We continue to review some of the most important materials in heat treatment and metallurgy.
Sodium (chemical symbol: Na)
Sodium is a soft, silvery-white metal that exhibits surface oxidation in air to form grayish-white sodium oxide (Fig. 1). For this reason, it is typically stored in oil or inert gas. Sodium metal is soft enough to be cut with a knife by hand. It is one of only three metals light enough to float on water; the other two being lithium and potassium. Like the other alkali metals (metals on the left edge of the periodic table) – lithium, potassium, rubidium, cesium and francium – sodium has a single electron in its outermost shell, causing it to readily bond with other atoms. As a result, sodium and the other alkali metals are so reactive that they are never found alone in nature.
Sodium reacts aggressively when placed in contact with water, changing into sodium hydroxide and hydrogen in an exothermic (heat-generating) reaction. Because the hydrogen is flammable, it spontaneously ignites (Fig. 2).
Compounds of sodium have been known and used since ancient times. Ancient Egyptians, for example, stuffed mummies with a material called natron to dry them out and preserve the flesh. Natron is a mixture of sodium-containing soda ash (sodium carbonate) and baking soda (sodium bicarbonate) that occurs naturally. Salt (sodium chloride) was in widespread use before the beginning of recorded history – starting around 6000 B.C.
Although sodium compounds had been known for many years, sodium metal was isolated for the first time in 1807 by Cornish chemist and inventor Sir Humphry Davy through the electrolysis of sodium hydroxide. Davy exposed caustic soda (sodium hydroxide) to an electric current, forming samples of sodium metal. He had done the same thing when isolating potassium, except that a higher current was necessary for sodium.
Davy was a prolific chemist. In addition to potassium and sodium, he was the first to isolate calcium, strontium, barium, magnesium and boron. He also discovered the elemental nature of chlorine and iodine and founded the new field of electrochemistry.
In 1799, Davy discovered the potential anesthetic properties of nitrous oxide, experimenting with it and noticing it made him laugh, whereupon he named it "laughing gas." Sodium was named after the English “soda.” It was given the atomic symbol Na because it was originally called “Natron,” after the Natron Valley in Egypt, one of the earliest sources of soda (sodium carbonate).
Here are some interesting facts about sodium.
- Atomic number: 11
- Element category: alkali metal
- Electrons per shell: 2, 8, 1
- Melting point: 370.944 K (97.794°C, 208.029°F)
- Boiling point: 1156.090 K (882.940°C, 1621.292°F)
- Density (near R.T.): 0.968 g/cm3
- Density when liquid (at m.p.): 0.927 g/cm3
- Critical point: 2573 K, 35 MPa (extrapolated)
- Heat of fusion: 2.60 kJ/mole
- Heat of vaporization: 97.42 kJ/mole
- Molar heat capacity: 28.230 J/(mole·K)
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