1.1 Atomic Structure-Cont.
Atomic Structure
At the beginning, Dalton’s atomic theory was not universally accepted by scientists. Further experimentation succeeded in adding more evidence that substances combine in fixed ratios, but failed to support the existence of atoms. Scientists wondered whether it would be possible to probe the nature of atoms themselves.
Nearly 100 years later, British scientist J. J. Thomson made an important breakthrough in understanding the structure of atoms. Through his experiments, using a device called cathode-ray tube, Thomson collected enough evidence to show that atoms contain extremely small particles carrying a negative charge. Thomson had discovered electrons.
A few years later, in England, a group of scientists led by Ernest Rutherford discovered that the atom consisted mostly of empty space! Rutherford’s group was studying the properties of certain ores, dug out of Earth’s crust, that emitted streams of highly energetic particles. The team placed a sample of the ores in front of a thin sheet of gold foil and studied the behavior of the emitted particles as they hit the foil. They observed that most of the particles passed right through the gold, but a very small percentage bounced backwards or glanced off at an angle.
Rutherford suggested that each atom of gold contains a very dense nucleus located at the center and surrounded by a large, empty space where electrons can move. At the time, the nucleus was thought to consist of one or more particles having positive charges, known as protons.
Refining the Model of the Atom
The discovery of a nucleus at the center of an atom was a big breakthrough for scientists in the understanding of the atomic structure. However, many questions remained. For instance, it was not clear how the electrons were arranged in the space around the nucleus or how electrons were involved in chemical reactions.
While Rutherford published the results of the gold-foil experiment, Danish scientist Niels Bohr joined Rutherford’s research group in England. Bohr took up the investigation into the atomic structure and soon after made an important discovery. In 1913, he found that each electron in an atom could be assigned a specific amount of energy that increases as the distance from the nucleus increases. Moreover, he wrongly proposed that these electrons orbit the nucleus in specific circular paths as shown in Figure 6. This model of the atom is known as the Bohr model.
Bohr’s model of the atom failed to fit the data collected for all elements known at that time, but, for many years, scientists had no better model for the atom. Finally, in 1926, Austrian scientist Erwin Schrodinger proposed a new model that is still used today. In this model, the electrons carry a specific amount of energy and move about a central nucleus, but not in fixed orbits. This model is called the electron cloud model because the electrons move about the nucleus in cloud-like regions. In Figure 7, the darker the region, the greater the probability of an electron being present at that point.
It is important to understand that the Bohr model is an energy diagram and not a picture of an atom. Electrons do not orbit the nucleus in circular paths like planets orbiting the sun. Also, electron clouds represent probabilities, but an electron itself is a discrete particle and not a diffuse cloud.
Fill in the blank.
According to Rutherford's model the atom consists of a very small but very dense
nucleus made up of one or more positively charged protons.
Fill in the blank.
An electron in orbit three would have
energy than an electron in orbit one.
The important aspect of Rutherford’s model which continued on in the electron cloud model of the atom is the fact that
The important aspect of Rutherford’s model which continued on in the electron cloud model of the atom is the fact that
