Page 75 - Physical chemistry understanding our chemical world

42 INTRODUCING INTERACTIONS AND BONDS

electrodes of the LCD causes the ‘magnet’ to reorientate in just the same way as a
magnet moves when another magnet is brought close to it.
These dipoles form because of the way parts of the molecule
Atomsorgroupsare attract electrons to differing extents. The power of an element
‘electronegative’ if they (when part of a compound) to attract electrons is termed its ‘elec-
tend to acquire neg- tronegativity’ χ. Highly electron-attracting atoms tend to exert
ativechargeatthe control over the outer, valence electrons of adjacent atoms. The
expense of juxtaposed most electronegative elements are those placed near the right-hand
atoms or groups. Groups side of the periodic table, such as oxygen and sulphur in Group
acquiring a positive
VI(b) or the halogens in Group VII(b).
charge are ‘electropos- There have been a large number of attempts to quantify elec-
itive’.
tronegativities χ, either theoretically or semi-empirically, but none
has been wholly successful. All the better methods rely on bond
strengths or the physical dimensions of atoms.
Similar to the concept of electronegativity is the electropositivity of an element, which
is the power of its atoms (when part of a compound) to lose an electron. The most
electropositive elements are the metals on the far-left of the periodic table, particularly
Groups I(a) and II(a), which prefer to exist as cations. Being the opposite concept to
electronegativity, electropositivity is not employed often. Rather, we tend to say that an
atom such as sodium has a tiny electronegativity instead of being very electropositive.

Why does dew form on a cool morning?

Van der Waals forces

Many people love cool autumn mornings, with the scent of the cool air and a rich
dew underfoot on the grass and paths. The dew forms when molecules of water
from the air coalesce, because of the cool temperature, to form minute aggregates
that subsequently nucleate to form visible drops of water. These water drops form a
stable colloid (see Chapter 10).
Real gases are never wholly ideal: there will always be some extent of non-ideality.
At one extreme are the monatomic rare gases such as argon and neon, which are non-
polar. Hydrocarbons, like propane, are also relatively non-polar, thereby precluding
stronger molecular interactions. Water, at the opposite extreme, is very polar because
some parts of the molecule are more electron withdrawing than
The symbol δ means ‘a others. The central oxygen is relatively electronegative and the two
small amount of ...’, so hydrogen atoms are electropositive, with the result that the oxygen
‘δ ’ is a small amount is more negative than either of the hydrogen atoms. We say it has
−
of negative charge. a slight excess charge, which we write as δ . Similar reasoning
−
shows how the hydrogen atoms are more positive than the oxygen,
+
with excess charges of δ .
These excess charges form in consequence of the molecule incorporating a variety
of atoms. For example, the magnitude of δ − on the chlorine of H–Cl is larger than
the excess charges in the F–Cl molecule, because the difference in electronegativity