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Chapter 2

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BLG 143
Lynda Mc Carthy

Chapter no. 2 Water and Carbon-The Chemical Basics of Life Ions,Atoms and Molecules • all matter is 96% Hydrogen, carbon, oxygen and nitrogen. • Dalton-a unit ot represent mass of proton, neutron and electron in grams. Aarrangement of electrons • Electrons move around around in atomic nuclei in specific regions called orbits. • each orbital can hold upto two electrons. • Orbitals are grouped into levels called electron shells. • Electron shells are numbered with the smallest number being closest to nucleus. • Each electron shell contains a specific number of orbits. • The electrons of an atom fill the innermost shell first,before filling outer shells. • Valence Shells-outermost shell of an atom. • Valence Electrons- electrons in the outermost shell of an atom. • Chemical Bonds-strong sttractions that bind atoms together. • An atom is most stable when it valence shell is filled. Shell can be filled through the formation of chemical bond. • Molecules-substances held together by covalent bond. • Two types of covalent bonds: 1. Polar covalent bond-an unequal sharing of electrons between atoms,bond is asymmetric e.g. in H2O due to high electronegativity of oxygen, it holds the electrons more closesly and gains a partial negative charge whereas hydrogens gain a partial positive charge.However, the net charge is zero. 2. Non-Polar covalent bond-electrons are shared equally between atoms, bond is symmetrical e.g.H2. • electronegativity-the ability of an atom to attract electrons. • oxygen is highly electronegative atom. • ionic bonds-the bond formed between two atoms by completely transfering an electron from one atom to the other. • Ion-an atom or molecule that carries charge. • Two types of ions: 3. Cations-positively charged ions. 4. Anions-negatively charged ions. • Ionic Bonding When an ionic bond is formed, electrons are completely transferred from one atom to another.As a result, the atom donating its electron gains a positive charge and becomes a cation. On the other hand, the atom gaining this electron gains a negative charge and becomes an anion. • the elctrical attraction between the ions is very strong. • Electron sharing continum-degree to which electrons are shared in chemical bonds. • equal sharing (non-polar covalent) same electronegativity. • unequal sharing (polar covalent) one atom more electronegative. • total transfer (ionic bond) very strong electronegativities. • Methane-CH4: Tetrahedral in shapes, structure with four triangular surfaces. • Water-H20: bent and two dimensional or planar. Representing Molecules • Molecular Formulas-no. and type of atoms in a molecule. • Structural Formulas-indicate which atoms are bonded together and whether the bonds are single, double or tripls-good for planar molecules. • Ball and Stick Models-provide information on 3D structures of molecules, also indicate the relatives sizes of atoms involved. • Space Filling Models-difficult to read but accurately depict the spatial relationships between atoms. • When a chemical reaction occurs, one substance is combined with other or broken down into another substance.Atoms are rearranged in molecules; in most cases, chemical bonds are broken and new bonds bonds form. The Early Oceans and Properties of Water • 75% of a cellès volumes is water. • 70% of human body is water. Properties of Water 5. Great Solvent- e.g. NaCL dissolving in H2O. 6. Cohesion,Adhesion and Surface Tension- i.e. binding between water molecules. 7. Density- becomes lighter upon becoming solid. 8. High capacity for absorbing energy, 9. Ability to dissociate • Hydrogen bond-weak electrical bond between partial positive charge on one H2O colecule and partial negative charge on another H2O. • forms,breaks apart with addition or subtraction of heat. • Hydrophilic (water-loving): substances that for H-bonds with water. • Hydrophobic (water-fearing): substances that do not interact with water. • Hydrogen bonding makes it possible for almost any charged or polar mol
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