MASS NUMBERS

Atomic Number

In the modern periodic table, the elements are actually arranged in order of increasing atomic number--that's the number of protons in one atom of a particular element. An undisturbed atom is electrically neutral, so the number of electrons in it is the same as its atomic number.

Atomic weight almost always increases with atomic number, so Mendeleev's sequence of elements was almost exactly the same as the one used today, though there are a couple of weird exceptions. In general, it's correct to think of atoms getting heavier as you go down a column or to the right across a row.

Mass number

A mass number is the total number of protons and neutrons found in an atom.

To find the mass number, you must know the number of neutrons in that particular isotope of the element and then add the atomic number (same as the number of protons) to it.

When this number is definite it is called a nuclide and the symbol is written with a superscript giving the mass number and a subscript giving the atomic number (number of protons) placed in front of atomic symbol: 2311Na. It can also be written after the elements name, sodium-23.

The Periodic Table of Elements

Many of the same principles we learned in the last lesson (atomic structure) can be applied to the third element in the series, lithium. Lithium has 3 protons in its nucleus and an atomic number = 3. To keep these protons stable and glued together, lithium normally has 4 neutrons in its nucleus. Recall that in a neutral atom the number of protons equals the number of electrons, so a neutral lithium atom will have 3 electrons spinning around it. When we look at the atomic structure of lithium though, we see a significant difference between it and the 1st two elements discussed in the last lesson. Lithium's 3rd electron spins at a different level (called a shell) than the 1st two electrons.

A Lithium Atom has 2 electron shells

        This 3rd electron is forced to spin around the nucleus in a second electron shell because of the repulsive forces between the 3 negatively charged electrons. Just as 2 positively charged protons repel each other if they are brought too close together, so will negatively charged electrons repel each other. No neutral particles exist however to help hold electrons together in their shells (neutrons only exist in the nucleus). When an electron will not fit into an existing level because of repulsive forces between negative charges, the atom will work around this problem by adding the electron to a new shell, more distant from the nucleus than the existing shell (or shells).
        In other words, electron shells have a limited capacity for electrons. As you might expect, the farther an electron shell is from the nucleus, the larger it is. You can calculate the total capacity of an electron shell using the formula 2n², where n equals the number of the electron shell. For example, for the 1st electron shell n = 1 and 2 x 1² = 2, telling us that the capacity of the 1st shell is 2 electrons as we have already seen. For the 2nd shell (n = 2) and 2 x 2² = 8. For an atom to fill its 2nd electron shell, it would need 10 electrons: 2 to fill the 1st shell and 8 to fill the 2nd. The 3rd shell has a total capacity of 2 x 3² = 18 electrons. But things get a bit tricky here. Electron shells actually have sublevels. The first sublevel (the s sublevel) holds 2 electrons. The second, p, sublevel holds 6. The third, d, sublevel holds 10. After levels 3s and 3p are filled, electron shell #3 acts as if it has reached capacity with only 8 total electrons. In other words, in an atom with 20 electrons (which is the element calcium, Ca) the first 2 electrons are located in the 1st shell, the next 8 in shell #2, the following 8 in shell #3 and the remaining 2 electrons are located in shell #4.
        As you can see, at this point atomic theory begins to get complicated. Just as we saw in the last lesson that the electron is not a simple particle but a more complex wave, here we find that filling electron shells is not as simple as stacking books on a shelf. This is because at the atomic level things are just plain wierd. Particles travel through time and space like something out of a bad Star-Trek re-run. It all boils down to something called quantum theory. We'll try to keep things as simple as possible, but if you would like, you can learn more about atomic structure and quantum theory using some of the links listed at the bottom of this page.

