Elements and the Periodic Table • Classification is arranging items into groups or categories according to some criteria. • The act of classifying creates a pattern that helps you recognize and understand the behavior of fish, chemicals, or any matter in your surroundings. • Classifying Matter • Matter is usually defined as anything that has mass and occupies space. Total disorder Lots of empty space Disorder Some space Particles closer together Order Particles fixed in position Gas Liquid Solid • Solids, Liquids, and Gases – Gases have no defined shape or defined volume • Low density – Liquids flow and can be poured from one container to another • Indefinite shape and takes on the shape of the container. – Solids have a definite volume • Have a definite shape. • Mixtures and Pure Substances – A mixture has unlike parts and a composition that varies from sample to sample – A heterogeneous mixture has physically distinct parts with different properties. – A homogeneous mixture is the same throughout the sample – Pure substances are substances with a fixed composition • A classification scheme for matter. – A physical change is a change that does not alter the identity of the matter. – A chemical change is a change that does alter the identity of the matter. – A compound is a pure substance that can be decomposed by a chemical change into simpler substances with a fixed mass ratio – An element is a pure substance which cannot be broken down into anything simpler by either physical or chemical means. • Sugar (A) is a compound that can be easily decomposed to simpler substances by heating. (B) One of the simpler substances is the black element carbon, which cannot be further decomposed by chemical or physical means. EXAMPLE Isopropyl alcohol is a A. heterogeneous mixture B. homogeneous mixture C. pure substance D. Compound E. pure substance and compound E • Elements • Reconsidering the Fire Element – The phlogiston theory viewed phlogiston as a component of all matter. – The burning of a material was considered to be the escaping of phlogiston from the matter. – If a material did not burn, it was considered to contain no phlogiston. • The phlogiston theory. (A) In this theory, burning was considered to be the escape of phlogiston into the air. (B) Smelting combined phlogiston-poor ore with phlogiston from a fire to make a metal. (C) Metal rusting was considered to be the slow escape of phlogiston from a metal into the air. • Discovery of Modern Elements – Antoine Lavoisier suggested that burning was actually a chemical combination with oxygen. – Lavoisier realized that there needed to be a new concept of elements, compounds, and chemical change. – We now know that there are 89 naturally-occurring elements and at least 23 short-lived and artificially prepared. • Priestley produced a gas (oxygen) by using sunlight to heat mercuric oxide kept in a closed container. The oxygen forced some of the mercury out of the jar as it was produced, increasing the volume about five times. • Lavoisier heated a measured amount of mercury to form the red oxide of mercury. He measured the amount of oxygen removed from the jar and the amount of red oxide formed. When the reaction was reversed, he found the original amounts of mercury and oxygen. • Names of Elements – The first 103 elements have internationally accepted names, which are derived from: • The compound or substance in which the element was discovered • An unusual or identifying property of the element • Places, cities, and countries • Famous scientists • Greek mythology • Astronomical objects. • Here are some of the symbols Dalton used for atoms of elements and molecules of compounds. He probably used a circle for each because, like the ancient Greeks, he thought of atoms as tiny, round hard spheres. • The elements of aluminum, Iron, Oxygen, and Silicon make up about 88 percent of the earth's solid surface. Water on the surface and in the air as clouds and fog is made up of hydrogen and oxygen. The air is 99 percent nitrogen and oxygen. Hydrogen, oxygen, and carbon make up 97 percent of a person. Thus almost everything you see in this picture us made up of just six elements. Atomic theory Dalton’s Postulates 1. Every element is composed of tiny particles called atoms 2. All atoms of a given element are identical 1. Atoms of different elements have different properties 3. Atoms of an element are NOT changed into atoms of another element by chemical processes 1. Matter can neither be created nor destroyed 4. Compounds are formed when atoms of more than one element combine Dalton’s Laws 1. The Law of Constant Composition: “Any given compound always consists of the same atoms and the same ratio of atoms. For example, water always consists of oxygen and hydrogen atoms, and it is always 89 percent oxygen by mass and 11 percent hydrogen by mass” 2. The Law of Conservation of Mass: “The total mass of materials before and after a chemical reaction must be the same. For example, if we combine 89 grams of oxygen with 11 grams of hydrogen under the appropriate conditions, 100 grams of water will be produced—no more and no less.” Dalton’s Laws 3. The Law of Multiple Proportions: “If two elements combine to form more than one compound, the masses of one of the elements that can combine with a given mass of the other element are related by factors of small whole numbers” For example, water has an oxygen-to-hydrogen mass ratio of 7.9:1. Hydrogen peroxide, another compound consisting of oxygen and hydrogen, has an oxygen-to-hydrogen mass ratio of 15.8:1. The ratio of these two ratios gives a small whole number. MODERN ATOMIC THEORY – Chemical Symbols • There are about a dozen common elements that have s single capitalized letter for their symbol • The rest, that have permanent names have two letters. – the first is