In this periodic table, elements are arranged in increasing order of their atomic numbers. The periodic table consists of 18 vertical columns and 7 horizontal rows. These columns are called as groups and horizontal rows are known as periods.
There are two series of elements at the bottom of main body. Each series consists of 14 elements and they are called as f-block elements or inner transition elements. Due to unique chemical and physical properties they are placed at the bottom of main body.
Entire table can be classified a metal, non-metal and metalloids. Metals are placed at the left side of periodic table whereas non-metals are at the right side. The zig-zag intermediate line between metals and non-metals are called as metalloids as they exhibit intermediate properties. The bottom lines of two series of elements also consist of metals. Metals are hard, solid, malleable and ductile whereas non-metals are electronegative elements which can exist as solid, liquid or in gaseous state.
Now see the position of hydrogen in periodic table. It is placed at the left side as the first element of group-1. It indicates that it must be an alkali metal but the color of box indicates that is a non-metal. If it is a non-metal, why does it place at left side with alkali metals? We cannot say that hydrogen is a metal or non-metal or metalloid. Since hydrogen has one electron in its valance shell so it must be an alkali metal as all alkali metals have one valence electron. But it does not exhibit other metallic properties like malleability, ductility etc.
At the same time, it exhibits few properties like halogen such as formation of hydride ion (H-). That is the reason; we cannot assign a perfect position of hydrogen in periodic table. You may find some periodic table with hydrogen at the top center of main body as it does not show similarity with other elements.
The atomic number of hydrogen is 1; it means it has 1 electron and 1 proton in it. There is no neutron in hydrogen as the mass number of hydrogen atom is also 1. One of the most common compounds of hydrogen is water in which two hydrogen atoms form bond with one oxygen atom. Since hydrogen atom has 1 electron to it tends to share its one electron to get two paired electrons. In other words, each hydrogen atom can form one bond with oxygen atom to for H2O molecule.
Like other elements, hydrogen also has some isotopic forms which have same atomic number but different mass number. Since isotopes of same element have same atomic numbers so they are placed at the same position in the periodic table.
Three isotopes of hydrogen are named as protium, deuterium and tritium. Protium is most abundant isotope of hydrogen with natural abundance > 99.98 %. The atomic mass of protium is 1.00782504 u and it is symbolized as 1H. It has 1 proton and 1 electron in it. Hydrogen-2 or deuterium is another isotope of hydrogen with 1 electron, 1 protons and 1 neutron. Therefore the mass number of deuterium is 2 and atomic mass is 2.01410178 u. It is symbolized as 2H. The natural abundance of it is very less compare to protium that is around 0.0026-0.0184 %. Tritium or hydrogen-3 is least abundant isotopes which are found in trace amount only. It has 1 proton, 2 neutrons in its nucleus therefore is symbolized as 3H. The atomic mass of it is 3.0160492 u.
We have seen the structure of water molecule. In water molecule, two hydrogen atoms are bonded with one oxygen atom through single covalent bond which is formed by equal sharing of electrons between H and O. So we can say that water is oxide of hydrogen since oxygen is more electronegative than H and bonded here with hydrogen atoms. If we say hydrogen it stands for protium as it is most abundant isotope of hydrogen. So we can say that water has protium isotope of hydrogen. Now if we replace protium with other isotopes of hydrogen says deuterium, it will form D2O here D represents deuterium or hydrogen-2. The chemical name of D2O is deuterium oxide just like hydrogen oxide which is commonly called as water.
Deuterium oxide is also called as heavy water as the atomic mass of deuterium is more than protium of water. Deuterium is a stable isotope of hydrogen which forms D2O with oxygen.
Just like water molecule, deuterium oxide is oxide of deuterium in which two deuterium atoms form covalent bond with oxygen atom. The chemical formula for deuterium oxide is D2O which indicates that it is composed of two deuterium atoms and one oxygen atom. Since the mass number of deuterium is just double of protium so deuterium oxide is also called as heavy water. The bond length of D-O bond is around 3% shorter than H-O bond in water molecule. We may say that the D-O bonds are stronger than H-O bonds that decrease the strength of D-O intermolecular bonds with different molecules.
In liquid phase, the bond angle of D2O molecule is about 106Âº which is slightly more than H2O (104.5 Âº) which makes the molecule little less bent compare to water molecule that has V-shape. Unlike water molecules, D2O forms less hydrogen bond than water molecule. But it is denser than water and freezes at 4ÂºC and boils at 101ÂºC.
