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Chapter 2

Chapter 2.docx

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Lydia W.L.Chen

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Chapter 2:Atoms, Molecules and Ions 2.1 – TheAtomic Theory • In 500 BC, the Greek philosopher Democritus expressed his belief that all matter is composed of small, indivisible particles (which he called atomos). • In 1808, John Dalton generated an atomic theory which defined what we call atoms. The main ideas of his atomic theory are summarized below: 1. Elements are composed of tiny, indivisible particles called atoms. 2. All atoms of a given element are identical (same size, mass, and chemical properties). However, the atoms of one element are different from atoms of other elements. 3. Compounds are composed of atoms of more than one element. In any compound, the ratio of the number of atoms of any two elements will be in whole numbers or fractions. 4. Achemical reaction involves the separation, combination and rearrangement of atoms. It does not involve the creation or destruction of any atoms. • Rule 3 supports the law of definite proportions and law of multiple proportions. o Law of Definite Proportions → Different samples of a compound contain the elements that the compound is composed of in the same proportions by mass. o Also, in a compound, the ratios of the atoms of the elements that make up the compound must be the same.  Every molecule in carbon dioxide must have a 2:1 ratio of O atoms to C atoms.Also each molecule must be 73% O and 27% C by mass. o Law of Multiple Proportions → When two elements, A and B combine to form more than one compound, in which the mass of elementAremains constant, the ratio of the masses of element B in the compounds will be in whole numbers.  The ratio of O in CO to the O in CO2is 1:2. • Rule 4 support the law of conservation of mass. o Law of Conservation of Mass → Matter can neither be created nor destroyed.  In a chemical reaction, the atoms get rearranged to form new compounds. No atoms are created or destroyed. 2.2 – The Structure of theAtom • Dalton proposed that atoms were indivisible. However, it was later discovered that atoms consisted of three subatomic particles: protons, neutrons, and electrons. Electron • The cathode ray tube experiment, performed by JJ Thomson led to the discovery of the electron. • When two metal plates are connected to a high voltage source, the negatively charged plate, called the cathode emits an invisible ray called the cathode ray. • The cathode ray is drawn towards the positively charged plate called the anode and passes through a hole in it. • The cathode ray passes through the anode and then to the end of the tube. Here, it strikes the fluorescent screen (specially coated surface) and produces a bright light. • When a magnet (magnetic field) and two electrically charged plates (electric field) are added to the tube, the cathode ray experienced a magnetic force and an electric force. o When the magnetic field is on and the electric field is off, the ray deflected toA. o When the magnetic field is off and the electric field is on, the ray deflected to C. o When the magnetic and electric fields both were on, the forces exerted by both fields on the ray got cancelled and the ray continued straight to B. • JJ Thomson said that since the cathode ray got attracted to the positively charged plate and repelled the negatively charged plate, the cathode ray consisted of negatively charged particles he called electrons. Any moving charged body behaves like a magnet and can interact with external magnetic and electric fields. • When a magnet was brought close to the cathode ray tube, the cathode rays deflected either up or down, depending on the polarity of the magnet. Reversing the polarity caused the cathode rays to deflect in the opposite direction • JJ Thomson then used the cathode ray tube to determine the charge to mass ratio of an electron. He calculated it to be -1.76 × 10 C/g. http://www.youtube.c om/watch?v=GzMh4q- 2HjM • Between 1908 and 1917, Robert Millikan performed an experiment that helped him to determine the charge of an electron. o In his experiment, an atomizer sprayed a fine mist of oil into the upper chamber. o Then, some of these oil droplets moved through a hole in the upper, positive plate and down into the chamber below. Millikan used the terminal velocity of the oil droplets to determine their mass. o X-rays were used to ionize gas particles in the second chamber. Electrons lost by the gas particles combined with oil droplets, giving them a negative charge. o When the voltage across the plates increased, the oil drops started to attract to the positive plate and began to slow down. o The voltage was adjusted in such a way that the F aed F cougteracted each other, causing the oil droplets to remain floating in the second chamber.  When this happened, he used the formula qε = mg and solved for the charge, q. He noticed that all the charges were multiples of -1.6022 × 10 -19 -19 C and concluded that -1.6022 × 10 C is the charge of an electron.  