3INTRODUCTION Chemistry is the science of substances. Chemistry studies the structure, properties and transformation of substances.Substances are charaterized by 2 essential properties: homogeneity and constant composition.
4INTRODUCTIONHomogeneity = is the property of substances to show same characteristics in all the volume.Constant composition = in any given portion of a substance there are the same particles which interact in the same way.Substances can not be separated into other substances by physical methods.
5INTRODUCTIONExemple of substances: water, sugar, oxygen, hydrogen, sodium chloride, copper, hydrochloric acid, sodiu hydroxide.Air = substance?Air is not a substance. By distillation of liquefied air one obtains O2, N2 and other gases.
6INTRODUCTION Gasoline = substance? Gasoline is not a substance, because by distillation it can be separated into the component hydrocarbons.NaCl solution = substance?Sodium chloride solution is not a substance, because by boiling the water is removed and the NaCl crystals remains.
7INTRODUCTION In nature, substances are found only rarely in pure form. Materials are made of substances or mixtures of substances.Materials can be homogeneous sau heterogeneous.Heterogeneous materials are composed of so-called "phases" which are homogeneous portions separated from other phases by areas where the properties vary sharply.
8INTRODUCTION Exemple of materials: metals and alloys. Metals are homogeneous materials containing a single metallic element. Alloys are homogeneous or heterogeneous mixtures of two or more metals.Alloys:Brass = copper and zinc alloyBronze = copper-tin alloy
9INTRODUCTION Exemple of materials: Wood = homogeneous material, composed of cellulose, lignin, resins and other substances.Glass = usually a homogeneous amorphous material (component particles are not included in a crystal lattice)Granite = homogeneous material composed of three different minerals: quartz, feldspar and mica.
10INTRODUCTIONSolutions = homogeneous mixture composed of two or more substances. Solutions are obtained by dissolving a substance called solute into another substance called solvent.Exemplesliquid solutionssolid solutionsgaseous solutions
11INTRODUCTION Liquid solutions: gas dissolved in liquid (CO2 in water) liquid dissolved in liquid (ethanol in water)solid dissolved in liquid (copper sulfate in water, naphtalene in benzene)Solid solutions = homogeneous alloysGaseous solutions = homogeneous mixtures of gases, i.e. air.<a href="http://www.publicdomainpictures.net/view-image.php?image=15830&picture=bare-de-aur">Bare de aur</a> de Petr Kratochvil
12INTRODUCTIONEach substance is distinguished from the other substances by its properties. A substance is characterized by its physical properties that can be expressed by numerical values. For pure substances these properties are called physical constants.melting point, boiling pointdensityrefractive indexdielectric constant
13International System of Units (S.I.). S.I. has 7 fundamental units INTRODUCTIONInternational System of Units (S.I.).S.I. has 7 fundamental unitsQuantityUnitSymbolLengthmetermMasskilogramkgTimesecondsThermodynamic temperatureKelvinKElectric currentAmpereALuminous intensitycandelacdAmount of substancemolemol
14Derived units Quantity Equation Name of the unit Symbol Area m2 Volume S = l2l – lengthsquare meterm2VolumeV = l3cubic meterm3Speedv = l / tt – timemeter per secondm s-1Acelerationa = v / tmeter per second squaredm s-2Molar concentrationcM = n / Vn- amount of substanceV-volumemole per cubic metermol m-3ForceF = m·am – massa – accelerationkilogram· meter per second squared (Newton)kg· m s-2( N )PressureP = F / SS – areakilogram· meter per square second per square meterkg· m s-2· m-2N· m-2= PaDensityρ = m / Vkilogram per cubic meterkg· m-3
15I. ATOMIC STRUCTURE OF SUBSTANCES Since antiquity there is the idea that things in nature are composed of very small particles, indivisible, called atoms (atom, gr. = indivisible).Atomic conception was resumed by J. Dalton in 1805, who showed on the basis of experimental observations accumulated until then that matter is made of atoms. Avogadro's research related to gaseous state, led to the discovery of molecules, where the atoms are bound together in different ratios.
