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Chapter 2

BLG 143 Chapter Notes - Chapter 2: Electron Shell, Ionic Bonding, Hydrogen Bond


Department
Biology
Course Code
BLG 143
Professor
Lynda Mc Carthy
Chapter
2

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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.

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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
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