Chapter 1 - BASIC CONCEPTS - ISOTOPES

ISOTOPES AND THEIR RELATIVE ABUNDANCE

Atoms of same element having same atomic number but different atomic weights are called isotopes. This phenomenon is called isotropy. It was discovered by Soddy. Isotopes have same number of elections, protons and same electronic configuration. They differ in number of neutrons present in nucleus. The isotopes have same chemical properties. Consider example of hydrogen, it has three isotopes, i.e.

Thus we can define isotopes as atoms of same element which differ in number of neutrons in nucleus. Carbon has three isotopes, ₆C¹², ₆C¹³, ₆C¹⁴. Each one of them have 6 electrons and 6 protons but they have 6, 7, 8 neutrons respectively. The number of isotopes of some other elements are O = 3, Ni = 5, Ca = 6, Pd = 6, Cd = 9, Sn = 11.

RELATIVE ABUNDANCE OF ISOTOPES

The isotopes of all elements have their own natural abundance. This relative abundance is determined from mass spectrometry. The properties of element which are written in books are that of most abundant isotope of an element.

There are 280 stable naturally occurring isotopes in nature. There are 40 radioactive isotopes. Besides 300 unstable radioactive isotopes have been produced by artificial radioactivity.

The number of isotopes of an element is a complex property. Some general information is given over hare.

MONOISOTOPIC ELEMENTS: These elements only single isotope for example Gold, Iodine, Fluorine, Arsenic.

ODD ATOMIC NUMBER: Elements having odd atomic number never possess more than two stable isotopes.

EVEN ATOMIC NUMBER: Elements with even atomic no have large number of isotopes.

MASS NUMBER MULTIPLE OF FOUR: The isotopes with mass number which is multiple of four are quite abundant. For example ₈O¹⁶, Mg²⁴, Si³², Ca⁴⁰, Fe⁵⁶ are almost 50% of earth crust.

EVEN MASS NO AND ATOMIC NO: Out of 280 naturally occurring isotopes, 154 have even mass number and even atomic number.

MASS SPECTROMETRY: Determination of relative atomic masses of isotopes.

Mass spectrometer is an instrument which is used to measure the exact mass of different isotopes of an element.

BASIC PRINCIPLE: The substance whose mass is to be determined is first charged into vapours. These vapours are then ionized in an ionization chamber, with the beam of high energy electrons. The positive ions are separated on the basis of their m/e ratio in a magnetic analyzer. These ions are detected by ion collector. The results are represented in the form of a spectrum. m/e is plotted on x-axis and relative no. of ions along y-axis.

Chapter 1 - BASIC CONCEPTS (Contd.)

MOLECULE, ITS ATOMICITY AND CHARACTERISTICS

The smallest particle of a pure substance which can exist independently is called molecule.

ATOMICITY: The numbers of atoms present in a molecule is called its atomicity. Thus a molecule can be:

• MONO ATOMIC: It contains only one atom like He, Ne, Ar etc.
• DI ATOMIC: It contains two atoms like H₂, O₂, N_2, etc.
• TRI ATOMIC: It contains three atoms like H₂O, CO₂, etc.
• POLY ATOMIC: It contains many atoms. Such molecules are called Macromolecules.

MACROMOLECULES: These are very large molecules containing very large number of atoms. For example HAEMOGLOBIN contains 10,000 atoms in one molecule and it is 68,000 times heavier than H-atom.

A molecule may contain atoms of same element or atoms of different elements.

• HOMOATOMIC molecules: H₂, O₂, P₄, S₈
• HETEROATOMIC MOLECULES: HCL, NH₃, C₆H₁₂O₆, H₂SO₄

PROPERTIES OF MOLECULES

1. Molecules of same substance are similar in all respects.
2. Molecules have empty spaces between them. These are maximum in gases and minimum in solids.
3. The molecules are in the state of constant motion. These movements are maximum in gases, lesser in liquids while solids have no vibration motion.
4. Molecules have attractive forces for each other
5. The molecules have definite kinetic and potential energy.
6. For a chemical reaction, molecules must collide with each other. The molecules exchange atoms as a result of collision and new compounds are formed.

