Listen and Learn Science/Atomic Units

Atomic Size.
An atom, is very very small in size, and mass. An atom is so small, that we cannot see it, even in a microscope. When we discuss things in an atomic scale, we need to visualise, very large, and very small numbers.

Very Large numbers.
Very large numbers can be expressed as, 10 to the power of some number. Ten to the power of three, is thousand. A Thousand, is also called a kilo. So, a kilogram is thousand grams. A kilometre, is thousand meters. Ten to the power of six, is a million, which has a prefix of mega. A megabyte, is million bytes. Ten to the power of nine, is a billion, which has a prefix of giga. A gigabyte, has a billion bytes. There are about, 1.2 billion human beings, in India. Ten to the power of 12, is one trillion, which has a prefix, of terra. A terabyte, has a trillion bytes. There are trillions of living cells, in our body. Ten to the power of 15, is one quad trillion, which has a prefix of peta. Ten to the power of 18, is one quintillion, which has a prefix of exa. Ten to the power of 21, is one sextillion, which has a prefix of zetta. Ten to the power of 24, is one septillion, which has a prefix of yota. One septillion, is one followed by 24 zeros. We have to stretch our imagination, to understand such large numbers. To get an idea how big this number is, let us assume, it will take one second, to count one number. To count up to 10, will take ten seconds. To count up to 1000, we will need 1000 seconds, or about 17 minutes. To count up to 1 million, we will require 278 hours, that is more than 10 days. To count up to a billion, we will require 27 years. To count up to a trillion, we need 27000 years. Now we have an idea, of how big 10 to the power of 12, or a trillion is. We should now try and imagine how big a number, a septillion or 10 to the power of 24 is.

Very Small numbers.
Numbers less than one, are decimals. We can express decimals and small numbers as, 10 to the power of some negative number. Ten to the power of minus three, is one by 1000, or point zero zero one. Ten to the power of minus three, is mille. So mille meter, is one thousandth of a meter. A mille gram, is one thousandth of a gram. Ten to the power of minus six, is micro. Very fine dust size is about 2.5 microns. The presence of such particles is used, as a measure of atmospheric pollution. Our blood cells, will be a few microns, in size. We can see them only, with a micro scope. Ten to the power of minus nine, is a billionth. It is also called a nano. Ten to the power of minus 12, is a trillionth. Ten to the power of minus 15, is a quintillionth. Ten to the power of minus 21, is a sextillionth. Ten to the power of minus 24, is a septillionth. These are obviously very very small. Now we are ready to understand, the scale of of an atom.

Constituents of an atom.
A atom has protons, neutrons and electrons. Any element has the same protons, neutrons and electrons. The size of a proton, a neutron and a electron, is always the same. The mass of a proton, a neutron and a electron, is always the same. A light element like hydrogen, and a heavy element like Iron, has the same protons, neutrons and electrons.

What makes one element different from another ? It is the number of protons and neutrons, in the nucleus, which makes them different. A heavier atom has more protons and neutrons, packed in the nucleus. The size of the atom remains the same. A heavier atom has a heavier nucleus.

A atom of an element, is defined by it atomic number, and atomic mass. Carbon, C 12, has 6 protons and 6 neutrons, or 12 nucleons. The weight of each nucleon is about the same. The mass of an electron, is very small compared to a nucleon. The mass of an atom, is the sum of the weight of the nucleons, and electrons.

Let us imagine, each nucleon to be a very tiny ball. The mass of each ball is the same. You know the mass of each ball. Imagine you are carrying a small bag, of weight say 12 grams, We can calculate the number of nucleons, it contains. If we say carbon has 12 balls, We can compute the number of carbon atoms. With some minor refinement, this concept can be, extended, to define the concept of a Mole.

