Chapter 5 Atomic Structure The atom is the smallest particle of an element that retains the properties of that element. A typical atom has a diameter of cm and a mass of grams. The word atom comes from the Greek word atomos meaning indivisible.

The idea of the atom was 1 st developed by Democritus about 400 B.C.The idea of the atom was 1 st developed by Democritus about 400 B.C. In the 1700’s, John Dalton, an English chemist, first stated his atomic theory which was based on the ideas set by Democritus almost 2000 yrs. ago.In the 1700’s, John Dalton, an English chemist, first stated his atomic theory which was based on the ideas set by Democritus almost 2000 yrs. ago.

The atom is made up of the following subatomic particles: 1. Electrons (e - ) : Are negatively charged particles Are located in the outer region of the atom called energy levels (electron clouds) Its mass is about 1/1840 amu or 9.11 x grams. The number of electrons in a neutral atom is determined by the element’s atomic number. 2. Protons (p + ) Are positively charged particles Are located in the dense nucleus of the atom It has a mass of 1 amu or 1.67 x grams. The number of protons in the atom is determined by the element’s atomic number.

3. Neutrons (nº) Are neutral particles having no charge. Are located in the nucleus of an atom. It has a mass of 1 amu or 1.67 x grams. The # of neutrons are calculated by subtracting the atomic number from the mass number. Remember: a.m.u. stands for atomic mass unit. The nucleus is the dense center of the atom. It contains protons and neutrons. Since the protons are (+) and the neutrons have no charge, the nucleus is therefore positive (+).

Atomic number: (see periodic table) designates the # of protons in the nucleus of an atom designates the number of electrons in the energy levels of a neutral atom Atomic #

Atomic mass: corresponds to the weighted average of the masses of the isotopes for a certain element ATOMIC MASS

Mass number: corresponds to the total number of protons and neutrons in the nucleus of an atom it is the rounded-off atomic mass round-off to whole # (70) this would be the mass #

The number of energy levels that an atom contains is given by the period # on the sides of the periodic table. Valence electrons are: the outermost electrons in an atom responsible for chemical bonding and thus chemical reactions group number given by the group number in the periodic table

The chemical properties of atoms, ions, and molecules are related to the arrangement of electrons. The energy level of an electron is the region around the nucleus where the electron is likely to be moving. Electrons in an atom cannot exit between energy levels. A quantum of energy is the amount of energy required to move an electron from its present energy level to the next higher one.

 Four forces govern the behavior of the subatomic particles in an atom: Electrons are kept in orbit around the nucleus by the electromagnetic force by acting as a force of attraction between the protons(+) in the nucleus and the electrons(-) in the energy levels. The electromagnetic force acts as a force of repulsion between the positively charged protons inside the nucleus. This force is the greatest of the four forces The strong force opposes the electromagnetic force of repulsion between the protons. The strong force “glues” protons together to form the nucleus. Without the strong force, there would be no atoms. The strong force works only when the protons are very close, hence, it has a limited range. This force is the greatest of the four forces.

3.The weak force is the key to the power of the sun. This force is responsible for a process called radioactive decay. During this process, a neutron in the nucleus changes into a proton and an electron. 4. The final force is gravity. This is the weakest force known in nature. Its effect on the atoms is very small compared to the other three forces. Its role in the atom is not clearly understood.

History of the Development of the Atomic Theory 400 B.C. Democritus 1 st proposed the idea of the atom 1803 John Dalton proposed the 1 st elaborate “atomic theory” 1897 J.J. Thomson discovered the electron using cathode ray tubes 1909 Millikan used the oil drop experiment that determined the charge and mass of the electron

1911 Rutherford discovered the nucleus using the gold foil experiment 1930 Shrodinger introduced “wave mechanics” as a mathematical model of the atom 1932 Chadwick discovered the neutron using alpha particles 1981 Bennig & Rohrer completed the construction of the STM

Energy from the Nucleus Nuclear energy, often mistakenly called atomic energy, is the most powerful kind of energy known. It produces the tremendous heat and light of the sun and the shattering blast of thermo-nuclear bombs. Nuclear energy results from changes in the nucleus of atoms. The first atomic bomb, or A-bomb, exploded on July 16, 1945 in Alamogordo, New Mexico. It produced and explosion equal to that of 19,000 short tons (17,000 metric tons) of TNT.

Transmutation The conversion of an atom of one element to an atom of another element is called transmutation. Radioactive decay is one way in which transmutation occurs. A transmutation can also occur when high-energy particles bombard the nucleus of an atom. The high-energy particles may be protons, neutrons, or alpha particles. The elements with atomic numbers above 92 are called transuranium elements and all undergo transmutation. None of them occurs in nature, and all of them are radioactive.

Define the following in your notes: Isotopegroup Ionperiod Cationelectron configuration Anionelectron dot structure Cathode rayelectronegativity STM (see p. 107) monatomic ion Metal polyatomic ion Nonmetal paramagnetic Metalloid Fission Fusion Molar mass Mole Gram atomic mass Ground state