WHAT IS ION? ENERGY CHANGES DURING FORMATION OF A POSITIVE AND NEGATIVE ION

ION: the species which carry positive or negative charge are called ions. There are two types of ions:

1. Positive ion (cations)
2. Negative ion (anion)

POSITIVE ION: positive ion is formed when one or more electrons are removed from a neutral atom.

A – e^- → A¹⁺

If an atom loses one electron, unipositive ion is formed.

If an atom loses two electrons, dipositive ion is formed.

If an atom loses three electrons, tripositive ion is formed.

Energy is required to remove an electron from an atom to produce positive ion. It is called ionization energy. Thus formation of positive ion is an endothermic process. The most common positive ions are Na¹⁺, K¹⁺, Mg²⁺, Al³⁺, Fe³⁺, Sn⁴⁺, etc.

NEGATIVE ION (ANION): Negative ION is formed when an atom gains one, two or more electrons.

B + e^- → B^1-

The formation of negative ion is exothermic process. the most common negative ions are Cl^1-, Br^1-, F^1-, S^2-, etc. uninegative, dinegative and trinegative ions are formed due to addition of one, two or three electrons in an atom.

Some negative ion consists of groups of atoms. For example OH^1-, SO₄^2-, PO₄^3-, MnO₄^1-, CrO₄^2- etc. Positive ions having group of atoms are less common. For example NH₄¹⁺ and some carbonations (carbon having +ve charge). Properties of ions are different from corresponding atoms.

MOLECULAR ION AND ITS SIGNIFICANCE

When a molecule gains or losses electron, a molecular ion is formed. For example CH₄¹⁺, CO₃^2-, N₂¹⁺ etc. Cationic (+ve) molecular ions are more abundant than anionic (-ve) molecular ions.

These molecular ions are produced by passing high energy electron beam or alpha-particles through a gas.

SIGNIFICANCE: The molecular ions are quite unstable. The breaking of molecular ion gives useful information about structure of natural products.

RELATIVE ATOMIC MASS: The mass of an atom of an element as compared to mass of carbon atom taken as 12 is called relative atomic mass. For example on C-12 scale mass of H = 1/12*12=1.0078 amu and mass of carbon is 12.0000 amu. The masses of atoms are extremely small. We don't have a balance to weight atoms. Thus we use relative atomic mass unit scale. The elements have fractional relative atomic masses due to their different isotopic abundance.

Chapter 1 - BASIC CONCEPTS

what is an atom? brief history of atom.

Definition: Atom is defined as smallest particle of an element which may or may not exist independently. e.g He, Ne, Ar etc. but Hydrogen, Oxygen atoms cannot exist independently.

The modern researches have clearly shown that atom is further composed of subatomic particles like electrons, protons, neutrons, hyperons, neutrino, anti-neutrons etc. more than 100 particles are present in atom.

history of atom

Greek philosopher thought that when matter is broken into smaller and smaller particles, finally a smallest particle is obtained which cannot be further subdivided. Democritus called these particles exiled atoms. This term has been derived from word "atomis" which means indivisible.

In late 17^th century it was found that atoms of same element are present in different substances. It was also discovered how atoms of different elements combine to form compounds and how to break a given compound into its elements.

atomic theory of matter

Dalton showed that law of conservation of matter and law of constant proportions could be explained on the basis of atoms. He developed an atomic theory according to which all matter is composed of atoms of different elements differ in their properties.

J. Berzelius determined atomic masses of elements. Berzelius also developed a system of giving symbols to each element.

EVIDENCE OF ATOMS

It is not actually possible to see atoms. An optical microscope can measure size of an object of size 500nm or above. The size of an atom is almost 0.2nm. The objects of size of an atom could be observed in an electron microscope. It uses a beam of electrons instead of visible light. The wavelength of electrons is much shorter than that of visible light. An electron microscope photograph of a piece of graphite is shown over here. The bright bands in fig are layers of c-atoms.

SIZE AND MASS OF ATOM:

In twentieth century x-ray work has shown that the diameter of atoms are of the order of 2x10 ^-10 meter or 0.2nm.

Masses of atoms range from 10^-25 kg to 10^-27 kg. The masses of atom are generally expressed in terms of amu. 1 amu = 1.661x10^-27 kg. A full stop "." can bear two million atoms present in it. It gives an idea about very small size of an atom.