Mole.
A mole is the number of atoms present, in 12 grams of pure carbon C 12. The unit the mole is m o l. Scientists have calculated this number, to be 6.022 multiplied by 10, to the power of 23. That is there are 602 sextillion atoms, in 12 grams of carbon. This gives us an idea of, how large a number of atoms are present, in a small amount of substance. It also gives us an idea, how small an atom is. Carbon is the same element, which is present in your pencil. When you write your name with a pencil, you are probably using trillions of carbon items. This might give you some idea, of some atomic scales. This number of atoms in a mole, is called the Avogadro constant.

Gram Atomic Mass.
Gram atomic mass of an element, is the atomic mass of the element, in grams. Examples. Mass number of Hydrogen is 1. The gram atomic mass, of hydrogen, is one gram. Mass number of Carbon is 12. Gram atomic mass, of carbon, or C 12 is 12 grams. Mass number of oxygen is 16. Gram atomic mass, of oxygen, or O 16 is 16 grams. Mass number of Iron is 55. Gram atomic mass, of Iron, or F e 55 is 55 grams.

The beauty of the Avogadro constant, is that it is the same for all elements. The mass of one mole of a element, expressed in grams, is equal to the element’s, gram atomic mass. 1 gram of hydrogen will have 602 sextillion atoms, and is called as 1 mole of Hydrogen. 12 gram of Carbon will have  602 sextillion atoms, and is called as 1 mole of Carbon. 16 grams of oxygen will have 602 sextillion atoms, and is called as 1 mole of Oxygen. 55 grams of Iron will have   602 sextillion atoms, and is called as 1 mole of Iron. 1 mole of any substance will always have, 602 sextillion atoms. This property is widely used in science.

Atomic Mass Unit.
The mass of an atom is very small. The mass of an atom is roughly, the mass of all the protons, neutrons and electrons. A pure carbon atom is C 12. Its mass number is 12. It has 12 nucleons, or 6 protons, 6 neutrons, plus 6 electrons. One twelfth the mass, of a pure carbon C 12 atom, is defined as one atomic unit. One atomic mass unit, is also called as A M U.

Mass of an A M U.
We know that, 12 grams of C 12 has, 6.022 multiplied by 10, to the power of 23 atoms. Carbon has 12 nucleons and 12 electrons. Let us say that one basic atom has 1 nucleon and 1 electron. We will call this as 1 Atomic mass unit, or 1 A M U. So the mass of one A M U will be, one divided by, 6.022 multiplied by 10, to the power of 23 atoms. One A M U, is equal to 1.66 multiplied by 10, to the power of minus 24 grams. This gives an idea how small an atom is, and how little mass it has. More interestingly, it means, we can compute the number of atoms, in a given mass, of any substance.

Relative Atomic Mass.
Different elements have different atomic mass units. A simple way, to understand the relative mass, is through the mass number. We know that, the mass number represents the number, of protons and neutrons. The mass number, is a good way to approximately know, how heavy an atom is, compared to a hydrogen atom. The mass number, of hydrogen is one. The mass number, of carbon is twelve. So, we can say that, carbon atom will be twelve times heavier, than a hydrogen atom. The mass number, of oxygen is 16. We can expect oxygen to be approximately, 16 times heavier than a hydrogen atom.

Elements have small quantities of isotopes. The mass numbers of isotopes are different, from the mass number of the pure element. For example pure carbon, C 12, has an atomic mass number, of 12. Carbon has an isotope C 14, which is also present, in natural carbon. The relative atomic mass, is calculated taking into account, the pure substance and the isotopes. The relative atomic mass, of carbon is 12.01. This is slightly different, from the mass number, which is 12. This is to account for very small amount of carbon Isotopes. Many other elements, also have isotopes. The relative mass of many elements, are not whole numbers.

Precise Relative atomic mass.
The relative atomic mass of an element, is the ratio of the mass of atoms, to one 12th the mass, of a carbon C 12 atom. Scientists have calculated precisely, the relative atomic mass, of all elements. This is based on the amount of isotopes present, of the element in nature, and other factors. This relative atomic mass, of each element is widely published. We can use, these published values, as it is.

