History of Chemistry Chemistry has been important since ancient (very old) times. The Greeks about 400 B.C. first explained why chemical changes occur. Greeks had proposed that all matter was composed of four fundamental substances fire, earth, water, and air. The foundations of modern chemistry were laid in the sixteenth century. By the late eighteenth century, combustion had been studied extensively. Antoine Lavoisier (1743–1794), a French chemist , who finally explained the true nature of combustion.

History of Chemistry Antoine Lavoisier in 1783 presented law of conservation of mass (Mass is neither created nor destroyed in a chemical reaction). He is Known as “father of modern chemistry.” John Dalton (1766–1844), an English school teacher stated that elements were composed of tiny individual particles (atoms), a given compound should always contain the same combination of these atoms.

Question The first people to attempt to explain why chemical changes occur were a) alchemists. b) metallurgists. c) physicians. d) physicists. e) the Greeks.

Question The Greeks proposed that matter consisted of four fundamental substances: a) fire, earth, water, air b) fire, metal, water, air c) earth, metal, water, air d) atoms, fire, water, air e) atoms, metal, fire, air

Question The scientist who discovered the law of conservation of mass and is also called the father of modern chemistry is a) Proust. b) Boyle. c) Priestly. d) Bauer. e) Lavoisier.

Dalton’s Atomic Theory In 1808 Dalton published his theory of atoms; 1- Each element is made up of tiny particles called atoms. 2. The atoms of a given element are identical; the atoms of different elements are different. 3. Chemical compounds are formed when atoms of different elements combine with each other. 4. Chemical reactions involve reorganization of the atoms—changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction.

Question Which of the following statements from Dalton’s atomic theory is no longer true, according to modern atomic theory? a)Elements are made up of tiny particles called atoms. b) Atoms are not created or destroyed in chemical reactions. c) All atoms of a given element are identical. d) Atoms are indivisible in chemical reactions. e) All of these statements are true according to modern atomic theory.

In 1809 Gay-Lussac performed experiments in which he measured (under the same conditions of temperature and pressure) the volumes of gases that reacted with each other. For example, Gay-Lussac found that 2 volumes of hydrogen react with 1 volume of oxygen to form 2 volumes of gaseous water and that 1 volume of hydrogen reacts with 1 volume of chlorine to form 2 volumes of hydrogen chloride.

Avogadro’s Hypothesis In 1811 Avogadro interpreted that; At the same temperature and pressure, equal volumes of different gases contain the same number of particles. (1 mole of any gas contains 6.022 x 1023 atoms/ions or molecules) Joseph Proust (1754–1826) presented Law of Constant Composition, also known as the law of definite proportions. • It states that the elemental composition of a pure substance never varies.

DISCOVERY OF ELECTRON The English physicist J. J. Thomson (1897), performed experiments to understand composition of the atom. He studied electrical discharges in partially evacuated tube called cathode-ray tube. Thomson found that when high voltage was applied to the tube, a “ray” he called a cathode ray (because it emanated from the negative electrode, or cathode) was produced. J. J. Thomson experiment resulted in the discovery of electron.

Because this ray was produced at the negative electrode and was repelled by the negative pole and attracted towards positive pole of applied electric field, Thomson postulated that the ray was a stream of negatively charged particles, now called electrons. Thompson also measured the charge/mass ratio of the electron to be 1.76 108 coulombs/g.

Cathode-ray Tube Experiment

The first scientist to show that atoms emit any negative particles was Question The first scientist to show that atoms emit any negative particles was J. J. Thomson. Lord Kelvin. c) Ernest Rutherford. d) William Thomson. e) John Dalton

Robert Millikan Experiment In 1909 Robert Millikan, performed an experiments involving charged oil drops. These experiments allowed him to determine the magnitude of the electron charge. With the charge-to-mass ratio determined by Thomson, Millikan was able to calculate the mass of the electron as me = 9.11 X 10─31 kilogram or me = 9.11x10 ─ 28 g. Charge-to-mass ratio of electron is given below; Magnitude of Charge on electron = ─1.60 x 10 ─19 C Magnitude of Charge/mass ratio (e/m) = 1.76 x 108 C/g

In 1911 Ernest Rutherford (Discover of Nucleus), shot a beam of α-particles at a thin sheet of gold foil and observed the pattern caused by their scattering. α particles are helium nuclei,4He2+.He observed the particles to travel through the foil with, at the most, very minor deflections in their paths. The results of the experiment were very different from those Rutherford anticipated.

