Thursday 28 May 2009

Chemistry – bonding (Ionic and covalent bond)

(I can't show the chemical formula properly here, but it should be right in the download version.) (sorry for stupid mistakes before... edited) 1) Elements combine and form compounds, holding together by chemical bonds, which is a force that holds atoms together. 2) Atoms become stable when their electronic arrangements are same as noble gases, and so that the aim of forming bonds is to let atoms contain electronic arrangements like noble gases. (or fully filled the outermost electron shell.) 3) Ionic bonds form when metallic atoms combine with non-metallic atoms, and covalent bonds forms between non-metallic atoms. 4) Ions are electrically charged particles, and form when atom loss or gain electrons. Group 1,2,3,6,7 have +, 2+, 3+, - and 2- charge respectively because losing or gaining such number of electrons make them become stable. The number and position of protons and neutrons don’t change. 5) Cation (positive ion) usually forms among metals because they loss electrons to contain the noble gas electronic configuration. e.g. Na→Na++e- 6) Anion (negative ion) usually forms among non-metals because they gain electrons to contain the noble gas electronic configuration. e.g. Cl+e-→Cl- 7) Ionic bond is the strong non-directional electrostatic force of attraction between oppositely charged (cation & anion) ions. When ionic bond is formed, there’s a complete transfer of electrons from metal to non-metal atom. In Chemical formula, the metals always go first in ionic bond. e.g. Na+ (a cation)+Cl-(a anion)→NaCl e.g.2 Mg2+(a 2+ charged ion)+2F-→MgF2 8) Some ions are polyatomic ions formed by covalent bond, and some metals have different oxidation numbers will have different number of charge. For example, Iron(III) is 3+ charged but iron(II) is 2+ charged. 9) Formulae of ionic compounds show the type of different ions present and ratio of ions of compound. Since the ionic compounds are electrically neutral, so that the total positive charge must be equal to the total negative charge. 10) A way to calculate the ratio between ions is to compare their charge. For example there’re two ions, with a+ and b- charge, the ratio will be b:a (assume this is the simplest form, otherwise simplify it first.) such that the positive charge (ab+) and negative charge (ab-) are equal. 11) If some ions are polyatomic ions and their ratio isn’t 1, use a bracket to show that “two times of the whole ion”. e.g.: we use Mg(OH)2 instead of MgOH2 or MgO2H2. 12) The naming of ionic compounds – Simply cation come first, then anion. The bracket after metal (e.g. Iron(II)) is also needed. Simple non-metal anions end with –ide, polyatomic ions which contain oxygen end with –ate or –ite, which –ate contains more oxygen. (e.g. SO4 is sulphate and SO3 is sulphite.) 13) Covalent bond is the strong directional electrostatic force of attraction between the shared electrons (negative) and the two nuclei (positive). 14) Covalent bond involves sharing of electrons instead of complete transfer of electrons, and they are found among non-metals. Molecule is formed when two or more atoms combined together by covalent bond only. The pairs of electrons shared between two atoms are called bond pair electrons. For example, H2 is formed by covalent bond (but this is still considered as “element”), they shared their electrons, and the outermost electron shell of both Hydrogen is fully filled. The bond pair can be expressed by H-H. Each hydrogen atoms forms H2 and forms diatomic molecule. The formula of covalent compounds is also called molecular formula. The molecular formula of a substance gives the actual number of atoms of each element in each molecule of the substance. 15) There’s a double bond formed in oxygen, and they shared 2 pairs of electrons. The pairs of electrons in the outermost shell and not involved in sharing are called lone pair electrons. Double bond is represented by O=O. Triple bond formed in nitrogen can be represented by N≡N. Two hydrogen and one oxygen combine together to become H2O, and it can be expressed as H-O-H. Further examples include CO2: O=C=C and H2O2 (hydrogen peroxide) H-O-O-H. 16) Formulae of covalent compounds – First, calculate the numbers of electrons needed to fill the outermost electron shell. Then found the ratio just like ionic bond. The formula just shows the amount and species of elements shown. For example, the molecular formula of H-S-H is H2S. 17) Note that this way of calculating ratio is just one of the ways to make covalent bonds. There maybe different types of covalent bonds form by the same type of elements. For example, hydrogen and oxygen can form H2O and H2O2. 18) The relative molecular mass and formula mass equal to the sum of atomic masses of all atoms present in the molecule of that substance or in the formula unit of it, but relative molecular mass can be only applied onto molecule. For example, molecular mass of H2O = mass of 2 H + mass of O = 18

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