capitalized and the second is lower case. • Some elements have symbols from their Latin names. • Ten of the elements have symbols from their Latin or German names. – Symbols and Atomic Structure • A molecule is a particle that is composed of two or more atoms held together by a chemical bond. • Isotopes are atoms of an element with identical chemical properties, but different masses due to a difference in the number of neutrons. • The atomic mass of an element is the average of all the atomic masses of the isotopes. – an isotopes contribution is determined by its relative abundance. • The mass of an element is the mass of the element compared to an isotope of carbon Carbon 12. – Carbon 12 is assigned an atomic mass of 12.00 g. – 12.00 is one atomic mass unit • The number of protons and neutrons in an atom is its mass number. • Atomic numbers are whole numbers • Mass numbers are whole numbers • The atomic mass is not a whole number. Atomic Mass Atomic Number Symbol Charge (if ion) Hydrogen Protons: 1 Neutrons: 0 Electrons: 1 1 1 H Sodium 23 11 Na Protons: 11 Neutrons: 12 Electrons: 11 Rhenium 186 75 Re Protons: 75 Neutrons: 111 Electrons: 75 Rhenium isotope 187 75 Re Protons: 75 Neutrons: 112 Electrons: 75 EXAMPLE How many protons, neutrons and electrons are found in an atom of 133 55 Cs Atomic number = protons and electrons There are 55 protons and 55 electrons Mass number = sum of protons and neutrons 133 – 55 = 78 There are 78 neutrons • The Periodic Law • Dmitri Medeleev gave us a functional scheme with which to classify elements. – Mendeleev’s scheme was based on chemical properties of the elements. – It was noticed that the chemical properties of elements increased in a periodic manner. – The periodicity of the elements was demonstrated by Medeleev when he used the table to predict to occurrence and chemical properties of elements which had not yet been discovered. • Mendeleev left blank spaces in his table when the properties of the elements above and below did not seem to match. The existence of unknown elements was predicted by Mendeleev on the basis of the blank spaces. When the unknown elements were discovered, it was found that Mendeleev had closely predicted the properties of the elements as well as their discovery. • The Periodic Law – Similar physical and chemical properties recur periodically when the elements are listed in order of increasing atomic number. • The Modern Periodic Table • Introduction – The periodic table is made up of rows of elements and columns. – An element is identified by its chemical symbol. – The number above the symbol is the atomic number – The number below the symbol is the rounded atomic weight of the element. – A row is called a period – A column is called a group • (A) Periods of the periodic table, and (B) groups of the periodic table. • Periodic Patterns – The chemical behavior of elements is determined by its electron configuration – Energy levels are quantized so roughly correspond to layers of electrons around the nucleus. – A shell is all the electrons with the same value of n. • n is a row in the periodic table. – Each period begins with a new outer electron shell – Each period ends with a completely filled outer shell that has the maximum number of electrons for that shell. – The number identifying the A families identifies the number of electrons in the outer shell, except helium – The outer shell electrons are responsible for chemical reactions. – Group A elements are called representative elements – Group B elements are called transition elements. • Chemical “Families” – IA are called alkali metals because the react with water to from an alkaline solution – Group IIA are called the alkali earth metals because they are reactive, but not as reactive as Group IA. • They are also soft metals like Earth. – Group VIIA are the halogens • These need only one electron to fill their outer shell • They are very reactive. – Group VIIIA are the noble gases as they have completely filled outer shells • They are almost non reactive. • Four chemical families of the periodic table: the alkali metals (IA), the alkaline earth metals (IIA), halogens (VII), and the noble gases (VIIIA). Metal: Elements that are usually solids at room temperature. Most elements are metals. Non-Metal: Elements in the upper right corner of the periodic Table. Their chemical and physical properties are different from metals. Metalloid: Elements that lie on a diagonal line between the Metals and non-metals. Their chemical and physical properties are intermediate between the two. – When an atom or molecule gain or loses an electron it becomes an ion. • A cation has lost an electron and therefore has a positive charge • An anion has gained an electron and therefore has a negative charge. – Elements with 1, 2, or 3 electrons in their outer shell tend to lose electrons to fill their outer shell and become cations. • These are the metals which always tend to lose electrons. – Elements with 5 to 7 electrons in their outer shell tend to gain electrons to fill their outer shell and become anions. • These are the nonmetals which always tend to gain electrons. – Semiconductors (metalloids) occur at the dividing line between metals and nonmetals. EXAMPLE What would the charge be on a sodium ion? Since sodium in in Group IA it is a metal and so would LOSE an electron You can tell how many would be lost by the group number Group 1A elements lose 1 electron So the charge would be +1 Remember an electron is negatively charged. When you lose them atom becomes positively charged… when you gain them it becomes negatively charged EXAMPLE How would you right the symbol for the sodium CATION? How many outer electrons does sodium have before it loses one? Na +1 It has 1…remember the group number!
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