GROUP 3 - CHEMISTRY .pdf

LESSON 3.3 INSIDE THE ATOM (The Subatomic Particles, Atomic Number and Mass Number, and Isotopes) 30 Zn Zinc 12...

LESSON 3.3 INSIDE THE ATOM (The Subatomic Particles, Atomic Number and Mass Number, and Isotopes) 30 Zn Zinc 12 Mg agnesium M  The Subatomic Particles Dalton’s Atomic Theory: Advances in experimentation and theory led to a deeper understanding of atomic structure and the discovery of subatomic particles— electrons, protons, and neutrons. 30 12 n  Electrons In 1897, British physicist Joseph John Thomson (1856-1940) discovered electron while studying the nature of cathode rays. He observed that cathode rays in cathode ray tube (CRT) are deflected by a negatively charged plate and attracted by a positively charged plate as if the rays consisted of negatively charged particles.  Electrons These particles were ejected from the atom of the cathode (the source of the cathode ray) and that the atom is divisible after all. Negative particles were more than 1000 times lighter than the hydrogen atom, but its mass is the same regardless of the element it came from. Fig. 3-1. The blue line represents the cathode ray deflected away from the negatively charged plate, which means that the ray is also negatively charged.  Electrons He proposed the plum pudding model of the atom the bread represents the atom having a diffuse positive charge, while the plums depict the negative electrons.  Electrons In 1910, American physicist Robert Millikan (1868-1953) published the result of his oil-drop experiment through which he determined the actual charge of the electron to be 1.592×10-19 coulomb, just slightly lower than the presently accepted value of 1.602×10-19 coulomb. Nucleus: Protons and Neutrons The discovery of X-ray by German physicist Wilhelm Conrad Roentgen (1845- 1923) and the radioactivity of uranium by French physicist Henri Becquerel (1852-1908) prompted many scientists to investigate The first X-ray machine discovered by Wilhelm radiation. Conrad Roentgen in 1895.  Nucleus: Protons and Neutrons Ernest Rutherford (1871-1937) Discovered and described the alpha and beta rays as positively and negatively charged radiations, respectively. Performed the gold foil experiment in 1909, with the hypothesis that alpha rays should pass through the plum pudding-like structure of the gold atoms.  Nucleus: Protons and Neutrons But in the experiment, Rutherford observed that some alpha rays were deflected at a regular pattern, some bounced back to the alpha particle source, and others passed through the foil. He then concluded that the atom has a very tiny positive nucleus at its center, an idea that led to the emergence of the nuclear model of an atom. Nucleus: Protons and Neutrons Nucleus: Protons and Neutrons The nucleus at the center consists of protons and neutrons, collectively known as nucleons. Rutherford’s nuclear model of the atom depicts the nucleus at the center, with electrons revolving around it. The neutrons and protons were later found to be inside the nucleus.  Atomic Number and Mass Number An atom is represented by its element symbol with the atomic number (Z) as the left subscript and the mass number (A) as the left superscript. The atomic number of an element represents the number of protons in its nucleus.  Atomic Number and Mass Number The mass number indicates the total number of protons and neutrons; it estimates the element's atomic mass. For an electrically neutral atom (i.e., with zero electric charge), the number of electrons is the same as the number of protons.  Atomic Number and Mass Number For example, Carbon (C) has an atomic number of 6 and a mass number of 12, meaning it has 6 protons, 6 neutrons, and 6 electrons (in a neutral atom).  Atomic Number and Mass Number Since each element has a specific number of protons in their nucleus, the atomic number therefore serves as the identity of an element. Before the discovery of the proton, elements were arranged and assigned numbers based on increasing atomic mass. The first periodic table arranged by increasing atomic mass.  Atomic Number and Mass Number Henry Moseley (1887-1915) Found a relation between the X-ray spectra of certain elements and their atomic numbers. Later on, the atomic number became associated with the number of protons in the nucleus of an atom. Thus, the positions of the elements in the earlier versions of the periodic table were either corrected or retained. The next element in the periodic table always has one proton more than the previous element.  Isotopes Definition of Isotopes: Atoms of the same element can have different numbers of neutrons. This results in different mass numbers but the same atomic number. Isotopes have the same electron configuration. Examples of Hydrogen Isotopes: Protium: Hydrogen-1 (H-1) — 1 proton, 0 neutrons. Deuterium: Hydrogen-2 (H-2) — 1 proton, 1 neutron. Tritium: Hydrogen-3 (H-3) — 1 proton, 2 neutrons.  Isotopes Applications of Isotopes: Uranium Isotopes: Naturally occurring uranium: 99.3% Uranium-238, 0.711% Uranium-235, 0.006% Uranium-234. Used as principal fuels in nuclear reactors. Uranium-238 helps determine the age of marine sediments. Carbon Isotopes: Naturally occurring carbon: 98.93% Carbon-12, 1.07% Carbon-13. Carbon-14 is used in carbon dating of archaeological materials. Medical Applications: Iodine-131: Used in radioisotope therapy to treat thyroid cancer. Technetium-99: Employed to produce images of specific organs for medical diagnosis and treatment.  End of Presentation. 30 Zn Zinc 12 Mg agnesium M