Download Elements, Compounds, and Mixtures: A Basic Introduction to Chemistry and more Summaries Chemistry in PDF only on Docsity!
TOPIC 1.
ELEMENTS, COMPOUNDS AND MIXTURES.
What is chemistry? Chemistry is the study of matter and the interconversion of matter. Matter is anything which has mass and occupies a volume. Sciences often begin by collecting and classifying. Subsequent observations lead to generalisations and laws. In the science of chemistry, there are numerous bases for classifying matter but the most fundamental is to subdivide matter into pure substances and mixtures. Pure substances can be further regarded as consisting of either elements or compounds.
MATTER
PURE SUBSTANCES MIXTURES
An infinitely large number of mixtures is ELEMENTS COMPOUNDS possible 90 naturally occurring An infinitely large elements + about 30 number of compounds man-made elements is possible
What characterises each of these groups? An ELEMENT is a substance which cannot be broken down into simpler component substances. An ATOM is the smallest possible unit of an element. Atoms are extremely small so any visible specimen of an element contains enormous numbers of atoms. Each element’s atoms are unique to that element. There are only 90 naturally occurring elements so it follows that there are only 90 different types of naturally occurring atom. What distinguishes the atoms of each different element is the subject of another Topic. Elements are conveniently further subdivided into two groups, METALS and NON-METALS , based on their physical and chemical properties. The physical properties of metals are probably already familiar - shiny when freshly cut, conduct heat and electricity well, malleable and ductile. The non-metals have the opposite properties to metals - they are usually powders or gases, do not conduct well and, if solids, are brittle. The chemical properties of each group will be discussed in future Topics.
Elements as they occur in nature rarely consist of discrete, individual atoms. Usually they consist of two or more atoms joined together by CHEMICAL BONDS of various types. For example, oxygen, the essential life-supporting element in air, is not present as individual oxygen atoms but instead, consists of two oxygen atoms bonded together. This unit is called a MOLECULE of oxygen. Some elements occur as discrete molecules containing even larger numbers of atoms such as 4, 6, 10 or 12 bonded atoms. However, most elements including all the metals do not normally exist as discrete molecules containing a fixed number of atoms but instead, consist of very large aggregates of atoms bonded together.
I - 1
Regardless of whether an element occurs as single atoms or as atoms bonded together, each element always consists of the same type of atom which imparts its own unique properties to that element. For example, aluminium is recognised as a silver-coloured metal while copper has a different colour. Apart from the obvious difference in colour, there are many unique physical and chemical properties which each of these two elements possess and which are attributable to the differences between their atoms.
Table 1 (Page I-20) gives a complete alphabetical list of all the elements, including some of those synthesised. In Table 2 (Page I-21), most of those elements are listed in groups containing four to six elements. Some of the groups are given names - for example, the elements of the first group collectively are known as the alkali metals, the seventh group is the halogens and the eight group is the noble gases. Elements within each group have many properties in common. In addition, another 11 elements which are part of a much larger grouping known as the transition elements are listed in Table 2. Note that hydrogen does not belong to any group and is regarded as being an exceptional individual element.
Compounds. A COMPOUND differs from an element in that an element contains only a single type of atom while a compound consists of smallest units which contain at least two different types of atom (i.e. atoms of at least two different elements) joined together by chemical bonds. Compounds always contain atoms of their constituent elements in the same numerical ratio regardless of how the compound was prepared. Consequently analysis of any pure compound always returns the same result. Therefore a compound when pure is HOMOGENEOUS. Many compounds exist as discrete molecules. The smallest unit of the compound carbon dioxide is the carbon dioxide molecule, each of which consists of one carbon atom bonded to two oxygen atoms - this molecule is the smallest particle of carbon dioxide that can exist. The ratio of one carbon atom to two oxygen atoms in the molecule of this compound applies equally well to the carbon dioxide which may originate from burning of natural gas or petrol or from chemical reactions such as when an acid is mixed with a substance such as limestone or washing soda. Carbon dioxide molecules always consist of one carbon atom bonded to two oxygen atoms regardless of the source of the compound.
However, not all compounds exist as discrete molecules - for example the compound sodium chloride also known as table salt exists not as individual sodium chloride molecules but as a highly structured crystal lattice, familiar as the rock salt commonly used in salt grinders. There are several types of chemical bonds that join atoms in elements and compounds and the type of bond present determines in part whether a given compound exists as discrete molecules. Chemical bonds are examined in later Topics.