The Precise relative atomic mass of some elements are given below. Hydrogen. Symbol H.

Atomic mass number, equals 1. Relative atomic mass, equals 1.008.

Carbon. Symbol C.

Atomic mass number, equals 12. Relative atomic mass, equals 12.01.

Nitrogen. Symbol N.

Atomic mass number, equals 14. Relative atomic mass, equals 14.01.

Oxygen. Symbol O.

Atomic mass number, equals 16. Relative atomic mass, equals 16.

Sodium. Symbol N a.

Atomic Mass number, equals 22. Relative atomic mass, equals 22.98.

Aluminium. Symbol A l.

Atomic Mass number, equals 26. Relative atomic mass, equals 26.98.

Chlorine. Symbol C l.

Atomic Mass number, equals 35. Relative atomic mass, equals 35.45.

Iron. Symbol F e.

Atomic Mass number, equals 55. Relative atomic mass, equals 55.84.

For convenience and ease of understanding, we will discuss the pure form, or commonly occurring form, of an element. For example, when we say carbon, we will refer to C 12. When we say oxygen, we will refer to O 16.

Valence.
Atoms of an element have a tendency to combine, with atoms of another element. This tendency to combine varies, from one element to another. This tendency to combine with another element, is called the Valence of the element. The Valence of hydrogen is different, from the Valence of carbon, which is different from, the Valence of oxygen. So is the case with other elements. Interestingly, the Valence of an atom, of an element, correlates to the way, the electrons are distributed in the shells, surrounding the nucleus. The first shell, called the K shell, can hold 2 electrons. The second shell, called the L shell, can hold 8 electrons. The third shell, called the M shell, can hold 18 electrons. If the number of electrons, in the outer most shell, is occupied to full capacity, the atom does not have a tendency, to combine with other atoms. Also, if the number of electrons, in the outer most shell is equal to 8, the atom does not have a tendency, to combine with other atoms. In all other cases, the atom has a tendency, to combine with other atoms.

Noble Gases.
Some gases do not combine with other elements. They are called Neutral, Inert or Noble Gases.

Helium has atomic number of 2. It has two electrons, in the first, or K shell. The K shell is full. So, helium, a inert gas, does not combine, with other elements. We can also say, the Valence of helium, is 0.

Neon has a atomic number of 10. It has a total of 10 electrons. The K shell has 2 electrons. The L shell has 8 electrons. Since the outer most shell has 8 electrons, Neon does not have a tendency to combine, with other elements. We can also say, the Valence of neon, is 0. Other examples of inert gases are, Argon, and Krypton.

Valence of elements.
Hydrogen has an atomic number of 1. It has one electron, in the K shell. The K shell has a capacity of 2, so there is scope for hydrogen, to accommodate one electron. When there is such a capacity, atoms tend to share an electron, from another atom. In this case, it has a capacity to share, one electron. Its Valence or combining power, is 1.

Carbon has an atomic number of 6. It has two electrons in the K shell. It has 4 electrons in the L shell. The capacity of the L shell is 8. So carbon has the capacity, to share four more electrons. The Valence of carbon, is 4.

Oxygen has an atomic number of 8. It has 2 electrons in the K shell. It has 6 electrons in the L shell. The capacity of the L shell is 8. So oxygen has the capacity, to share two more electrons. The Valence of oxygen is 2.

Molecules.
Molecules are combinations, of atoms. Usually molecules, are combinations of atoms, of different elements. In some cases, it can be the combination of atoms, of the same element. Hydrogen exists in nature, as a molecule, of two hydrogen atoms. Two atoms of hydrogen, share one electron. So the K shell of the hydrogen atom, has 1 of its own electron, and 1 shared electron. This fills the K shell. So hydrogen occurs, as H 2 molecules in nature. 2 is written as a subscript. So H subscript 2, is the natural way hydrogen exists.

