Geiger–Marsden experiment facts for kids

The Geiger–Marsden experiment (also called the Gold foil experiment or the Rutherford experiment) was a very important science experiment done in 1909. Two scientists, Hans Geiger and Ernest Marsden, carried it out. They were working under the guidance of Ernest Rutherford at the Physical Laboratories of the University of Manchester in England. The results of this experiment were quite surprising and changed how scientists understood the atom. It showed that the atomic nucleus exists, which was a huge discovery! This experiment proved that the old idea of the plum pudding model of the atom was wrong. Instead, it led to the Rutherford atomic model (also known as the planetary model), which is much closer to what we know about atoms today.

What Was the Geiger-Marsden Experiment?

The Geiger-Marsden experiment was designed to test what atoms were made of. Before this experiment, many scientists believed in the "plum pudding model" of the atom. This model suggested that atoms were like a blob of positive charge with tiny negative electrons scattered inside, like plums in a pudding. Rutherford wanted to see if this model was correct by firing tiny particles at a very thin piece of gold foil.

The "Plum Pudding" Idea

In 1904, J.J. Thomson, who discovered the electron, proposed the plum pudding model. He thought that an atom was a sphere of positive charge. Inside this sphere, there were small, negatively charged electrons. These electrons were spread out evenly, just like plums in a dessert. This model suggested that the positive charge was spread out and not very dense.

Rutherford's Big Question

Ernest Rutherford was curious about the structure of the atom. He believed that if the plum pudding model was right, then tiny, fast-moving particles should pass straight through atoms. He expected them to be only slightly deflected, like a bullet going through soft butter. He asked his students, Hans Geiger and Ernest Marsden, to perform an experiment to test this idea.

How the Experiment Worked

Geiger and Marsden set up a special apparatus for their experiment.

• They used a source that shot out tiny, positively charged particles called alpha particles. These particles are very fast and energetic.
• They aimed these alpha particles at a very thin sheet of gold foil. This foil was only a few atoms thick.
• Around the gold foil, they placed a screen coated with a material that would glow when an alpha particle hit it. This allowed them to see where the particles went after hitting the gold.

Most of the alpha particles were expected to go straight through the gold foil. Some might be deflected a little bit, but not much. This was based on the plum pudding model, where the positive charge was thought to be spread out.

What They Discovered

The results of the Geiger-Marsden experiment were truly amazing and unexpected.

• Most of the alpha particles did pass straight through the gold foil, just as expected. This meant that atoms are mostly empty space.
• However, a small number of particles were deflected at very large angles. Some even bounced almost straight back! This was like firing a cannonball at a piece of tissue paper and having it bounce back. Rutherford famously said it was "almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

The New Atomic Model

The surprising results meant that the plum pudding model could not be correct. For alpha particles to bounce back, they must have hit something very small, very dense, and very positively charged inside the atom. This led Rutherford to propose a new model for the atom in 1911: the Rutherford atomic model.

• In this model, the atom has a tiny, dense, positively charged center called the nucleus.
• Almost all the mass of the atom is concentrated in this nucleus.
• The negatively charged electrons orbit around this nucleus, much like planets orbit the sun. This is why it's also called the planetary model.
• The rest of the atom is mostly empty space.

When alpha particles hit the tiny, dense nucleus, they were strongly repelled and bounced back. Particles that passed far from the nucleus went straight through. Particles that passed close to the nucleus were deflected at various angles. This new model explained all the observations from the experiment.

Why This Experiment Was Important

The Geiger-Marsden experiment was a landmark moment in science.

• It completely changed our understanding of the atom's structure.
• It proved the existence of the atomic nucleus, which is a fundamental part of modern physics.
• It paved the way for future discoveries in quantum physics and nuclear science.
• This experiment showed that even simple experiments can lead to revolutionary insights when scientists are open to unexpected results.

Images for kids

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  A replica of one of Geiger and Marsden's apparatus

See also

In Spanish: Experimento de Rutherford para niños