Note that when elements have combined to form compounds, they lose their original properties and the compound formed has its own characteristic properties. For example, carbon - a black solid - combines with another element, the colourless gas oxygen which supports combustion, to form the compound carbon dioxide which is a colourless gas that is incapable of supporting combustion. Compounds can only be converted back to their component elements by breaking the chemical bonds which hold the atoms together in the compound. For example, the compound water, which contains two hydrogen atoms and one oxygen atom
Chemical symbols. Each type of atom (i.e. each element) is conveniently represented by a symbol which denotes a single atom of that element. For example,
H hydrogen O oxygen C carbon N nitrogen S sulfur Cl chlorine He helium Al aluminium Cu copper
Note that each symbol always starts with an upper case letter, and that when a second letter is used, it is always written in lower case. Table 1 (page I-20) includes the symbols of all elements and Table 2 (page I-21) contains those elements whose symbols are frequently used in basic chemistry courses and which must be committed to memory.
Symbols can be used not just to represent single atoms of elements, but also in combinations to represent molecules of elements and also the formulas of compounds. In these cases, when there is more than a single atom of any element present, a subscript is used to show how many. For example, as water consists of molecules each containing two hydrogen atoms bonded to one oxygen atom, then the formula for the water molecule is H O. 2 Similarly, carbon dioxide molecules each contain one carbon atom bonded to two oxygen atoms, so the formula for its molecule is CO. 2 Each subscript specifies the number of atoms of the element immediately preceding that subscript. As another example, the formula for glucose, C H 6 12 O , shows that each molecule of this compound contains 6 carbon 6 atoms, 12 hydrogen atoms and 6 oxygen atoms bonded together.
As mentioned earlier, most elements do not occur naturally as single atoms ( MONATOMIC ), but instead as discrete molecules containing 2, 3 or more atoms ( DIATOMIC, TRIATOMIC ) or as large numbers of atoms bonded together. As an example, the usual form of the element oxygen in the atmosphere is as diatomic molecules of formula O 2 rather than as individual O atoms. The few elements which do occur in nature as monatomic species are those listed in the eighth group of Table 2, viz helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). This group of elements is known as the NOBLE GASES because they are almost inert. The only elements which occur naturally as diatomic molecules are hydrogen, nitrogen, oxygen, plus all the halogens, fluorine, chlorine, bromine and iodine. These eight elements normally exist as the molecular species H , N ,O , 2 2 2 F , Cl , Br 2 2 2 and I 2 respectively rather than in the form of single atoms. All metals and some non-metals usually occur as extremely large aggregates of atoms bonded together and are not usually considered to be composed of discrete molecules, so generally just the formula of the atom is used to represent these elements. For example, the metallic element sodium consists of large numbers of sodium atoms bonded together and this element is represented just by its atomic symbol, Na, with no subscript. Similarly the symbol of the single atom is used to represent all other metals such as iron (Fe), copper (Cu) and aluminium (Al). While those non- metallic elements listed above such as chlorine (Cl ), oxygen (O ) and hydrogen 2 2 (H ) which occur as diatomic molecules are shown as such in their formulas, some 2 other non-metals occur as large aggregates of atoms like the metals (although bonded differently) and are represented by their atomic formulas alone. A common example is the element carbon which occurs in nature as diamond, charcoal and graphite. Each of these forms of carbon contain large numbers of bonded carbon atoms but not discrete molecules, so carbon is represented by the atomic symbol C.
Check your understanding of this section. Which elements occur as diatomic molecules in nature? Which elements occur naturally as monatomic species? How would one recognise that the formula NaCl applied to a compound? Why can’t a mixture have a unique chemical formula? What does the formula for the molecule HCl indicate? Why would it be incorrect to write 2H rather than H 2 as the formula for the molecule of hydrogen?
Allotropes. A given element may occur with more than one arrangement of its constituent atoms. For example, carbon occurs naturally as the black amorphous powder called charcoal, as graphite which is used as the "lead" in pencils, and also as diamond. All three forms of carbon contain only carbon atoms, but the arrangement of those atoms in each form is different resulting in the very different physical properties of charcoal, graphite and diamond. The various forms in which an element may occur are called ALLOTROPIC MODIFICATIONS or simply ALLOTROPES of that element.
As another example, oxygen usually occurs in the atmosphere is a diatomic molecule. However, particularly in the upper atmosphere, a small amount of oxygen occurs as another, triatomic allotrope called ozone. What would the formulas be for these two allotropes?