Although most of the particles passed straight through, many of the particles were deflected at large angles, and some were reflected, never hitting the detector. This outcome was a great surprise to Rutherford. The large deflections of the particles could be caused only by a center of concentrated positive charge that contains most of the atom’s mass.

Atoms are made up of three major parts: Part Found Mass Charge Protons Part of Nucleus 1.7x10-24g (1.0 AMU) +1 Neutrons Part of Nucleus 1.7x10-24g (1.0 AMU) 0 Electrons Outside Nucleus 9.1x10-28g (small) -1 Notes: 1) Neutral atoms contain equal number of electrons and protons. 2) Atoms can loose or gain electrons to become charged = ions 3) Electrons are reshuffled in a chemical reaction. 4) Number of protons (Atomic number) determine the identity of the atom or ion. 5) Mass of atom = Number of Protons plus Number of Neutrons

Question The scientist whose alpha-particle scattering experiment led him to conclude that the nucleus of an atom contains a dense center of positive charge is a) J. J. Thomson. b) Lord Kelvin. c) Ernest Rutherford. d) William Thomson. e) John Dalton.

Question Rutherford’s experiment was important because it showed that: a) radioactive elements give off alpha particles. b) gold foil can be made to be only a few atoms thick. c) a zinc sulfide screen scintillates when struck by a charged particle. d) the mass of the atom is uniformly distributed throughout the atom. e) an atom is mostly empty space.

Atomic Structure The nucleus has small size compared with the overall size of the atom and its extremely high density. The tiny nucleus represents the atom’s mass (A). Question: “If all atoms are composed of these same components, why do different atoms have different chemical properties?” Answer: Different atoms of different elements have different number and the arrangement of the electrons.

The Mass Number (A): The Mass Number of an atom is the total number of protons and neutrons in the atom. The Atomic Number (Z): It is Number of the protons or number of electrons in an atom. All atoms of the same element have the same number of protons or electrons.

Neutral atoms have the same number of protons and electrons. Atomic Structure Neutral atoms have the same number of protons and electrons. Ions are charged atoms as Cl ─, OH─ , Na+, H+. Cations = They have more protons than electrons and are positively charged as Na+, H+ Anions = They have more electrons than protons and are negatively charged as Cl ─, OH─. 23

Question All of the following are true except: a) Ions are formed by adding electrons to a neutral atom. b) Ions are formed by changing the number of protons in an atom’s nucleus. c) Ions are formed by removing electrons from a neutral atom. d) An ion has a positive or negative charge. e) Metals tend to form positive ions.

Question Which one of the following statements about atomic structure is false? a) The electrons occupy a very large volume compared to the nucleus. b) Almost all of the mass of the atom is concentrated in the nucleus. c) The protons and neutrons in the nucleus are very tightly packed. d) The number of protons and neutrons is always the same in the neutral atom. e) All of the above statements (a-d) are true.

Isotope The term isotope is formed from the Greek roots isos (equal) and topos (place). Hence: "the same place," meaning that different isotopes of a single element occupy the same position on the periodic table. The number of nucleons (protons and neutrons) in the nucleus is the mass number, and each isotope of a given element has a different mass number due to different number of neutrons.

Isotope Isotops of Hydrogen Nucleons Carbon Chlorine 1H 1 Proton 12C 6 Protons 6 Neutrons 35Cl 17 Protons 18Neutrons 2H 1 Neutron 13C 7 Neutrons 37Cl 20Neutrons 3H 2 Neutron 14C 8 Neutrons -

Isotopic Effect A neutral atom has the same number of electrons as protons. Thus, different isotopes of a given element all have the same number of protons and share a similar electronic structure. Because the chemical behavior of an atom is largely determined by its electronic structure, different isotopes exhibit nearly identical chemical behavior. However, heavier isotopes tend to react somewhat more slowly than lighter isotopes of the same element.

Question The element rhenium (Re) exists as two stable isotopes and 18 unstable isotopes. Rhenium-185 has in its nucleus a) 75 protons, 75 neutrons. b) 75 protons, 130 neutrons. c) 130 protons, 75 neutrons. d) 75 protons, 110 neutrons. e) not enough information is given.