This page discusses the fads in some atomic and also physical properties of the group 1 elements - lithium, sodium, potassium, rubidium and cesium. Sections below cover the patterns in atom radius, first ionization energy, electronegativity, melting and also boiling points, and also density.

You are watching: Why do alkali metals have very low densities


Trends in atomic Radius

The chart listed below shows the increase in atomic radius down the group.

*

The radius of one atom is administer by two factors:

The variety of layers the electrons approximately the cell nucleus The attraction the outer electrons feel from the nucleus

Compare the electronic configurations of lithium and also sodium:

Li: 1s22s1 Na: 1s22s22p63s1

In every element, the external electron experience a net charge of +1 native the nucleus. The optimistic charge top top the cell core is canceled out by the an unfavorable charges the the inner electrons. This result is depicted in the number below:

*

This is true because that each of the other atoms in team 1. The only factor affecting the dimension of the atom is the variety of layers of inner electrons which surround the atom. More layers the electrons take up more space, as result of electron-electron repulsion. Therefore, the atoms increase in size down the group.

The first ionization energy of an atom is characterized as the power required to remove the many loosely organized electron from each of one mole of gas atoms, creating one mole that singly fee gaseous ions; in other words, that is the power required for 1 mole the this process:

\< X(g) \rightarrow X^+ (g) + e^-\>

*

A graph reflecting the first ionization energies that the team 1 atom is shown above. Notice that first ionization energy decreases down the group. Ionization energy is governed by 3 factors:

the fee on the nucleus, the lot of screening by the inside electrons, the distance between the outer electrons and the nucleus.

Down the group, the boost in nuclear fee is specifically offset by the boost in the number of inner electrons. As discussed before, in every of the aspects Group 1, the outermost electrons experience a net fee of +1 indigenous the center. However, the distance in between the nucleus and the external electrons rises down the group; electrons come to be easier come remove, and the ionization power falls.


Trends in Electronegativity

Electronegativity is a measure up of the propensity of an atom to entice a bonding pair the electrons. The is normally measured ~ above the Pauling scale, on i m sorry the many electronegative facet (fluorine) is provided an electronegativity that 4.0 (Table A2).

*

A graph showing the electronegativities of the team 1 elements is displayed above. Every of these elements has a really low electronegativity when compared with fluorine, and also the electronegativities decrease from lithium to cesium.

Picture a bond between a salt atom and a chlorine atom. The bond can be thought about covalent, created of a pair of mutual electrons. The electron pair will certainly be pulled toward the chlorine atom because the chlorine nucleus contains many more protons than the salt nucleus. This is shown in the figure below:

*

The electron pair is therefore close to the chlorine the an efficient electron deliver from the salt atom come the chlorine atom occurs—the atoms are ionized. This strong attraction native the chlorine nucleus describes why chlorine is much much more electronegative than sodium.

See more: Peter Berger Seeing The General In The Particular Mean? The Sociological Perspective

Now compare this through a lithium-chlorine bond. The net pull indigenous each finish of the bond is the same as before, but the lithium atom is smaller sized than the sodium atom. That method that the electron pair is going to be an ext strongly attractive to the net +1 charge on the lithium end, and thus closer to it.

*

Trends in Density

The densities of the team 1 elements increase under the group (except because that a downward fluctuation at potassium). This tendency is displayed in the number below: