Notes for Standard III
Valence electrons are the electrons in the outermost main energy level. The valence
electrons are the electrons most directly involved in forming bonds with other atoms.
Elements in group 1 have one valence electron. Group 2 elements have 2 valence
electrons. Group 13 has 3, group 14 has 4 and so on. Atoms are most stable if their outer
main energy level is filled with electrons. Group 18 elements have a full outer shell so
they are very stable.
An atom will react (bond) with another atom if it makes it more stable. For example, Na
(sodium) has one valence electron in its 3rd energy level. If sodium loses its one outer
electron by giving it to another atom, sodium will be more stable because now its
outermost level will be level 2 and will be full. But now sodium has one less electron
compared to the number of protons in its nucleus so it has a positive charge and is now
called a sodium ion. Chlorine is in group 17, so it has 7 valence electrons. If chlorine
gains one more electron it will have a full outer shell and will be more stable. But now
chlorine has a negative charge because it has an extra electron compared to the number of
protons in its nucleus and is now called a chloride ion.
A positively charged sodium ion and a negatively charged chloride ion will form an ionic
bond. Ionic bonds are formed when atoms gain or lose electrons to form ions. Metals
tend to give up electrons to non-metals to form ionic compounds.
Covalent compounds are formed when atoms share the electrons in the bond. Nonmetals tend to form covalent compounds with other non-metals by sharing electrons.
Metallic bonds are formed when metals bond with metals. In a metal, the electrons are
shared with all the atoms in the metal. The electrons in the outer levels of metal atoms
can freely move from one metal atom to another. This is why metals conduct
electricity—electrons can move through the metal.
Study page 193 in your chemistry book!
A polar molecule is a molecule where the electrons in the bonds between the atoms are
pulled more strongly toward one side of a molecule than the other.
Water (H2O) is a polar molecule because the oxygen has a stronger attraction for the
electrons in the bond than do the two hydrogens. Therefore, the electrons spend more
time near the oxygen side of the molecule, making the oxygen side slightly more
negatively charged than the hydrogen side. The molecular geometry of a water molecule
is bent.
Ammonia (NH3) is also a polar molecule because nitrogen has a stronger attraction for
the electrons than do the three hydrogens. The nitrogen side of the molecule is slightly
more negatively charged than the hydrogen side. The molecular geometry of an
ammonia molecule is pyramidal.
Methane (CH4) is non-polar because it is a symmetrical molecule. The carbon is in the
center so even if it pulls the electrons toward itself, the molecule as a whole will not have
a negative side and a positive side. The molecular geometry of methane is tetrahedral.
Polar molecules such as water tend to stick together because the positive side of one
molecule is weakly attracted to the negative side of another molecule. These weak
attractions are called hydrogen bonds and give water unique properties such as:
surface tension (this is why belly-flops hurt), capillary action (water can creep up thin
tubes), and a high boiling point (the “stickiness” of polar molecules make it more
difficult to separate them from each other to form a gas. This is why water has a high
boiling point compared to other molecular (covalent) compounds. (The more polar the
molecules, the higher the boiling point).