We know that water exist in liquid state due to presence of hydrogen bonds between water molecules. Since hydrogen bonding is weaker in D2O so the boiling point must be lower than H2O but due to increased mass, its boiling point a little higher than H2O.
Deuterium oxide or heavy water can be produce by distillation or electrolysis of regular water with large cascade of stills and electrolysis chambers which consumes large amounts of power. Due to this, chemical methods are more preferable compare to these methods. Dual temperature exchange sulphide process which is also known as Girdler sulfide process is the most cost-effective.
Dissociation of deuterated hydrofluorocarbons also forms deuterium fluoride that can be separated by physical methods is also used as an alternative process. Commercial heavy water is referred as deuterium oxide and is sold in various grades of purity. Letâs discuss one of the most common preparation methods of heavy water.
Regular water is main source of heavy water and prolonged electrolysis or fraction distillation can be used to isolate D2O from it. Prolonged electrolysis of regular water is a multistep process in which electrolysis of alkaline water is done in cylindrical vessels. These cylindrical vessels are made of steel which act as cathode and sheet of nickel acts as anode. Here anode has a number of holes in it.
First stage includes three electrolytic cells which are filled with 3% of NaOH solution. At 110 V, electrolysis is carried out at 72-hrs that reduced the volume to 1/6th of original volume. During this process hydrogen and oxygen gases are evolved with 2.5 % of heavy water.
Second stage conducts with 6 electrolytic cells with the residue left from the first stage. The evolved gases are burnt and water sent to first stage for recycling. It improves the concentration of heavy water up to 12% which must be a part of third stage.
Third and fourth stages also improve the concentration of heavy water up to 99 % and residual water returns to previous of electrolysis. Last stage of electrolysis is used to remove alkali and other impurities with the help of distillation. Here D2 and O2 gases are formed as residual which are burnt to get 100% pure D2O.
Let’s discuss some common uses of deuterium oxide.
- As a neutron moderator – We know that nuclear fission of U-235 is brought by slow speed neutron particles which are slow down with the help of heavy water. So D2O acts as moderator in nuclear reactors.
- Preparation of deuterium- Another use of D2O is production of deuterium (D2) by electrolysis or decomposition in the presence of metal.
- A tracer compound- D2O is also used as trace compound to study different mechanism of chemical reactions. It is also used to study the structure of oxyacids of phosphorus and for the determination of number of ionizable H atoms.
- In living organisms- D2O retards the growth of plants and animals like tobacco seeds cannot grow in D2O. Presence of D2O can also kill small fishes, tadpoles and mice. No doubt small amount of D2O in water acts as tonic and stimulates vegetables.
- In Fourier transform spectroscopy- D2O is also used in place of water in FTIR during collection of spectra of proteins. This is because regular water can form hydrogen bonds with amide region of proteins which affect the spectra of proteins.
- As neutrino detector and tritium producer- D2O or heavy water can be used as neutrino detectors and precursor of tritium.
- For the determination of basicity of acids.
Deuterium oxide is a tasteless, colorless and odorless liquid with high viscosity compare to regular water. That is the reason; ionic compound ionic solids are less soluble in heavy water.
Both H2O and D2O exhibit little difference in chemical nature. Some common differences between these two liquids are listed below.
|| Regular water
|| Heavy water
| Density at 20°C
| Freezing point
| Boiling point
|| 1.000 g/mL at 4°C
|| 1.1073 g/mL at 11.6°C
| Specific heat at 20°C
|| 1.00 cal g-1 K
|| 1.01 cal g-1 K
| Surface tension at 20°C
|| 72.8 dynes/cm
|| 67.8 dynes/cm
| Dielectric constant
| Viscosity at 20°C
|| 10.09 millipoises
|| 12.6 millipoises
Since deuterium is not a radioactive element therefore heavy water does not decay and no harmful radiations are there from it. Since D2O is heavier than regular water therefore the chemical and biochemical effects are different for it. If we drink heavy water, you will not notice the effect of it until 25% of water of your body is replaced by heavy water. That may take around a week of drinking heavy water nothing else. Another form of heavy water is made from tritium rather than deuterium that is definitely harmful as tritium is a radioactive element.