Atoms and Molecules

The Atom

When scientists started exploring matter, they realised that matter can be divided into smaller and still smaller particles. What was the ultimate particle like? They discovered that the smallest particle of an element that maintains its chemical identity through all chemical and physical changes is called and 'atom'. John Dalton (1766 - 1844) can rightly be called the father of the Modern Theory on Atoms. He proposed his Atomic Theory in 1808, i.e., almost 200 years back. He did not have the help of sophisticated instruments that are available today to the scientists. Hence, many of his proposals, have been modified and updated. Over the years, substantial changes have taken place regarding the atomic theory, yet some of the assumptions that Dalton made are still held valid.

Atom was derived from the Greek word 'atoms', meaning 'indivisible'. Thus, it was considered to be indivisible. Molecules

Dalton's Atomic Theory

John Dalton provided a simple theory of matter to provide theoretical justification to the laws of chemical combinations in 1805. The basic postulates of the theory are:

• All substances are made up of tiny, indivisible particles called atoms.
• Atoms of the same element are identical in shape, size, mass and other properties.
• Each element is composed of its own kind of atoms. Atoms of different elements are different in all respects.
• Atom is the smallest unit that takes part in chemical combinations.
• Atoms combine with each other in simple whole number ratios to form compound atoms called molecules.
• Atoms cannot be created, divided or destroyed during any chemical or physical change.

Nature of Atom

At present we know that the atom is the smallest particle of an element, which may or may not have independent existence. It is made up of sub-atomic particles like electrons, protons and neutrons. Atoms of one type of element differ from those of the other due to different number of sub-atomic particles.

Representation of an Atom by a Symbol

A symbol signifies a shorthand representation of an atom of an element. The symbol of any element is based on the English name or Latin name (written in English alphabets) and is represented by using only its first letter or the first letter and another letter.

• The first letter is written in capital and the second letter in small.
• The second letter can be any letter in the name of the element, which is internationally accepted.

Symbols of Common Metals

Sl. No	Name in English	Name in any other Language other than English	Symbol
1.	Lithium	-	Li
2.	Sodium	Natrium (latin)	Na
3.	Magnesium	-	Mg
4.	Aluminium	-	Al
5.	Potassium	Kalium (latin)	K
6.	Calcium	-	Ca
7.	Vanadium	-	V
8.	Chromium	-	Cr
9.	Manganese	-	Mn
10.	Iron	Ferrum (latin)	Fe
11.	Cobalt	-	Co
12.	Nickel	-	Ni
13.	Copper	Cuprum (latin)	Cu
14.	Zinc	-	Zn
15.	Gallium	-	Ga
16.	Strontium	-	Sr
17.	Molybdenum	-	Mo
18.	Silver	Argentum (latin)	Ag
19.	Cadmium	-	Cd
20.	Tin	Stannum (latin)	Sn
21.	Antimony	Stibium (latin)	Sb
22.	Barium	-	Ba
23.	Tungsten	Wolfram (German)	W
24.	Platinum	-	Pt
25.	Mercury	Hydrargyrum (latin)	Hg
26.	Lead	Plumbum (latin)	Pb
27.	Bismuth	-	Bi
28.	Polonium	-	Po
29.	Radium	-	Ra
30.	Uranium	-	U

Symbols of Chemically Active Non-metals

Sl. No	Non - metal	Physical State	Symbol
1.	Hydrogen	Gas	H
2.	Nitrogen	Gas	N
3.	Oxygen	Gas	O
4.	Fluorine	Gas	F
5.	Chlorine	Gas	Cl
6.	Bromine	Liquid	Br
7.	Astatine	Solid	At
8.	Carbon	Solid	C
9.	Iodine	Solid	I
10.	Sulphur	Solid	S
11.	Phosphorus	Solid	P
12.	Silicon	Solid	Si

Symbols of Chemically Inactive Non-metals or Noble Gases

Sl. No	Nobel Gas	Symbol
1.	Helium	He
2.	Neon	Ne
3.	Argon	Ar
4.	Krypton	Kr
5.	Xenon	Xe
6.	Radon	Rn

 

The names of the elements had originated from different sources in different ways. Some of them are discussed here. For example,

• Hydrogen was named by Lavoisier using the Greek words 'Hydro' ie., water and 'Genes' meaning forming.
• Chlorine has its origin from Greek word - 'Chloros' - meaning greenish yellow, 'Bromos' means stench in Greek and 'Iodes' in Greek means violet.
• The word nickel has originated from a German word meaning 'Satan' or 'Old nick'. Cobalt comes from a German word - 'Globlin' or 'Evil Spirit'.
• Argon comes from a Greek word meaning 'Inactive'.
• Gold is an Anglo-Saxon word - 'Aurum'.