SUMMARY
Chemistry is the study of matter and its interconversion. All matter is ultimately
composed of atoms. Matter which consists of only one type of atom is called an
element while matter whose constituent particles contain atoms of more than one
element is called a compound. Elements can be classified in many different ways. Some classifications rely on simple physical properties while others also
incorporate chemical properties. One useful classification of elements is into
metals vs non-metals.
The basic units of most substances however consist not of single atoms but usually of atoms which are joined to each other by chemical bonds of various types. These
basic units may consist of two or more atoms joined by chemical bonds to form
discrete entities called molecules or they may consist of extremely large and
indeterminate numbers of bonded atoms such as found in the metals or in crystalline compounds like sodium chloride. Pure substances contain just a single
element or compound and are homogeneous while the mixtures which constitute
most of the matter around us consist of more than one element and/or compound
physically mixed rather than chemically bonded and are heterogeneous. A given compound always has the same numerical ratio of its constituent atoms of
each component element, regardless of the way that compound was produced.
When elements are converted to compounds or compounds converted back to their
constituent elements, chemical bonds are broken and new bonds formed between the constituent atoms - this is termed a chemical change. Chemical change results
in the loss of the properties which characterised the original substances while the
products resulting have their own characteristic properties.
Mixtures are inherently impure, their composition varying with the sample analysed. Mixtures can be separated into their pure components by physical
processes such as distillation as there are no chemical bonds between those
components. Such procedures are called physical changes.
Atoms of elements are conveniently denoted by a symbol which represents a single atom of the particular element. Compounds can then be represented by a
combination of the symbols of the constituent atoms by using subscripts to show
the number of each atom present in the formula. Thus H O 2 represents two
hydrogen atoms and one oxygen atom combined to form a molecule of water. Of the elements, only the six noble gases occur in nature as the monatomic species.
The elements hydrogen, oxygen, nitrogen, fluorine, chlorine, bromine and iodine
occur naturally as the diatomic molecules of their atoms. Most elements including
all metals do not occur as discrete molecules at all but instead consist of extremely
large numbers of atoms bonded together. The number of constituent atoms in a given sample of such elements depends only on the size of the sample.
Some elements occur with more than one possible arrangement of the bonds
between their constituent atoms, and these different forms are called allotropes. An
example is the element carbon which occurs naturally as charcoal, graphite and diamond. The various allotropic modifications of an element normally result in
different physical properties and also may have some differing chemical properties.
TUTORIAL QUESTIONS - TOPIC 1.
- Define the following terms:
Element Atom Compound Mixture Electrolysis Chemical change Physical change Diatomic molecule Allotropes Molecule Monatomic element
- Give the name of each of the following elements. Avoid looking up the information in Table 2 as much as possible.
Li ........................ Ga ........................
Al ........................ B ........................
F ........................ Na ........................
Ne ........................ Ar ........................
I ........................ S ........................
Pb ........................ N ........................
K ........................ Cl ........................
Be ........................ As ........................
He ........................ Rn ........................
Cs ........................ Mg ........................
Br ........................ Se ........................
Ba ........................ O ........................
P ........................ C ........................
Kr ........................ Ge ........................
Tl ........................ Te ........................
Xe ........................ Sr ........................
Bi ........................ Sn ........................
Ca ........................ Rb ........................
In ........................ Si ........................
Sb ........................
- The following formulas apply to either an element or to a compound. Write beside each formula "element" or "compound" as appropriate. If the formula is for a compound, write the names of all the component elements in the compound and the number of atoms of each element present in the molecule.
CO 2 ............................................................
I 2 ............................................................
C H 6 12 O 6 ............................................................
Na ............................................................
N 2 ............................................................
SO 2 ............................................................
He ............................................................
B ............................................................
HCN ............................................................
H CO 2 3 ............................................................
H SO 2 4 ............................................................
Cu ............................................................
Pt ............................................................
Al ............................................................
Be ............................................................
SiI 4 ............................................................
PCl 3 ............................................................
Rb ............................................................
H PO 3 4 ............................................................
Ag ............................................................
N O 2 3 .............................................................
As O 2 3 ............................................................
HF ............................................................
ClO 2 ............................................................
Ba ............................................................
Bi ............................................................
- A sample of some elements will be provided for your inspection during the tutorial session. A competition within the group based on recognition of these elements will be organised.
CHEMICAL CROSSWORD No. 1(a) ELEMENTS AND THEIR SYMBOLS
Rules for this crossword:
- Give the symbol for any element where the name is provided or give the name of the element where a symbol is provided.
