Introduction to compounds and elements
Compounds:
The compound is defined as a pure substance containing two or more elements which are combined together in a fixed proportion by mass.
Elements:
An element is the simplest or basic form of a pure substance which cannot be broken into anything simpler than it by physical or chemical methods. The pure substance which is made up of one kind of atoms only. The common examples of elements are hydrogen, carbon, , sulphur, gold etc.
Elements are further classified into three types.
Metals
Non-metals
Semi-metals
There are few elements which possess the characteristics of both metals and non-metals. These are actually border-line elements and are known as semi-metals. Semi-metals are also called as metalloids. A few common examples are: Arsenic, Antimony and Bismuth.
Types of elements based on physical states:
Based on physical states, the elements have been classified as solids, liquids and gases.
Solid elements: Most of the elements are solids at room temperature. For example, copper, silver, gold, potassium, carbon (diamond, graphite), iodine, phosphorous etc.
Liquid elements: Only mercury and bromine exist as liquid at room temperature. Gallium and cesium become liquids at a temperature 302 K and 303 K respectively. These are slightly higher than the room temperature (298 K).
Gaseous elements: Eleven elements exist in the gaseous state at room temperature. These are hydrogen, oxygen, nitrogen, fluorine, chlorine, helium, neon, argon, krypton, xenon and radon.
A compound is also a pure substance like elements. But it represents a combination of two or more elements which are combined chemically.
Types of compounds
The compounds have been classified into two types
Organic compounds
Organic compounds are the compounds which are obtained from living beings (plant and animal). It has been found that all the organic compounds contain carbon as their essential constituent. The organic compounds are quite often known as carbon compounds.
Examples: Methane, ethane, propane, alcohol, etc.
Inorganic compounds
Inorganic compounds have mostly obtained from non-living sources such as rocks and minerals.
Example: Salt, marble, washing soda, baking soda, etc.
Characteristics of compounds:
John Dalton, a British school teacher, published his theory about atoms in the year 1808. His findings were based on experiments and also from laws of chemical combination.
Main assumptions or postulates of Dalton
* All matter consists of indivisible particles called atoms.
* Atoms of the same element are similar in shape and mass, but differ from the atoms of other elements.
* Atoms cannot be created or destroyed.
* Atoms of different elements may combine with each other in a fixed, simple, whole number ratio to form compound atoms.
* Atoms of same element can combine in more than one ratio to form two or more compounds.
* Atoms are the smallest unit of matter that can take part in a chemical reaction.
Drawbacks of Dalton’s atomic theory of matter
* The indivisibility of an atom was proved wrong, for, an atom can be further subdivided into protons, neutrons and electrons. However an atom is the smallest particle, which takes part in chemical reactions.
* According to Dalton, the atoms of same element are similar in all respects. This is wrong because atoms of some elements vary in their mass and density. Such atoms of the same element having different masses are called isotopes. For example, chlorine has two isotopes having mass numbers 35 a.m.u and 37 a.m.u.
* Dalton also said atoms of different elements are different in all respects. This has been proved wrong in certain cases like argon and calcium atoms, which have the same atomic mass of 40. Such atoms of different elements that have the same atomic mass are called isobar.
* According to Dalton atoms of different elements combine in simple whole number ratio to form compounds. This is not seen in complex organic compounds like sugar C12H22O11.
* The theory completely fails to explain the existence of allotropes. The difference in properties of charcoal, graphite, diamond went unexplained in spite of being made up of same kind of atoms.
Merits of Dalton’s atomic theory
* It has enabled us to explain the laws of chemical combination.
* Dalton was the first person to recognize a workable distinction between the ultimate particle of an element (atom) and that of a compound (molecule).
This is the 2007 list of atomic weights of the elements, as accepted by the IUPAC. The table is based on the 2005 table at Pure Appl. Chem., 78, 2051-2066 (2006), including the 2007 changes to the atomic weight values for lutetium, molybdenum, nickel, ytterbium and zinc. The elements are listed by increasing atomic number. The number in parenthesis indicates the uncertainty in the last digit of the atomic weight.
Atomic Number – Symbol – Name – Atomic Weight
1 H – Hydrogen – 1.00794(4)
2 He – Helium – 4.002602(2)
3 Li – Lithium – 6.941(2)
4 Be – Beryllium – 9.012182(3)
5 B – Boron – 10.811(7)
6 C – Carbon – 12.0107(8)
7 N – Nitrogen – 14.0067(2)
8 O – Oxygen – 15.9994(3)
9 F – Fluorine – 18.9984032(5)
10 Ne – Neon – 20.1797(6)
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Here’s a list of chemical elements ordered by increasing atomic number. The names and element symbols are provided.
1 – H – Hydrogen
2 – He – Helium
3 – Li – Lithium
4 – Be – Beryllium
5 – B – Boron
6 – C – Carbon
7 – N – Nitrogen
8 – O – Oxygen
9 – F – Fluorine
10 – Ne – Neon
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What does the p in pH stand for?
The term pH has been in use for more than a century. It is a logarithmic measure of the hydrogen ion concentration ([H+]): pH = -log10[H+]. (Technically, there aren’t bare protons (H+) floating around in solutions, but that wasn’t known when pH was introduced!) The original symbol used by Sorensen was pH+.
Theories vary as to the origin of the p – most agree it means power but whether in Latin, French or German, seems in dispute. Thinking it would be either French or Latin as the original paper was published in French, I was surprised to find that it’s neither, though the legend is both old and persistent. By 1920, many authors were assuming that it meant “power”, but Jens Norby returned to the original sources and points out that it was the arbitrary choice of the letters p and q for two variables in the work-up of the experimental data. The variable p eventually ends up in the formula arrived at for the concentration of the hydrogen ion.
The modern form pH was introduced in 1920, “as a matter of typographical convenience”.
For the full explanation, see Jens G. Norby, The origin and the meaning of the little p in pH, Trends in Biochemical Sciences 25, 36-37 (2000). The illustration is a selection from the original paper: Sorensen, Compt. redn. du Lab. de Carlsberg 8 1-168 (1909).