Water is a molecule, of 2 hydrogen atoms, and 1 oxygen atom. It is represented, as H 2 O. The oxygen atom, shares 1 electron, with each hydrogen atom. The K shell of each hydrogen atom, gets full filled with, one shared atom with oxygen. K shell, has one own electron, and one shared electron. The L shell of the oxygen atom, gets full filled with, one shared atom, from each hydrogen atom. L shell has six of its own electrons, and two shared electrons, making a total of 8 electrons. 8 in the outer most shell, we know has a stabilising effect. The valence of both oxygen and hydrogen, are satisfied, So, Water is a stable molecule.

Carbon dioxide is a molecule, of 1 carbon and 2 oxygen atoms. It is represented, as C O 2. Atomic number of carbon is 6. The K shell has 2 electrons. The L shell has 4 electrons. The atomic number of oxygen is 8. The K shell has 2 electrons. The L shell has 6 electrons. Carbon shares 2 electrons with each, of the two oxygen atoms. The L shell of the carbon atom, has 4 of its own electrons, and 4 shared electrons, for a total of 8 electrons. The L shell of the oxygen atom, has 6 of its own electrons, and 2 shared electrons, for a total of 8 electrons. So C O 2 is a stable molecule.

This is the way, all the molecules, are formed. Elements combine with other elements to satisfy, their valence.

Molecular mass of a compound.
The molecular mass of a compound, is the total of the mass, of the atoms in it. Water is H 2 O. H 2 O has two hydrogen atoms, and one oxygen atom. Molecular mass of hydrogen is 1. Molecular mass of oxygen is 16. So the molecular mass of H 2 O, is 2 multiplied by one, which is two, plus sixteen, equal to 18.

Carbon dioxide, we breathe out is C O 2. C O 2 has one carbon atom and two oxygen atoms. Molecular mass of carbon is 12. Molecular mass of oxygen is 16. So the molecular mass of C O 2 is 12, plus, 2 multiplied by 16, equal to 44.

Relative Molecular Mass.
Relative Molecular mass of a substance, is the ratio of the mass, of the substance, to one twelfth, the mass of an atom, of C 12. Thus, the relative molecular mass shows, how many times a molecule is heavier than, one twelfth of a carbon, C 12 atom. If we take only, the pure atoms as discussed, the relative molecular mass will be the same, as the molecular mass.

Gram Molecular Mass.
Gram molecular mass of a substance, is the relative molecular mass, expressed in grams. Some examples, of gram molecular mass. Hydrogen exists in nature, as a combination of two hydrogen atoms, H subscript 2. The gram molecular mass, of hydrogen is 2 grams. Oxygen exists in nature, as a combination of 2 oxygen atoms, O subscript 2. The gram molecular mass, of oxygen is 32 grams. Molecular mass of water is 18. The gram molecular mass, of water is 18 grams. Molecular mass, of carbon dioxide is 44. The gram molecular mass, of carbon dioxide is 44 grams.

The mass of one mole of a substance, expressed in grams, is equal to the substance’s, gram atomic mass. This principle is applicable, for molecules also. The mass of one mole of a molecule, expressed in grams, is equal to the molecule’s, gram molecular mass.

A gram atomic mass of a substance always contain, 602 sextillion molecules. 1 mole of any substance will always have, 602 sextillion atoms. This is a very significant, and universal relationship. Eighteen grams of H 2 O, has 602 sextillion molecules of H 2 O, and is called a mole of H 2 O. Forty four grams of C O 2, has 602 sextillion molecules of C O 2, and is called a mole of C O 2. This concept becomes useful, in chemical reactions and equations. It is used to express, the amount of reactants, and products, in a chemical reaction. 2 Mols of H 2 reacts, with with 1 mol of O 2, to form 2 mols of H 2 O.