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Lecture 1

BSC 215 Lecture Notes - Lecture 1: Chemical Polarity, Ionic Bonding, Covalent Bond

Biological Sciences
Course Code
BSC 215
Jason Pienaar

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Atoms, Molecules, Ions, and Bonds
Matter is anything that takes up space and has mass. Matter consists of elements that
possess unique physical and chemical properties. Elements are represented by
chemical symbols of one or two letters, such as C (carbon), Ca (calcium), H (hydrogen),
O (oxygen), N (nitrogen), and P (phosphorus). The smallest quantity of an element that
still possesses the characteristics of that element is an atom. Atoms chemically bond
together to formmolecules, and the composition of a molecule is given by its chemical
formula (O 2, H 2O, C 6H 12O 6). When the atoms in a molecule are different, the
molecule is a compound (H 2O and C 6H 12O 6, but not O 2).
The atoms of the elements consist of a nucleus containing positively
charged protons and neutrally charged neutrons. Negatively charged electrons are
arranged outside the nucleus. The atoms of each element differ by their number of
protons, neutrons, and electrons. For example, hydrogen has one proton, one electron,
and no neutrons, while carbon has six protons, six neutrons, and six electrons. The
number and arrangement of electrons of an atom determine the kinds of chemical
bonds that it forms and how it reacts with other atoms to form molecules. There are
three kinds of chemical bonds:
Ionic bonds form between two atoms when one or more electrons are
completely transferred from one atom to the other. The atom that
gains electrons has an overall negative charge, and the atom that
donates electrons has an overall positive charge. Because of their
positive or negative charge, these atoms are ions. The attraction of the
positive ion to the negative ion constitutes the ionic bond. Sodium
(Na) and chlorine (Cl) form ions (Na + and Cl ), which attract one
another to form the ionic bond in a sodium chloride (NaCl) molecule. A
plus or minus sign following a chemical symbol indicates an ion with a
positive or negative charge, which results from the loss or gain of one
or more electrons, respectively. Numbers preceding the charges
indicate ions whose charges are greater than one (Ca 2+, PO 4 3).
Covalent bonds form when electrons are shared between atoms. That is,
neither atom completely retains possession of the electrons (as
happens with atoms that form ionic bonds). A single covalent bond
occurs when two electrons are shared (one from each atom). A double
or triple covalent bond is formed when four or six electrons are shared,
respectively. When the two atoms sharing electrons are exactly the
same, as in a molecule of oxygen gas (two oxygen atoms to form O 2),
the electrons are shared equally, and the bond is a nonpolar covalent
bond. When the atoms are different, such as in a molecule of water
(H 2O), the larger nucleus of the oxygen atom exerts a stronger pull on
the shared electrons than does the single proton that makes up either
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