Then he used this value and the charge to mass ratio determined by Thomson to calculate the mass of an electron, 9.10 × 10 -2g. http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi? it=swf::100%::100%::/sites/dl/free/0072512644/117354/02_Millikan_Oil_Drop.swf::Milikan%20Oil%20Drop Radioactivity • In 1895, Wilhelm Röntgen noticed that cathode rays emit highly energetic radiation which caused the photographic plates in the cathode ray tube experiment to darken. • Since the rays of this radiation could not be deflected by a magnet, they were not composed of charged particles. o Röntgen called these rays X-rays. • Antoine Becquerel noticed that thickly wrapped photographic plates started to darken when exposed to a certain Uranium compound. o Similar to the X-rays that Röntgen discovered, the rays from the Uranium compound were highly energetic and could not be deflected by a magnet. However, unlike X-rays, they were emitted spontaneously. • Any element that spontaneously emits radiation is said to be radioactive. • Three types of rays can be produced from the radiation emitted by radioactive substances. o Alpha rays consist of positively charged α particles. Therefore, they are deflected towards the negatively charged plate. o Beta rays consist of β particles (electrons). They are deflected towards the positively charged plate. o Gamma rays are composed of γ particles, which are electrically neutral. They do not get deflected towards any of the electrical plates as they are NOT affected by electrical/magnetic fields. The Proton and the Nucleus • In the early 1900s, Thomson’s plum pudding model of the atom was used. o In the plum pudding model of the atom, the electrons are embedded in a uniform positively charged sphere. o The positive charge of the sphere is enough to cancel the negative charge of all the electrons. • In 1910, Rutherford performed a series of experiments that helped him in discovering the structure of an atom. o He directed alpha particles at various metal foils (i.e. gold foil). Rutherford predicted that most of the alpha particles would be slightly deflected as they pass through the foil since an atom’s positive charge is evenly distributed throughout the atom in Thomson’s model. o However, most of the alpha particles went through the foil without getting deflected, or with a very slight deflection.Avery small number of particles had a huge deflection or bounced back. • Rutherford proposed a new model of the atom that explained the observations of his experiment. o Most of an atom is empty space.  This explains why many alpha particles were not deflected or only slightly deflected. o All the positive charge and most of the mass are in the centre of the atom called the nucleus. The nucleus only occupies 1/10 of the atom’s volume.  The few alpha particles that bounced back or had a huge deflection were directed at the nucleus. The great repulsion force between the alpha particle and the nucleus caused the particles to deflect greatly or bounce back. 13 o The nucleus takes up 1/10 of the volume of an atom. The Neutron • One major problem in Rutherford’s model of the atom was that it could not explain why the ratio of the masses in helium and hydrogen is 4:1. o Since a hydrogen atom has 1 proton and a helium atom has two protons, the ratio should be 2:1. o Rutherford said that the difference in ratio is because the nucleus probably consists of a third subatomic particle. • James Chadwick discovered the third subatomic particle, the neutron. o When he bombarded a thin sheet of beryllium with α particles, a high energy radiation of electrically neutral particles was emitted. o Chadwick called these neutral particles neutrons and said that they exist in the nucleus. Neutrons have a mass slightly greater than protons. • The three main subatomic particles are protons, neutrons, and electrons. Protons and neutrons are located in the nucleus, whereas electrons are located in orbitals at various distances from the nucleus. 2.3 –Atomic Number, Mass Number, and Isotopes • Atomic Number (Z) = Number of Protons • The number of protons in an atom determines the element the atom is from. o Ex:Any atom that contains 9 protons is a fluorine atom • In a neutral atom, Number of Protons = Number of Electrons • Mass Number (A) = Number of Protons + Number of Neutrons Isotopes:Atoms of the same element that have different numbers of neutrons (same atomic number, different mass number). o Ex: Hydrogen (1p, 0n), Deuterium (1p, 1n), and Tritium (1p, 2n) • All isotopes except for the ones for H are identified by their mass numbers. o Ex: carbon-12, carbon-13, carbon-14 • Since the numbers of protons and electrons in an atom determine its chemical properties, all isotopes have the same chemical properties (only the number of neutrons vary between isotopes). o This means that all isotopes are equally reactive and can form the same compounds. 2.4 – The Periodic Table Periodic Table:Achart in which elements with similar chemical and physical properties are grouped together. • In the periodic table, elements are arranged in terms of atomic number. • Horizontal rows are called periods. There are 7 periods in the periodic table. • Vertical columns are called groups or families. There are 18 groups in the periodic table. • The elements in the periodic table can be divided into 3 categories – metals, non-metals, and metalloids. o Ametal is a good conductor of heat and electricity. The majority of the elements
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