Welcome to You Ask Andy

Caroline Sturgeon, age 10, of Granger, Utah, for her question:

How do crystals  form?

Crystals are solid substances built from atoms and molecules. They are crystals because their building blocks are arranged in orderly patterns. A diamond is a dazzling crystal arrangement of ordinary carbon atoms. Most of the earth's minerals are crystals of some sort because their atoms are arranged in orderly patterns. But, strange to say, a vase of so called crystal glass is not a true crystal. Its disorderly atoms are not arranged in neat patterns.

We say that solid crystals grow from separate atoms. But they do not use outside energy. to model and remodel their particles, as living things do. They grow by arranging the particles of non living chemicals in definite designs. To grow into a solid crystal, the chemical particles need two things. They must be free to move around. And they must have time to arrange themselves in their proper places.

To grow a crystal, the chemical building blocks need help to move around: They may move through certain gaseous vapors. Salt crystals form when a bit of sodium is sealed in a flask of chlorine vapor. Some chemical particles use liquids to float around. Alum crystals grow when aluminum sulfate and potassium sulfate aye dissolved in water. Heat is another way to give atoms and molecules freedom to move. All sorts of chemicals slither around in molten lava. As the mixture cools, they may have time to arrange themselves in crystals. You can see various little crystals in certain samples of granite.

In ordinary salt, the atoms of sodium and chlorine arrange themselves in cubes. Each chlorine atom is inside a square made by four sodium atoms. Next door is a sodium atom in a square with a chlorine atom at each corner. The growing crystal stacks these basic cubes into tiny sugar lump shapes. All tiny crystals are alike and they grow together to form a grain of table salt. If you wish to note down the number of atoms used to build a grain of salt, write 10 plus 24 zeros.

An alum crystal grows a shape like two pointed pyramids, sharing the same base. Each chemical can grow one or perhaps several crystal designs. It grows the way it does because atoms have positive and negative charges of electricity. These opposite charges attract each other and cling together, somewhat like the opposite poles of two magnets. But two positive^. repel each other and pull apart So do two negative charges.

As the particles move freely around, their electrical charges either attract or repel each other. When two opposites come within range, they pull the particles together in a certain position. Other pairs of opposites repeat the same pattern and the solid crystal grows, particle by particle.

Crystals seem to grow very slowly. But the tiny atoms need time to find and arrange their opposite charges in the proper position. It takes a row of 100 million atoms to measure one inch. And each atom must be placed correctly to form the crystal's faces and angled corners.

 

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