Group I properties Revision Notes

Alkali metals are the elements found in Group I of the periodic table. The first three alkali metals are lithium (Li), sodium (Na), and potassium (K). These metals share similar chemical and physical properties due to their single electron in the outermost shell, which influences their behaviour.

One notable physical property is that alkali metals are relatively soft compared to many other metals. For example, sodium can be easily cut with a knife, revealing a shiny surface beneath the dull oxide layer. This softness is linked to their atomic structure and metallic bonding strength.

• Alkali metals have one electron in their outer shell.
• They are soft metals, easily cut with a knife.
• Examples: lithium, sodium, potassium.

As you move down Group I from lithium to potassium, the melting points of the alkali metals decrease. Lithium has the highest melting point among the three, followed by sodium, then potassium with the lowest.

This trend occurs because the metallic bonds become weaker down the group. Metallic bonding strength depends on the attraction between the positive metal ions and the delocalised electrons. As atoms get larger down the group, the outer electron is further from the nucleus, reducing the attraction and weakening the bond.

For example, lithium melts at about 180 °C, sodium at 98 °C, and potassium at 63 °C. This means potassium is the softest and easiest to melt among them.

• Melting point decreases from lithium to potassium.
• Weaker metallic bonds due to larger atomic size.
• Example melting points: Li ~180 °C, Na ~98 °C, K ~63 °C.

Density is mass per unit volume (density = mass/volume). As you move down Group I, the density of alkali metals generally increases from lithium to potassium.

Although the atoms get larger, their atomic masses increase more significantly, causing the overall density to rise. For instance, lithium has a density of about 0.53 g/cm³, sodium about 0.97 g/cm³, and potassium about 0.86 g/cm³. Note that potassium’s density is slightly less than sodium’s due to its larger atomic volume, but the general trend from lithium to potassium is an increase in density.

This increase in density reflects the balance between increasing atomic mass and atomic size.

• Density = mass/volume.
• Density generally increases down the group due to increasing atomic mass.
• Example densities: Li 0.53 g/cm³, Na 0.97 g/cm³, K 0.86 g/cm³ (with slight anomaly).

Alkali metals react vigorously with water to form a metal hydroxide and hydrogen gas. The reactivity increases down the group: potassium reacts more vigorously than sodium, which is more reactive than lithium.

This is because the outer electron is more easily lost as the atomic radius increases down the group. The electron is less tightly held by the nucleus, so it can be removed more readily, making the metal more reactive.

For example, lithium reacts slowly with water, producing bubbles of hydrogen gas and forming lithium hydroxide. Sodium reacts faster, melting into a ball and fizzing rapidly. Potassium reacts even more vigorously, often igniting the hydrogen gas produced, creating a lilac flame.

• Reactivity with water increases down the group.
• Outer electron is lost more easily due to larger atomic radius.
• Examples: Li reacts slowly; Na reacts faster and melts; K reacts violently with flame.

To summarise, as you move down Group I from lithium to potassium:

The metals become softer and easier to cut due to weaker metallic bonds.

Melting points decrease because metallic bonding weakens with increasing atomic size and electron distance from the nucleus.  - Density generally increases because atomic mass increases faster than atomic volume, though potassium shows a slight anomaly due to its larger size.  - Reactivity with water increases because the outer electron is less strongly held and lost more easily, making reactions faster and more vigorous.

• Softness increases down the group.
• Melting point decreases due to weaker metallic bonds.
• Density generally increases due to rising atomic mass.
• Reactivity with water increases as outer electron is lost more easily.

Group I elements, also called alkali metals, occupy the first column of the periodic table. They include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). These metals share similar chemical and physical properties due to their single electron in the outermost shell, which they readily lose to form +1 ions.

• All have one electron in their outer shell.
• They are metals with relatively low melting points.
• They are highly reactive, especially with water.

As you move down Group I from lithium to francium, several physical properties show clear trends. The metals become softer, their melting points decrease, and their densities generally increase. For example, lithium is a hard metal compared to potassium, which is soft enough to cut with a knife. The decrease in melting point is due to the weakening metallic bonds as atomic size increases, because the outer electron is further from the nucleus and less strongly held.

Density increases because the atomic mass increases faster than the volume of the atoms.

• Softness increases down the group.
• Melting point decreases down the group.
• Density increases down the group.

Group I elements react vigorously with water to form hydroxides and hydrogen gas. The reactivity increases down the group because the outer electron is more easily lost as atomic radius increases and the attraction between the nucleus and outer electron decreases. For example, potassium reacts more violently with water than sodium, which in turn reacts more vigorously than lithium.

• All react with water to produce hydrogen gas and metal hydroxides.
• Reactivity increases down the group due to weaker attraction for the outer electron.
• Example: K reacts more vigorously than Na, which reacts more vigorously than Li.

Using the trends observed in lithium, sodium, and potassium, predictions can be made about rubidium, caesium, and francium. These elements are expected to be even softer, have lower melting points, higher densities, and react more violently with water. For instance, caesium is known to be very soft and reacts explosively with water, consistent with the trend.

Such predictions rely on understanding the atomic structure and periodic trends rather than direct observation.

• Rubidium, caesium, and francium are predicted to be softer than potassium.
• Their melting points should be lower than potassium's.
• They should be denser and more reactive with water.

The trends in Group I properties are explained by changes in atomic structure down the group. Each successive element has an additional electron shell, increasing atomic radius. This greater distance between the nucleus and outer electron reduces the nuclear attraction, making it easier to lose the electron and increasing reactivity.

The weakening metallic bonds due to the larger atomic size cause melting points to fall, while increasing atomic mass causes density to rise.

• Atomic radius increases down the group due to added electron shells.
• Nuclear attraction on outer electron decreases, increasing reactivity.
• Metallic bonds weaken, lowering melting points.
• Atomic mass increases, raising density.