16I. STRUCTURE OF SUBSTANCES Atom = basic unit of matter, indestructible by ordinary chemical methods.Molecule = the smallest particle of a substance that can exist in the free state and manifests the properties of the substance.
17I. STRUCTURE OF SUBSTANCES At the end of the 19th century, the English physicist J. J. Thomson showed that the atoms of any element emit tiny negative particles. He called the particles "corpuscles", but later scientists preferred the name electron.Although atoms contain negative particles, the whole atom is not charged. So, Thomson concluded that atoms must also contain positive particles to balance the negative charge of the electrons. He imagined the “plum pudding model” of the atom, in which electrons are scattered like plums into the uniform “soup” or “cloud” of positive charges.
18I. STRUCTURE OF SUBSTANCES Plum pudding modelelectronscloud of positive chargeIn this model, the atom is composed of electrons surrounded by a “soup” of positive charge to balance the electrons' negative charges.
19The “plum pudding” model was disapproved by RUTHERFORD’s experience. I. STRUCTURE OF SUBSTANCESThe “plum pudding” model was disapproved by RUTHERFORD’s experience.Rutherford measured the deviation of alpha particles (helium ions with a positive charge) directed normally onto a sheet of very thin gold foil. Since the mass of the electrons is very small, and the α particles are heavy, considering the plum pudding model, the alpha particles should pass through the foil without significant deviation. The results of the experiment were very different from what Rutherford anticipated. Most of the α particles passed straight through the foil, but some of them were deflected at large angles and some were even reflected backward.
20Rutherford’s experiment Observed resultsExpected results for the “plum pudding model”“It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. … I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive center, carrying a charge” — Ernest Rutherford
21I. STRUCTURE OF SUBSTANCES Conclusions:the plum pudding model for the atom could not be correct;the atom must contain a very small (compared with the size of the atom) positive charge which causes the large deflections of the α particles;the atom is mostly empty space because most of the α particles passed directly through the foil.
22I. STRUCTURE OF SUBSTANCES Based on these results, Rutherford proposed the planetary model of the atom. According to this model the atom is composed of 2 parts: a small, compact center of positive charge called nucleus surrounded by a cloud of tiny negative particles called electrons, moving around the nucleus.
23I. STRUCTURE OF SUBSTANCES Planetary model of the atom
24I. STRUCTURE OF SUBSTANCES The planetary model of the atom is not perfect, because it can not explain why electrons are not colliding with the nucleus. It is known that an accelerating electric charge emits electromagnetic waves, so electrons will continuously lose energy, they will spiral towards the nucleus and finally they will collide with the nucleus.To explain why this did not occur, Niels Bohr (1913) postulated:
25I. STRUCTURE OF SUBSTANCES Electrons can move around the nucleus only on certain orbits, called allowed orbits.Electrons can gain or lose energy only by jumping from one allowed orbit to another. When an electron moves towards the nucleus energy is radiated and if it moves away from the nucleus energy is absorbed.For an electron to remain in its orbit the electrostatic attraction force between the electron and the nucleus must equal to the centrifugal force which tends to throw the electron out of its orbit.
26I. STRUCTURE OF SUBSTANCES Niels Bohr admitted that the orbits of the electrons are circular.Bohr’s model of the atom
27I. STRUCTURE OF SUBSTANCES A. Sommerfeld (1916), based on the atomic spectra of hydrogen, suggested that the permissive orbits of the electrons may be elliptic.Bohr – Sommerfeld model of the atom
28I. STRUCTURE OF SUBSTANCES In 1919 Rutherford discovered that the nucleus contains positive particles named protons. In 1932 James Chadwick discovered that nucleus contains also neutral particles – neutrons. Protons and neutrons are known as nucleons.+NucleusProtonNeutronNucleons
29Properties of the main subatomic particles I. STRUCTURE OF SUBSTANCESProperties of the main subatomic particlesElectric chargeMassSymbol[C]Relative chargekgamuProton+1.602·10-19+1·10-271.0073p; p+Neutron·10-271.0087n; noElectron-1.602·10-19-1·10-31e; e-Proton = a nucleon with positive charge (+1)Neutron = a nucleon with zero charge (0)Protons and neutrons have close weights, and electrons are much lighter.