MODELS OF AN ATOM

- It is important to realize that a lot of what we know about the structure of atoms has been developed over a long period of time. This is often how scientific knowledge develops, with one person building on the ideas of someone else. We are going to look at how our modern understanding of the atom has evolved over time.

Atomic Models In 1808, John Dalton described

the atom as an indivisible solid particle. He thought that each element has its own kind of atom. In the early 1900s several scientists made experimental investigations that reshaped Dalton’s idea of the atom.

Thomson’s Plum Pudding Model of the Atom

In 1987, J.J Thomson, an English scientist, first speculated that the atom is made up of even smaller particles.

Thomson was studying the effect of passing an electrical current through a gas when the gas gave off rays made of negatively charged particles. Since the gas was known to be made of uncharged atoms, he wondered where these charged particles came from.

He thought that these negatively charged particles came within the atom. It meant that a particle smaller than the atom does exist and that atom is not indivisible after all.

These particles are known today as electrons. This discovery of the negative electrons created another problem to solve.

Since the atom is known to be a neutral particle of matter, there must be something that balances the negative electron. So, Thomson thought that positively charged particles existed in the atom.

The atomic model Thomson proposed was like a plum pudding. In this model, the atom is made up of something like a positively charged pudding-like material in which negatively charged electrons were scattered like plums in the pudding. x

Thomson’s model was far from correct but was an important development in the understanding of the structure of the atom.

In 1908, Ernest Rutherford, an English physicist, was working on an experiment which was remotely related to the unfolding of the mysteries of the structure of the atom.

In his experiment, he fired a stream of alpha particles, which are tiny positively charged particles, at a thin gold foil (about 2000 atoms thick).

Rutherford’s Model of the Atom

He discovered that most of the positively charged ‘bullets’ easily passed through the gold foil without changing direction at all.

This tells that the atom is mostly an empty space, contrary to Thomson’s idea of a plum pudding model filled with positively charged materials.

• Some of the ‘bullets’ however bounced right back, away from the gold foil while other were deflected which meant that they hit something solid which repelled them.

• Rutherford thought that only positive particles could repel the positive bullets. He therefore proposed a new model the atomic structure, that the atom is mostly an empty space that has a dense positively charge center that repelled the positively charged alpha particles which he used as bullets in the experiment.

• This center of the atom was given the name ‘nucleus’ by Rutherford. The nucleus are very small compared to the size of the whole atom.

• Rutherford’s model is an atom where all of its positive charges are contained at its center in the nucleus and the negatively charged electrons are contained outside the nucleus around the edge of the atom.

Between the electrons and the nucleus is mostly empty space. You could imagine an atom to be similar to a football stadium where a marble is placed at the center.

Rutherford’s model of the atom did not adequately explain the arrangement of the electrons, but it was useful in many ways.

Models are not wrong. Rather they are the best we have at the time. Scientists change them as new information becomes available.

THANK YOU -

Garnette C. Villa