Groups 1, 2, 17 and 18

Position tells you the outer electrons

Elements in the same group (vertical column) have the same number of outer-shell electrons. Elements in the same period (horizontal row) have the same number of occupied main energy levels.

For Groups 1 and 2 and Groups 13 to 18, the number of outer-shell electrons is the last digit of the group number (helium has 2).

The outer electrons control how an atom reacts. Groups 1 and 2 lose electrons to form positive ions with a full outer level. Group 17 gains one electron to form a 1− ion. Group 18 already has a full outer level (the octet), so it has no need to react.

Group 1: the alkali metals

Lithium, sodium, potassium, rubidium and caesium are soft, shiny metals when freshly cut. They have low melting points and low density (lithium, sodium and potassium float on water). They conduct electricity because their outer electron is delocalised and free to move. Chlorine does not conduct, because it is made of molecules with no free charged particles.

They are so reactive that they tarnish quickly in air, so sodium and potassium are stored under oil to keep out oxygen and moisture.

Reaction with water. The metal skates across the surface, fizzes and forms hydrogen gas and a metal hydroxide solution, which is alkaline.

The solution turns red litmus blue (and universal indicator purple, about pH 13 to 14).

The trend: reactivity increases down the group. Lithium fizzes steadily, sodium melts into a ball and darts about, and potassium ignites with a lilac flame.

Why does it increase? Going down the group:

• There are more occupied energy levels, so the atomic radius increases.

• The outer electron is further from the nucleus.

• Inner electrons shield it from the nuclear attraction. The greater distance and extra shielding outweigh the increased nuclear charge, so the effective nuclear attraction on the outer electron is weaker.

• So the outer electron is held less tightly and is lost more easily (the first ionisation energy falls).

Group 2: the alkaline earth metals

Beryllium, magnesium, calcium, strontium, barium and radium have two outer electrons. They form 2+ ions, for example Ca²⁺ with the electronic structure 1s² 2s² 2p⁶ 3s² 3p⁶, the same as argon.

They are harder and denser than Group 1 and have higher melting points. They conduct electricity. They are reactive, but less so than the Group 1 metal in the same period.

Reaction with water. Calcium reacts steadily with cold water, giving hydrogen and an alkaline solution of calcium hydroxide.

Magnesium reacts very slowly with cold water but burns in steam. Barium reacts more vigorously than calcium.

The trend: reactivity increases down the group, for the same reasons as Group 1: larger atomic radius, more shielding, outer electrons lost more easily.

Why less reactive than Group 1? In the same period, a Group 2 atom has one more proton than its Group 1 neighbour, so the nuclear charge is greater, the atomic radius is smaller and the outer electrons are held more tightly. More energy is needed to remove them (higher ionisation energies).

Ions and hydroxides. Barium, with 56 electrons, forms Ba²⁺ with the electronic structure of xenon (1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶), because it loses its two outer electrons. Group 2 hydroxides are alkaline, so universal indicator turns purple (about pH 12 to 14) in the solution formed.

Group 17: the halogens

Fluorine, chlorine, bromine, iodine and astatine exist as diatomic molecules (F₂, Cl₂, Br₂, I₂). The molecules are non-polar and held together by London dispersion forces, which get stronger as the molecules get bigger. So melting and boiling points rise down the group.

The halogens do not conduct electricity, as they are covalent molecules with no free-moving charge carriers.

Reactivity decreases down the group. A halogen atom reacts by gaining one electron to complete its octet. Going down the group:

• The atomic radius increases and shielding increases.

• The incoming electron enters a level further from the nucleus and is attracted less strongly.

• So it is gained less easily and reactivity falls. Fluorine is the most reactive, iodine the least.

Displacement reactions. A more reactive halogen displaces a less reactive one from a solution of its halide.

The solution turns orange as bromine forms. Chlorine is the stronger electron acceptor, so it takes electrons from bromide ions, which become bromine.

If the halogen added is less reactive than the one in the halide, nothing happens: iodine added to potassium chloride gives no reaction.

Group 18: the noble gases

Helium, neon, argon, krypton, xenon and radon are colourless, odourless gases at room temperature. They are monatomic: each atom exists on its own.

Their outer level is full (an octet, or 2 for helium), a very stable arrangement. They have very high first ionisation energies and essentially no tendency to gain or lose electrons, so they are unreactive.

Because they are single atoms with only London dispersion forces between them, their boiling points are extremely low (helium boils at about −269 °C) and rise down the group as the atoms get larger.

They do not conduct electricity, as they have no free charged particles in the gas state.

Uses follow from their properties: argon fills light bulbs (it does not react with the hot filament), helium is used in balloons and neon in signs.

Mendeleev and the modern table

In 1869 Dmitri Mendeleev arranged the known elements in order of increasing atomic mass, starting a new row whenever the properties repeated. Elements with similar properties then lined up in columns.

His table had two big strengths:

• He left gaps for elements not yet discovered and predicted their properties. His "eka-silicon" was later found and named germanium, and its properties matched his predictions closely. This made the table a powerful predictive tool.

• He was willing to break the order of atomic mass to keep elements with similar properties in the same column. For example, tellurium (mass 127.6) is placed before iodine (126.9), because iodine behaves like the other halogens and tellurium does not.

A weakness was that ordering by atomic mass could not explain such exceptions. The modern table orders elements by atomic number (number of protons), discovered after Mendeleev. Tellurium (52) then comes before iodine (53) naturally, with no exception needed. The table is also arranged in blocks (s, p, d) according to the sublevel being filled.

Summary

• Elements in a group have the same number of outer electrons: Group 1 has 1, Group 2 has 2, Group 17 has 7 and Group 18 has 8 (helium 2).

• Group 1 and 2 metals react by losing electrons. Reactivity increases down each group, as atomic radius and shielding increase and the outer electrons are held less tightly. Group 1 metals react with water to give hydrogen and an alkaline hydroxide.

• Halogens (diatomic molecules; Cl₂ gas, Br₂ liquid, I₂ solid) react by gaining an electron. Reactivity decreases down the group, and a more reactive halogen displaces a less reactive one from its halide.

• Noble gases are monatomic and unreactive because of their full outer level.

• Mendeleev ordered by atomic mass, left gaps and predicted new elements. The modern table orders by atomic number, which removes the exceptions.

Back to the start: sodium reacts more violently than lithium because its outer electron is further from the nucleus and better shielded, so it is lost more easily.