Structure of the atom • Although John Dalton thought atoms were indivisible, investigators in the late 1800’s proved otherwise. • Scientific investigations of matter showed that atoms are actually composed of several basic types of smaller particles. • Today, an atom is defined as the smallest particle of an element that retains the chemical properties of that element. • All atoms consist of two regions. The nucleus is a very small region located at the center of an atom. Protons and neutrons are found in the nucleus. Surrounding the nucleus, is a region occupied by negatively charged particles called electrons.

Discovery of the electron • In 1897, J.J. Thomson provided the first hint that an atom is made of smaller particles. • When current is passed through a cathode ray tube, a stream of particles (cathode rays) travel from the cathode(- terminal) to the anode (+ terminal), producing a beam or ray. • When a magnetic or an electric field was placed around the tube, the rays were deflected away from the negatively charged end.

Discovery of the electron

Discovery of the electron • Thomson concluded that all cathode rays are composed of identical negatively charged particles, which were named electrons. • Thomson was also able to find the charge to mass ratio of the electrons. • In 1909, experiments conducted by Robert A. Millikan measured the charge of the electrons. • Scientists used the charge of electrons and the charge to mass ratio of the electron to determine the mass of an electron. • The mass of an electron was found to be about 1/2000 the mass of a hydrogen atom

Thomson’s plum pudding model • Based on what was learned about electrons, two other inferences were made about atomic structure: 1. because atoms are electrically neutral, they must contain a positive charge to balance the negative electrons. 2. Because electrons have so much less mass than atoms, atoms must contain other particles that account for most of their mass.

Thomson’s plum pudding model • Thomson proposed a model for the atom that is called the plum pudding model, where he believed the negative electrons were spread evenly throughout the positive charge of the rest of the atom.

Discovery of the atomic nucleus • After Thomson proposed his plum pudding model of the atom, new experiments disproved this model. • In 1911, Ernest Rutherford and his associates Hans Geiger and Ernest Marsden bombarded a thin piece of gold foil with fast moving, positively charged alpha particles. • Most of the alpha particles passed through the foil. However, a some particles were deflected and some were deflected back to the source.

Gold Foil Experiment Section 2 The Structure of the Atom Chapter 3

Discovery of the atomic nucleus • Rutherford reasoned that the alpha particles must have experienced some powerful force within the atom. • The source of this force occupies a very small amount of space, because so few of the total number of alpha particles had been deflected. • He concluded that the force must be caused by a very densely packed bundle of matter with a positive electric charge, which he called the nucleus. • Rutherford discovered that size of the nucleus was very small compared to the size of the atom.

Rutherford’s atom

The Bohr Model • Rutherford’s model proposed that negatively charged electrons were held in an atom by the attraction between them and the positively charged nucleus. • In 1913, scientist Niels Bohr proposed a model in which he placed each electron in a specific energy level. • According to Bohr’s atomic model, electrons move in definite orbits around the nucleus, much like planets circle the sun. These orbits, or energy levels, are located at certain distances from the nucleus.

Bohr’s atom Bohr’s atom

The Wave Model • Bohr’s model worked well in explaining the structure and behavior of simple atoms such as hydrogen. However, it did not explain more complex atoms. • Today’s atomic model is based on the principles of wave mechanics, which involve complex mathematical equations. • According to this model, electrons do not move about an atom in a definite path like planets around the sun. In fact, it is impossible to determine the exact location of an electron. Scientists can only predict where an electron is most likely to be found. • The probable location of an electron is based on how much energy the electron has.

The Wave Model • According to the modern atomic model, an atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make the atom neutral. • The region of space around the nucleus where an electron is most likely to be found is called an orbital. • Orbitals have different shapes and maximum numbers at any level • s (sharp) - spherical (max = 1) • p (principal) - dumb-bell shaped (max = 3) • d (diffuse) - four-lobe-shaped (max = 5) • f (fundamental) - six-lobe shaped (max = 7)

Orbitals

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