- The symbols must observe proper upper and lower case conventions for chemical symbols.
- Names may start with an upper case letter only where that letter is the initial letter of a formula to be entered in the other direction on the grid. All other letters in the name must be lower case.
ACROSS
1. N
- hydrogen
- Ar
- I
- tin
- manganese
- He
- barium
- phosphorus
- Al
- Sn
- caesium
- Bi
- Au
- chromium
- iodine
- Sb
- sulfur
- Li
- Ne
- aluminium
- potassium
- Br
- copper
- Ba
- gold
- Sr
- F
DOWN
- neon
- Rn
- mercury
- zinc
- As
- indium
- strontium
- Mg
- Ir
- boron
- Ra
- platinum
- Hg
- Cr
- B
- H
- carbon
- In
- silicon
- Te
- Na
- Kr
- Pb
- calcium
- bismuth
- selenium
- Fe
- thallium
- argon
CHEMICAL CROSSWORD No. 1(b) ELEMENTS AND THEIR SYMBOLS
Rules for this crossword:
- Give the symbol for any element where the name is provided or give the name of the element where a symbol is provided.
- The symbols must observe proper upper and lower case conventions for chemical symbols.
- Names may start with an upper case letter only where that letter is the initial letter of a formula to be entered in the other direction on the grid. All other letters in the name must be lower case.
ACROSS
- nitrogen
- phosphorus
- iodine
- arsenic
- Sb
- aluminium
- carbon
- Mn
- Cs
- silicon
- silver
- chromium
- Ca
- tin
- copper
- sulfur
- O
- C
- He
- manganese
- Sn
- bismuth
- Sr
- beryllium
- antimony
- strontium
- radon
- Be
- lithium
- gold
- hydrogen
- Mg
DOWN
- sodium
- platinum
- indium
- Ag
- Fe
- Ge
- Zn
- calcium
- Pb
- As
- Ar
- Hg
- sulfur
- Si
- cobalt
- Se
- B
- Xe
- F
- Ni
- Na
- bromine
- Ba
- In
- rubidium
- magnesium
- helium
- nickel
CHEMICAL CROSSWORD No. 1(b)
ELEMENTS AND THEIR SYMBOLS
1 2 3 4 5
6 7 8
9
10 11 12
13 14
15 16 17
18
19 20
21
22 23
24 25
26 27 28 29
30 31
32 33
34 35
36
37
38 39 40 41
42 43
44
45
46 47 48 49
50
- Mn manganese Ag silver
Fe iron Cu copper Ni nickel Zn zinc Cr chromium Pt platinum Au gold Hg mercury Cd cadmium H hydrogen Cl chlorine Br bromine N nitrogen S sulfur P phosphorus Si silicon
- Symbol Symbol
bromine Br helium He nitrogen N oxygen O sulfur S lithium Li beryllium Be rubidium Rb carbon C xenon Xe silicon Si selenium Se lead Pb antimony Sb neon Ne boron B thallium Tl aluminium Al iodine I radon Rn germanium Ge phosphorus P sodium Na magnesium Mg potassium K krypton Kr calcium Ca caesium Cs strontium Sr indium In tin Sn arsenic As bismuth Bi barium Ba gallium Ga chlorine Cl argon Ar fluorine F
- CO 2 compound; 1 carbon atom + 2 oxygen atoms
I 2 element
C H 6 12 O 6 compound; 6 carbon atoms + 12 hydrogen atoms + 6 oxygen atoms
Na element
N 2 element
SO 2 compound; 1 sulfur atom + 2 oxygen atoms
He element
B element
HCN compound; 1 hydrogen atom + 1 carbon atom + 1 nitrogen atom
H CO 2 3 compound; 2 hydrogen atoms + 1 carbon atom + 3 oxygen atoms.
