Atoms & Specific Gravity

8/6/2019 Atoms & Specific Gravity

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Specific Gravity

Specific gravity is the ratio of density of a substance compared to the density of fresh water at

4C (39 F). At this temperature the density of water is at its greatest value and equal 1 g/mL.Since specific gravity is a ratio, so it has no units. An object will float in water if its density is

less than the density of water and sink if its density is greater that that of water. Similarly, anobject with specific gravity less than 1 will float and those with a specific gravity greater than

one will sink. Specific gravity values for a few common substances are: Au, 19.3; mercury, 13.6;alcohol, 0.7893; benzene, 0.8786. Note that since water has a density of 1 g/cm3, the specific

gravity is the same as the density of the material measured in g/cm3.

The Discovery of Specific Gravity

The discovery of specific gravity makes for an interesting story. Sometime around 250 B.C., the

Greek mathematician Archimedes was given the task of determining whether a craftsman had

defrauded King Heiro II of Syracuse. The king had provided a metal smith with gold to make acrown. The king suspected that the metal smith had added less valuable silver to crown and keptsome of the gold for himself. The crown weighed the same as other crowns but due to its

intricate designs it was impossible to measure the exact volume of the crown so its density couldbe determined. The king challenged Archimedes to determine if the crown was pure gold.

Archimedes had no immediate answer and pondered this question for sometime.

One day while entering a bath, he noticed that water spilled over the sides of the pool, and

realized that the amount of water that spilled out was equal in volume to the space that his bodyoccupied. He realized that a given mass of silver would occupy more space than an equivalent

mass of gold. Archimedes first weighed the crown and weighed out an equal mass of pure gold.

Then he placed the crown in a full container of water and the pure gold in a container of water.He found that more water spilled over the sides of the tub when the craftsmans crown wassubmerged. It turned out that the craftsman had been defrauding the King! Legend has it that

Archimedes was so excited about his discovery that he ran naked through the streets of Sicilyshouting Eureka! Eureka! (Which is Greek for I have found it!).


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Smaller Than the Atom

So, are atoms made of even smaller stuff? The answer to thisquestion is yes. Atoms are mostly empty space, but in the center ofthe atom is a structure called a nucleus. The nucleus is acongregation of particles. These particles are called protons andneutrons. Neutrons are neutral, or have no electrical charge.Protons, however, carry a positive electrical charge of 1. So, in acarbon atom, which has 6 protons in its nucleus, the overall electriccharge of the nucleus would be 6. However, a regular atom iselectrically neutral. This is because swirling around the nucleus inwhat is called the "electron cloud". The electrons in the electron

cloud counteract the positive charges of the protons in the atomicnucleus with their negative electrical charges. This generates theneutral charge of the atom. The number of electrons and number of protons correlate in a one to oneratio. This means that there are the same number of protons and electrons in one atom. So, if an atomhas 6 protons, like carbon, it will also have 6 electrons. The 6 electrons each have a charge of -1. Thismeans that the total charge of all the electrons is -6, or -1x6. The charge of carbon's nucleus is 6 (fromthe protons), so when you add the two: 6 + -6, you get 0, which means that the atom, overall, has nocharge.

Note: Picture NOTto scale: An atom is more than 99% empty space, and the protons and neutrons makeup a very small amount of the volume of an atom. Additionally, the electrons are much smaller in

proportion to the nucleons (protons and neutrons) than we have depicted. The nucleons are actuallyabout 1800 times the size of an electron.

COOLER THAN ABSOLUTE ZERO!

Group 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Period

1

1

H

2

He

2 3 4 5 6 7 8 910


3/4

Li Be B C N O F 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

655

Cs

56

Ba

57

La

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

787

Fr

88

Ra

89

Ac

104

Rf

105

Db

106

Sg

107

Bh

108

Hs

109

Mt

110

Ds

111

Rg

112

Cn

113

Uut

114

Uuq

115

Uup

116

Uuh

117

Uus

118

Uuo

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

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

Periodic Table Key

X

Synthetic Elements

X

Liquids or melt

at close to

room temp.

X

SolidsXGases

Alkali Metals

Alkali Earth Metals

Transition Metals

Other Metals

Metalloids

Other Non Metals

Halogens

Halogens Noble Gases

Lanthanides

& Actinides


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The Periodic TableThe first periodic table was devised by Dmitri Mendeleev and published in 1869.

Mendeleev found he could arrange the 65 elements that were then known in a grid or table so that each element had:

1. A higher atomic weight than the one on its left.

2. Similar chemical properties to other elements in the same column.

He realized that the table in front of him lay at the very heart of chemistry. In his table he noted gaps - spaces where elements should be but none had yet been discovered.

In fact, just as Adams and Le Verrier could be said to have discovered the planet Neptune on paper, Mendeleev could be said to have discovered germanium (which he called eka-siliconbecause he observed a gap between silicon and tin), gallium (eka-aluminum) and sc andium (eka-boron) on paper, for he predicted their existence and their properties before their actual

discoveries.

Although Mendeleev had made a crucial breakthrough, he made little further progress because the Rutherford-Bohr model of the atom had not yet been formulated.

In 1913, Henry Moseley, who worked with Rutherford, showed that it is atomic number (electric charge) which is most fundamental to the chemical properties of any element. Mendeleev hadbelieved chemical properties were determined by atomic weight. Moseley correctly predicted the existence of new elements based on atomic numbers.

Today the chemical elements are still arranged in order of increasing atomic number (Z) as you go from left to right across the table. We call the horizontal rows periods and the vertical rowsgroups.

We also know now that an element's chemistry is determined by the way its electrons are arranged - its electron configuration.

The noble gases are found in group 18, on the far right of each period. The reluctance of the noble gases to undergo chemical reactions indicates that the a toms of these gases strongly prefertheir own electron configurations - f eaturing a full outer shell of electrons - to any other.

In contrast to the noble gases, the elements with the highest reactivity are those with the greatest need to gain or lose electrons in order to achieve a f ull outer shell of electrons.

Elements that sit in the same group (e.g. the alkali metals in Group 1) all have the same number of outer e lectrons, leading to si milar chemical properties.

Likewise the halogens in Group 17 a lso have similar properties to one another. When halogens react, they gain a n electron to form negatively charged ions. Each ion has the same electronconfiguration as the noble gas in the same period. The ions are therefore more chemically stable than the elements from which they formed.

There is a progression from metals to non-metals across each period.

The block of elements in groups 3 - 12 contains the transition metals. These are similar to one another in many ways: they produce colored compounds, have variable valency and are oftenused as catalysts.

Then we come to the lanthanides (elements 58 - 71) and actinides (elements 90 - 103). The lanthanides are often called the rare earth elements, although in fact these elements are not rare.The actinides include most of the well-known elements that take part in or are produced by nuclear reactions. No element with atomic number higher than 92 occurs naturally in large quantities.Tiny amounts of plutonium and neptunium exist in nature as decay products of uranium. These elements, and higher elements, are also produced artificially in nuclear reactors or particle

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Atoms & Specific Gravity