Why does this matter? What significance do electron shells have on the fact that "you'd rather be fishing"? As it turns out, the reason hydrogen atoms stick to oxygen atoms to form the water in the fish pond depends on electron structure. Actually, just about everything depends on electron structure. The chemical properties of an atom will depend on the number of electrons in the atom's outermost (or valence) electron shell. Thus lithium behaves more similarly to hydrogen than it does to helium because both lithium and hydrogen have 1 electron in their outermost shell. The atoms can be arranged according to their electron structure and chemical properties. This is where we encounter the Periodic Table of Elements.
        The Periodic Table of Elements (or the Periodic Table for short) was first proposed by a Russian chemist named Dmitri Mendeleev in 1871. A modern version of the Table appears below. Atoms are ordered by their atomic number in the Periodic Table. The Table is set up so as to indicate the number of electron shells in each atom and the number of valence electrons (electrons in the outermost shell) in the atom. As you descend rows in the Table, the number of electron shells in the atom increases. For example, hydrogen (H) in the 1st row has 1 shell, lithium (Li) in the 2nd row has 2 shells, sodium (Na) 3 shells, etc. As you read the Table from left to right in any one row, the number of valence electrons increases. For example, hydrogen has 1 electron (in the first shell). Helium (He), the 2nd element in the first row, has 2 electrons (thus filling its valence shell). Let's look at lithium (Li) again. From the atomic number we know that Li has 3 electrons. From its position on the Periodic Table (and from our discussion above) we know that Li has 1 valence electron: 2 electrons fill Li's 1st shell and 1 orbits in the second shell. From its position on the Table we know that berylium (Be) has 2 valence electrons in its 2nd shell. Can you predict the structure of the next element, Boron (B)?
        Each element in the Table below is linked to information on Chris Heilman's Pictorial Periodic Table. To return to this page hit the 'Back' button on your web browser
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The Periodic Table of Elements

< Group

VIIIA

1
H
1.01

IIA

Atomic number
Symbol
Atomic mass

	Metals
	Transition Metals
	Metalloids
	Nonmetals

IIIA

IVA

VIA

VIIA

2
He
4.00

3
Li
6.94

4
Be
9.01

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

IIIB

IVB

VIB

VIIB

VIIIB

IIB

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
63.55

30
Zn
65.38

31
Ga
69.72

32
Ge
72.59

33
As
74.92

34
Se
78.96

35
Br
79.90

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
(98)

44
Ru
101.07

45
Rh
102.91

46
Pd
106.4

47
Ag
107.87

48
Cd
112.40

49
In
114.82

50
Sn
118.69

51
Sb
121.75

52
Te
127.60

53
I
126.90

54
Xe
131.30

55
Cs
132.91

56
Ba
137.34

57
La*
138.91

72
Hf
178.49

73
Ta
180.95

74
W
183.85

75
Re
186.21

76
Os
190.2

77
Ir
192.22

78
Pt
195.09

79
Au
196.97

80
Hg
200.59

81
Tl
204.37

82
Pb
207.2

83
Bi
208.96

84
Po
(209)

85
At
(210)

86
Rn
(222)

87
Fr
(223)

88
Ra
226.03

89
Ac*
(227)

104
Rf
(261)

105
Db
(262)

106
Sg
(263)

107
Bh
(262)

108
Hs
(265)

109
Mt
(266)

110
Uun
(269)

111
Uuu
(272)

112
Uub
(277)

113
Uut
(282)

*Lanthanide series:

58
Ce
140.11

59
Pr
140.91

60
Nd
144.24

61
Pm
(145)

62
Sm
150.36

63
Eu
151.96

64
Gd
157.25

65
Tb
158.92

66
Dy
162.50

67
Ho
164.93

68
Er
167.26

69
Tm
168.93

70
Yb
173.04

71
Lu
174.97

*Actinide series:

90
Th
232.04

91
Pa
231.04

92
U
238.03

93
Np
237.05

94
Pu
(244)

95
Am
(243)

96
Cm
(247)

97
Bk
(247)

98
Cf
(251)

99
Es
(252)

100
Fm
(257)

101
Md
(258)

102
No
(259)

103
Lr
(260)