1.2 Neutral atoms attract and repel each other at close distances through van der Waals interactions

charge

An isolate neutral atom has no net ____

dipole

The presence of a charge near a neutral atom induces a ____ in the atom

dipole

a "_____" refers to a molecule or bond where there is a separation of electric charge, meaning one end of the molecule has a partial positive charge and the other end has a partial negative charge

polarized, attract, transient

When two neutral atoms approach each other the electron clouds of the two atoms are ____, which means that the electrons become redistributed so as to produce a dipole moment in the atom. Such dipoles ___ each other, but the charge polarization is ____ (impermanent)

dipole moment

A ______ _____ is a measurement of the separation of two opposite electrical charges. Dipole moments are a vector quantity. The magnitude is equal to the charge multiplied by the distance between the charges and the direction is from negative charge to positive charge: μ = q · r.

distance

a dipole is a pair of opposite charges separated by a ______

induced dipole

When two neutral atoms (that is, atoms with no net electrical charge) approach each other closely, they attract each other. This attraction is due to an ________ ____

electrical field

Induced dipoles occur when an atom is subjected to an ________ ______ that causes the redistribution of electrons in an asymmetric way. For example, when a neutral atom is next to a positive charge, the external charge will attract the electron cloud of the atom towards itself and tend to push the nucleus of the atom away. The result is the formation of an induced dipole in the atom (see Figure 1.5B).

London force, van der Waals attraction

When two neutral atoms approach each other, transient fluctuations in the electron clouds of each atom set up transient dipoles (see Figure 1.5C). The transient dipoles mutually reinforce each other, leading to an attractive force between the atoms. This attractive force is called the _____ _____, after the physicist Fritz London, or the ____ _____ ________ _____, after Johannes van der Waals

optimal, 5

The spatial range of the van der Waals attraction depends on the nature of the interacting atoms. For the atoms commonly found in biological molecules, van der Waals attractions are ____ at distances between 3 and 4 Å. They are of negligible strength beyond ___ Å (Figure 1.6).

radius, sum

The van der Waals ____ is a measure of the size of an atom. The energy due to the van der Waals attraction between two atoms is optimal when they are separated from each other by the ___ of their van der Waals radii. If they move closer, the energy increases sharply

3, repulsion

Despite the attraction that brings atoms together, electron repulsion prevents atoms from getting much closer to each other than ~__ Å. This strong repulsion at small distances is called the van der Waals ___, and it causes atoms to behave like hard spheres when they approach each other very closely

-10, 0.1

angstrom (Å), unit of length, equal to 10^{^___} metre, or __ nanometre

contact

When two atoms approach each other, the energy is at a minimum (that is, the atoms interact optimally) when the distance between the two atoms is equal to the sum of their van der Waals radii (see Figure 1.6). If the atoms approach each other more closely, they begin to repel each other. Two atoms that are separated by the sum of the van der Waals radii are said to be in van der Waals ______

thermal

The magnitude of the _____ energy is greater than the stabilization afforded by the van der Walls interactions, so that the interactions between atoms can be easily disrupted by collisions

Electronegativity

______ is defined as the tendency of an atom participating in a covalent bond to attract the bonding electrons.

short

The repulsion between atoms at __ distances is responsible for many fundamental aspects of the structures of DNA, RNA, and proteins.

Steric effects

_______ ______ are the effects seen in molecules that come from the fact that atoms occupy space. When atoms are put close to each other, this costs energy. The electrons near the atoms want to stay away from each other. This can change the way molecules want to react.

lower

As you can see from Figure 1.6, when two atoms are in van der Waals contact, the stabilization energy is about −1 kJ•mol−1. The stabilization energy is the amount by which the energy at the optimal distance is ____ than when the atoms are far apart

collisions, temperature

thermal energy. This is the amount of energy that is readily transferred between molecules by random ______, and its value depends on the ______.

disrupted

At room temperature (which we shall take to be ~300 K), the value of the thermal energy is ~2.5 kJ•mol−1. Th is means that if an interaction between two atoms is stabilized by less than ~2.5 kJ•mol−1, then this interaction is very easily ______ by collisions at room temperature. It is by this criterion that the van der Waals attraction is very weak

additive

Even though van der Waals interactions are individually very weak, a considerable degree of stabilization can be achieved in the central structural core of proteins or in the stacked nucleotides of DNA by the ______ effect of many such interactions between closely (but not too closely) packed atoms.