Transcript lecture
Faculty of Biotechnology General Chemistry Lecture 3 Periodic Table of Elements Dr. M. Abd-Elhakeem Mendeleev • In 1869, Dmitri Ivanovitch Mendeléev created the first accepted version of the periodic table. • He grouped elements according to their atomic number, and as he did, he found that the families had similar chemical properties. • Blank spaces were left open to add the new elements he predicted would occur. • A periodic table is a tabular display of the chemical elements, organized on the basis of their atomic numbers. • The main body of the standard form of table is an 18 groups (columns) and 7 periods (rows). Families • Columns of elements are called groups or families. • Elements in each family have similar but not identical properties. • For example, lithium (Li), sodium (Na), potassium (K), and other members of family IA are all soft, white, shiny metals. • All elements in a family have the same number of valence electrons. Periods • Each horizontal row of elements is called a period. • The elements in a period are not alike in properties. • In fact, the properties change greatly across even given row. • The first element in a period is always an extremely active solid. The last element in a period, is always an inactive gas. • Three major classes are immediately recognized. – Metals – non-metals – Metalloids Properties of Metals • Good conductors of heat and electricity. • Shiny. • Ductile (can be stretched into thin wires). • Malleable (can be pounded into thin sheets). • A chemical property of metal is its reaction with water which results in corrosion. Properties of Non-Metals • Poor conductors of heat and electricity. • Not ductile or malleable. • Solid non-metals are brittle and break easily. • Dull. • Many non-metals are gases. Sulfur Properties of Metalloids Silicon • Metalloids (metal-like) have properties of both metals and non-metals. • They are solids that can be shiny or dull. • They conduct heat and electricity better than nonmetals but not as well as metals. • They are ductile and malleable. Famous groups and periods 1. 2. 3. 4. 5. 6. 7. Alkali metals (Group IA) Alkaline Earth Metals (Group IIA) Halogens (Group VIIA) Nobel gases (Group VIIIA) Transition metals Lanthanides Actinides. Alkali Metals • The alkali family is found in the first column of the periodic table. • Atoms of the alkali metals have a single electron in their outermost level (s subshell), in other words, 1 valence electron. • They are shiny, have the consistency of clay, and are easily cut with a knife. Alkali Metals • They are the most reactive metals. • They react violently with water to form alkali. • Alkali metals are never found as free elements in nature. They are always bonded with another element. Alkaline Earth Metals • They are never found uncombined in nature. • They have two valence electrons. • Alkaline earth metals include magnesium and calcium, that present commonly in earth crust. Transition Metals • Transition Elements locate in the middle of the periodic table • These are the metals you are probably most familiar: copper, tin, zinc, iron, nickel, gold, and silver. Transition Metals • The compounds of transition metals are usually brightly colored and are often used to color paints. • Transition elements have more than one valence and so you must calculate the oxidation state in each compound Transition Elements Many transition metals combine chemically with oxygen to form compounds called oxides. Halogen Family • The elements in this family are fluorine, chlorine, bromine, iodine, and astatine. • Halogens have 7 valence electrons, which explains why they are the most active nonmetals. They are never found free in nature. • Halogen atoms only need to gain 1 electron to fill their outermost energy level. i.e. oxidation state = -1 • They react with alkali metals to form salts. • They are gases Noble Gases • Noble Gases or inert gases are colorless gases that are extremely un-reactive. • One important property of the noble gases is their inactivity. They are inactive because their outermost energy level is full. • The family of noble gases includes helium, neon, argon, krypton, xenon, and radon. • All the noble gases are found in small amounts in the earth's atmosphere. • Note that There are four distinct rectangular areas or blocks. • s block • p block • d block • The f-block is usually not included in the main table, but rather is floated below. s block elements d block elements p block elements f block elements Periodicity of chemical properties 1- Atomic radius: the atomic radii decrease in going from left to right across a given period. Atomic radii increase from top to bottom down a group 2- Ionization Energy Ionization energy is the amount of energy required to remove an electron from a gaseous atom or ion. Ionization energy generally increases from left to right across periods and decrease from top to bottom down groups. Electronegativity • It is the tendency of an atom or a functional group to attract electrons • electronegativity increases on passing from left to right along a period, and decreases on descending a group Polarity • It is the charged pole in the compounds due to difference in electron negativity and the asymmetry of the compound's structure Water is a polar molecule Methane is nonpolar molecule Solubility • Solubility is defined as the maximum mass of a substance that can be dissolved in a fixed mass of a solvent at a given temperature. • A substance will have a different solubility in different solvents depending on polarity of both the substance and the solvent. • Still remember about ”like dissolves like”? Factors affecting solubility 1. Temperature 2. Pressure 3. Polarity of solvent and solute Name Solubility in water Exception Nitrates, acetates, Soluble perchlorates Ag+ for acetate sulphates Ba+2, Pb+2, Hg+2, Ca+2, Ag+1, and Sr+2 Soluble Chlorides, bromides, Soluble iodides Pb+2, Ag+2, Hg+2 Carbonates, phosphates, Sulfites, Sulphides Alkali metal ammonium Hydroxides, chromates , Insoluble Insoluble and Ba+2, Sr+2, and Ca+2 • In fact you can predict most of physical properties of any unknown element only by its position in the periodic table. • Ex. Predict the physical and chemical properties of 55Cs Atomic Mass Unit (AMU) • The unit of measurement for an atom is an AMU. It stands for atomic mass unit. • One AMU is equal to the mass of one proton. Atomic Mass Unit (AMU) • There are 6 X 1023 or 600,000,000,000,000,0 00,000,000 amus in one gram. • (Remember that electrons are 2000 times smaller than one amu). Calculation of molecular weight