Particle Theories. Flashcards
Dalton’s Model.
In the 1870s, John Dalton did experiments to measure how elements combined. He thought that he could explain the results of his experiments if everything is made of atoms. His model of an atom was a very small indestructible sphere.
He thought that all the atoms in an element are the same, and the atoms in one element are different from the atoms in all other
elements.
Thomson’s Model.
In 1904, Thomson suggested a new model. He said that atoms are like plum puddings (Figure 4), which are like blueberry muffins. This is called the plum-pudding model of the atom. In this model, the atom is thought to consist of positive mass with negative electrons embedded in it. He did not know what the positive mass was made of, or the structure of the atom. He thought that the positive mass might be solid, or like a cloud.
THOMSON HAD TO MAKE SENSE OF TWO IDEAS:
• An atom contains negative electrons, and overall, the atoms have no electrical charge. They are neutral.
The Bohr Model.
In 1913, Niels Bohr suggested that electrons can only move in fixed orbits, called electron shells, around the nucleus (Figure 4). The Bohr model of the atom has been improved over the past century, but it still provides a good basis for imagining how protons, neutrons, and electrons in atoms are arranged.
Democritus and Leucippus.
Two Greek philosophers, Democritus and Leucippus, thought that eventually you could not cut any more, and you would have an atom. The word ‘atomos’ means
‘indivisible’ in Greek. Atoms, they said, were too small to see.
Rutherford Model.
To explain his results, Rutherford suggested that an atom is made of a tiny, positively charged nucleus with electrons around the outside (Figure 3). This is called the Rutherford model of the atom. Almost all the mass of an atom is in its nucleus. When alpha particles hit a nucleus, they bounce back. Later, scientists discovered the nucleus is made of two different types of particle, which they called protons and neutrons.