H SO 2 4 compound; 2 hydrogen atoms + 1 sulfur atom + 4 oxygen atoms
Cu element
Pt element
Al element
Be element
SiI 4 compound; 1 silicon atom + 4 iodine atoms
PCl 3 compound; 1 phosphorus atom + 3 chlorine atoms
Rb element
H PO 3 4 compound; 3 hydrogen atoms + 1 phosphorus atom + 4 oxygen atoms
Ag element
N O 2 3 compound; 2 nitrogen atoms + 3 oxygen atoms
As O 2 3 compound; 2 arsenic atoms + 3 oxygen atoms
HF compound; 1 hydrogen atom + 1 fluorine atom
ClO 2 compound; 1 chlorine atom + 2 oxygen atoms
Ba element
Bi element
CHEMICAL CROSSWORD No.1(b)
ELEMENTS AND THEIR SYMBOLS
N P I A s
a n t i m o n y i g
r A l e
z o C l v r
i m a n g a n e s e m
n r a r a
c a e s i u m d n
r e e S i
A g n C r u
o i c a l c i u m
S n c C u
i C r s
l b o x y g e n f
i o e l l
c a r b o n h e l i u m
o o o n o
n n M n t i n r
n u B i
s t r o n t i u m n
o c B e
d S b k S r i
i R a e R n
u b e r y l l i u m d
m i L i
A u M H N u
m a g n e s i u m
TABLE 1 - ALPHABETIC LIST OF THE ELEMENTS
ELEMENT SYMBOL
ATOMIC NUMBER
ATOMIC MASS ELEMENT SYMBOL*
ATOMIC NUMBER
ATOMIC MASS* Actinium Ac 89 (227) Mendelevium Md 101 (256) Aluminium Al 13 26.98 Mercury Hg 80 200. Americium Am 95 (243) Molybdenum Mo 42 95. Antimony Sb 51 121.8 Neodymium Nd 60 144. Argon Ar 18 39.95 Neon Ne 10 20. Arsenic As 33 74.92 Neptunium Np 93 (244) Astatine At 85 (210) Nickel Ni 28 58. Barium Ba 56 137.3 Niobium Nb 41 92. Berkelium Bk 97 (247) Nitrogen N 7 14. Beryllium Be 4 9.012 Nobelium No 102 (253) Bismuth Bi 83 209.0 Osmium Os 76 190. Bohrium Bh 107 (262) Oxygen O 8 16. Boron B 5 10.81 Palladium Pd 46 106. Bromine Br 35 79.90 Phosphorus P 15 30. Cadmium Cd 48 112.4 Platinum Pt 78 195. Calcium Ca 20 40.08 Plutonium Pu 94 (242) Californium Cf 98 (249) Polonium Po 84 (209) Carbon C 6 12.01 Potassium K 19 39. Cerium Ce 58 140.1 Praseodymium Pr 59 140. Caesium Cs 55 132.9 Promethium Pm 61 (145) Chlorine Cl 17 35.45 Protactinium Pa 91 (231) Chromium Cr 24 52.00 Radium Ra 88 (226) Cobalt Co 27 58.93 Radon Rn 86 (222) Copper Cu 29 63.55 Rhenium Re 75 186. Curium Cm 96 (247) Rhodium Rh 45 102. Dubnium Db 105 (262) Rubidium Rb 37 85. Dysprosium Dy 66 162.5 Ruthenium Ru 44 101. Einsteinium Es 99 (254) Rutherfordium Rf 104 (261) Erbium Er 68 167.3 Samarium Sm 62 150. Europium Eu 63 152.0 Scandium Sc 21 44. Fermium Fm 100 (253) Seaborgium Sg 106 (266) Fluorine F 9 19.00 Selenium Se 34 78. Francium Fr 87 (223) Silicon Si 14 28. Gadolinium Gd 64 157.3 Silver Ag 47 107. Gallium Ga 31 69.72 Sodium Na 11 22. Germanium Ge 32 72.59 Strontium Sr 38 87. Gold Au 79 197.0 Sulfur S 16 32. Hafnium Hf 72 178.5 Tantalum Ta 73 180. Hassium Hs 108 (265) Technetium Tc 43 (98) Helium He 2 4.003 Tellurium Te 52 127. Holmium Ho 67 164.9 Terbium Tb 65 158. Hydrogen H 1 1.008 Thallium Tl 81 204. Indium In 49 114.8 Thorium Th 90 232. Iodine I 53 126.9 Thulium Tm 69 168. Iridium Ir 77 192.2 Tin Sn 50 118. Iron Fe 26 55.85 Titanium Ti 22 47. Krypton Kr 36 83.80 Tungsten W 74 183. Lanthanum La 57 138.9 Uranium U 92 238. Lawrencium Lr 103 (257) Vanadium V 23 50. Lead Pb 82 207.2 Xenon Xe 54 131. Lithium Li 3 6.941 Ytterbium Yb 70 173. Lutetium Lu 71 175.0 Yttrium Y 39 88. Magnesium Mg 12 24.31 Zinc Zn 30 65. Manganese Mn 25 54.94 Zirconium Zr 40 91. Meitnerium Mt 109 (266)
*All atomic masses are given to four significant figures. Values in parentheses represent the mass number of the most stable isotope.