Elements Named After Places

Scandium (Sc) - found in ScandinaviaThulium (Tm) - named after Thule the earlier name of ScandinaviaEuropium (Eu) - after the continent EuropePolonium (Po) - named after the Curies after their home town in Poland

Named After Planets

Selenium (Se) - 'Seles' Greek name for the moonPlutonium (Pu) - Neptunium (Np) Uranium (U)Mercury was named after a planet but derives its symbol Hg from the Latin word 'Hydragyrum' meaning liquid silver

Named After Scientists

Curium (Cm) after Pierre and Marie CurieFermium (Fm) after Enrico FermiEinsteinium (Es) after Albert EinsteinMendelevium (Md) after Dimitri Mendeleev

Significance of a Symbol of an Element

Qualitative

• It represents a specific element.
• It represents one atom of an element.

Quantitative

• It represents how many times an atom is heavier than one atomic mass unit.
• It represents one-gram atom of an element.
• It represents how many atoms are present in one-gram atom (1 mole)For example, 12 grams of carbon has 6.023 x 10²³ atoms in it.

Size of the Atom Elements

Atoms are infinitesimally small. Smaller than any thing we can imagine. One hydrogen atom, the smallest atom known, is approximately 5 x 10^-8 mm in diameter. To put that in perspective, it would take almost 20 million hydrogen atoms to make a line as long as this dash ' - '.

The extremely small sizes of atoms are exceedingly impractical to work with; special measuring units have been devised to determine the dimension of atoms in terms of mass. The mass or weight of an atom can be computed in the following ways.

Mass of the Atom

Experiments have determined that the mass of an atom is very small, ranging from 1.7x10^-24 g to about 4.0x10^-22 g. It is very difficult to accurately weigh a bunch of atoms, as they are extremely light, small, and invisible; they weigh too little for us to express their masses in grams or kilograms. As a consequence, it is necessary to reduce the atomic masses into simple figures.

Atomic Mass

An indirect method to determine the absolute mass of an atom has been devised. The mass of a particular atom is taken as a standard unit and the masses of other atoms are related to this standard. Hydrogen being the lightest element and being the smallest atom was chosen and assumed to have a mass of 1. An atom of hydrogen was assigned an atomic mass equal to one atomic mass unit (a.m.u). The number does not signify the mass of an atom in grams. It is just a pure number. The masses of atoms of other elements were compared to that of hydrogen, in order to find their atomic mass relative to it. If one atom of sulphur weighs as much as 32 atoms of hydrogen, then the relative atomic mass of sulphur is 32 a.m.u.

This way of defining the mass of one atom of hydrogen has its difficulties. While the mass of one atom of hydrogen is considered as 1 atomic mass unit, hydrogen gas in its natural state has 3 isotopes of atomic mass 1, 2 and 3 respectively. Thus average mass works out to be 1.008 a.m.u rather than 1 a.m.u. This in turn complicates the atomic masses of all other elements. Later on, an atom of oxygen was preferred as standard by taking its mass as 16 units. In 1961, the International Union of Chemists selected the most stable isotope of carbon (C-12 isotope) as standard for comparison with other atoms or molecules.

Atomic Mass Unit

The mass of 1/12^{th 12}C isotope of carbon is now the standard used for defining one atomic mass unit (a.m.u). It is equal to 1.66 x 10^-24 g.

Atomic Mass of an Element or Relative Atomic Mass

The atomic mass of an element is a relative quantity and is the mass of one atom of the element relative to the mass of one carbon-12 atom. Thus,Relative atomic mass (RAM) of an element is the number of times one atom of an element is heavier than 1/12 the mass of an atom of carbon [C-12].

For example, if 1 atom of Na weighs as much as 23 parts of isotopes, then the atomic mass of sodium is 23 a.m.u.

Remember:

Atomic Mass Unit Scale

The scale in which the relative atomic masses of different atoms are expressed is called atomic mass unit scale.

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Atomic masses are not expressed as whole numbers because natural elements are mixtures of two or more isotopes. The atomic weight of chlorine is 35.43 amu. Chlorine exists as two isotopes in the ratio 3:1 

Therefore average atomic masses are not whole numbers.

Gram Atomic Mass (GAM)

The atomic mass of an element expressed in grams is called the gram atomic mass of an element. It is also called gram atom.

Example: The gram atoms present in 46 grams of sodium

Gram atomic mass is the relative atomic mass of an element expressed in grams.