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Chapter 2: Elements, Compounds, and Chemical Reactions Elements and Atoms Ellen Fraser The Periodic Table Elements and atoms can be explained by Dalton’s atomic theory, which in turn also explains the chemical laws: Law of Conservation of Mass: No detectable gain or loss of mass occurs in chemical reactions. Mass is conserved. Law of Definite Proportions: In a given chemical compound, the elements are always combined in the same proportions by mass. Laws of Multiple Proportions: Whenever 2 elements form more than 1 compound, the different masses of 1 element that combine with the same mass of the other element are in the same ratio of small whole numbers. Atomic Structure: Atoms are composed of 3 types of subatomic particles called protons, neutrons and electrons. The dense nucleus of an atom contains positively charged protons and neutrally charged neutrons, while negatively charged electrons surround the nucleus and fill the remaining space in an atom. As seen in the image above, the modern periodic table arranges elements in rows (periods) in order of increasing atomic number. The rows are arranged so that the columns (groups or families) have similar chemical and physical properties. Group 1 (with the exception on hydrogen) is classified as the alkali metals, Group 2 is classified as the alkaline earth metals, Group 17 is classified as the halogens, Group 18 is classified as the noble gasses and many of the elements between Groups 3-12 are classified as transition metals. Metals, Nonmetals and Metalloids Most elements are metals and they occupy the entire left hand side of the periodic table (with the exception of hydrogen). Nonmetals can be found in the upper right hand region of the table, while metalloids occupy a narrow band between metals and non metals (also known as the “ladder”). As for individual properties, metals exhibit metallic luster and tend to be ductile and malleable as well as good conductors of electricity. Nonmetals, which are often gases, tend to be the opposite, being brittle, lacking in metallic luster and poor conductors of electricity. Metalloids contain properties that are somewhere in between the properties of metals and nonmetals. Reactions of Elements to form Compounds Almost every element has the ability to form a compound, however not all combinations of elements are possible. When a chemical reaction occurs, the properties of the substances present in the reactant begin to disappear and are replaced with chemical properties of the new substances in the product. Chemical formulas are used to show what happened during a chemical reaction. Element symbols are used as well as subscripts (little numbers) for either free elements (diatomic molecules), or chemical compounds. These subscripts show how many atoms of each element are present. Some compounds can also form solids called hydrates which contain water molecules in definite proportions. Chemical Equations A chemical equation is a scientific equation that shows a before-andafter description of what happened in a chemical reaction. Chemical equations need to be balanced on both sides, and coefficients (big numbers) allow you to balance an equation. Coefficients have the ability to change the number of atoms among both the reactants and the products. This allows the equation to support the law of conservation of mass. The physical state of the reactants and products can be identified by placing symbols beside the chemical formulas. The symbols are s (solid), l (liquid), g (gas), and aq (aqueous solution or something that can be dissolved in water). Example of a Balanced Chemical Equation: 3Ba9OH02 + 2Na3PO4 Ba3(PO4)2 + 6NaOH There are also different types of chemical reactions that can make different chemical equations such as combination, decomposition, single replacement, double replacement, and combustion. Ions and Ionic Compounds Binary ionic compounds are formed when metals react with nonmetals. In this type of reaction, electrons are transferred from the metal to the nonmetal. The metal atom becomes a positive ion (a cation) while the nonmetal atom becomes a negative ion (an anion). The formula of an ionic compound needs to be in the smallest whole number ratio that is possible. There are also many ionic compounds that contain polyatomic ions, which are atoms that are composed of 2 or more atoms. When creating a compound out of 2 different elements (a metal and a nonmetal) you need to switch or transfer the charges. Ex: Mg 2+ Cl 1- = MgCl2 Naming Ionic Compounds When naming ionic compounds (nomenclature), you need to specify the cation first, then follow with the anion. If there are 2 non metals in the compound, you can add prefixes such as mono, di, tri, penta to the beginning of the elements name depending on how many atoms there are of it. If there is a metal within the compound, you cannot add any prefixes. When naming, you also add “ide” onto the end of the name of the nonmetal, however you keep the metal name the same, for example, magnesium sulfide. There are some more complex rules to naming compounds as well (see below). Nomenclature Summary Formula Begins With…. Metal Non-metal Ionic Substance Molecular Substance Refer to common ions Use Prefixes: 1= mono 2= di 3= tri 4= tetra 5= penta 6= hexa 7= hepta 8= octa 9= nona 10= deca Hydrogen Acid Binary Hydro ic Acid Oxo-acids ate-ic ite-ous acids