MadSci Network: Physics

Re: What makes metal shiny?

Date: Thu Aug 24 02:13:13 2000
Posted By: Adrian Popa, Director Emeritus, Hughes Research Laboratories
Area of science: Physics
ID: 967008075.Ph


You have asked a very interesting question that is a bit complicated because it is related to the basic structure of the atoms that make materials in the universe.

All substances are composed of atoms which are very, very small. If we had a magnifying glass that could magnify an apple up to the size of the earth, the atoms would then be the size of an apple. Atoms are composed of a central core called the nucleus that is 1000 times smaller than the atom. The nucleus also has a strong positive electric charge.

The nucleus is surrounded by a cloud of whirling super small electrons each with a negative electrical charge. The positive charge of the nucleus attracts the negative electrons so that it is very hard for them to escape and fly away from the atom. The (neutral) atoms of each of the 100 or so different elements in the universe have a different number of electrons. For example hydrogen atoms have 1 electron, there are 6 electrons in carbon atoms in our pencils, 8 in the oxygen we breath, 13 in aluminum, 47 in silver, 79 in gold and 92 in uranium atoms.

Light is composed of very small packages of electromagnetic energy called photons. We are able to see objects because light photons from the sun (or other light source) reflect off of the atoms within the object and some of these reflected photons reach the light sensors in our eyes and we can see the objects. It takes many millions of photons entering our eyes each second for us to view the world.

When photons of light hit the atoms within an object three things can happen.

First, the photons can bounce back from the atoms in the object; we call this reflection.

Second, the photons can pass through an object such as glass and we call them transparent.

Three, the photons can be stopped by the atoms within the object and the photon energy is converted to heat; we call this absorption.

Objects that reflect many photons into our eyes make the objects appear shiny. Objects that absorb photons and reflect less photons appear dull or even dark black to our eyes. Many materials can appear shiny to our eyes. Metals are shiny, glass can be shiny, diamonds are shiny and even rocks and plastic can be shiny. However, of all of the different atoms in the materials in the universe, metals are the most shiny because they reflect the most photons of all of the elements.

Of all of the metals, aluminum and silver are the most shiny to our eyes. Gold is also one of the more shiny metals. However, gold is not as shiny as silver and aluminum. Mercury, a liquid metal, is also shiny and special telescope mirrors have been made of mercury.

So why are metals more shiny than materials made of other atoms?

Metals can transmit electricity and are called conductors of electricity because the outer electrons that are most distant from the nucleus of the metal atoms are not as tightly held by the positive nucleus as are the electrons in other elements. These electrons can fly off if there is an electrical pressure called a voltage. The flow of these electrons, called free electrons, through the metal produces the current called amperes that flows through the electrical wires in our homes.

These free electrons also reflect light photons right at the surface layers of atoms in an object so that the photons canít be absorbed by the deeper layers of atoms. These materials appear bright to our eyes. Atoms of nonmetallic elements may reflect some photons but most of the photons pass into the deep layers of atoms and are usually absorbed or they pass through the object.

So the answer to your question is that the free electrons in metal atoms reflect more photons than the atoms of nonmetallic elements. These objects appear more shiny (bright) because our eyes receive more photons from them when they are in light.

Best regards, Your Mad Scientist in California

Adrian Popa

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