The periodic table, Year 10: groups 0, 1 and 7

The outer shells of the atoms are full, which is a stable arrangement

HintNothing needs to be gained, lost or shared.

The whyOther atoms react in ways that give them the electron arrangement of a noble gas. A noble gas atom has that arrangement already, so it does not easily react, or even join with other atoms of its own kind.

Helium

HintIt is the first in the group, with room for just one shell.

The whyIts two electrons fill the first shell, which holds only two, so the arrangement is just as stable as the eight of neon (2,8) or argon (2,8,8).

Above

HintSee which way the figures are heading as you go down the list, remembering that −186 is warmer than −246.

The whyEach noble gas boils at a higher temperature than the one before it, so the pattern lets you predict a value you have not been given. Xenon in fact boils at −108 °C.

atoms

HintThe smallest particle of an element, on its own.

The whyOxygen and chlorine pair up as O2 and Cl2, because sharing electrons gives each of the pair a full outer shell. A noble gas has a full outer shell already, so its particles stay separate. That is why the formula of helium gas is simply He.

Relative atomic mass

HintIt tells you how heavy an element's atoms are on average.

The whyHelium (4) has the lowest boiling point, and each heavier noble gas boils at a higher temperature. Linking a trend to a property like this is what lets you estimate values for elements further down the group.

lithium hydroxide; hydrogen

HintOne product makes the water alkaline; the other is the gas you see as fizzing.

The whyThe metal fizzes steadily as the gas is given off, and the other product dissolves to make an alkaline solution, which is why Group 1 is called the alkali metals. Sodium and potassium give the same kinds of product, more vigorously.

Potassium chloride, a white solid

HintIts name follows the same pattern as table salt's, and it looks much the same.

The whyAll three alkali metals react vigorously with chlorine to form metal chlorides: lithium chloride, sodium chloride and potassium chloride. Each is a white ionic salt that dissolves in water to give a colourless solution.

oxygen

HintIt is the reactive gas that makes up about a fifth of air.

The whyThe dull layer is sodium oxide. Lithium, sodium and potassium all tarnish like this, potassium the fastest, which is why they are stored under oil.

Potassium's outer electron is further from the nucleus, so it is lost more easily

HintCompare 2,1 with 2,8,8,1 and count the shells lying between the centre and that final 1.

The whyGoing down Group 1 the atoms get bigger, with more shells between the nucleus and the outer electron. The attraction holding that electron is weaker, so the atom gives it up more readily and reacts faster.

1

HintLithium is 2,?; sodium is 2,8,?; the missing figure is the same each time.

The whyLithium is 2,1, sodium 2,8,1 and potassium 2,8,8,1. That single outer electron is lost when the metal reacts, which is why the alkali metals behave so alike.

7

HintThe group number is the clue, just as it is for Group 1.

The whyFluorine is 2,7 and chlorine is 2,8,7. Each halogen atom needs one more electron to fill its outer shell, so they react in the same kinds of way.

Br2

HintWrite the element's two-letter symbol, then the number in a pair.

The whyAll the halogens are made of two-atom molecules: F2, Cl2, Br2 and I2. So in a symbol equation chlorine gas is written Cl2, never Cl.

Both increase

HintAt room temperature the first is a gas, the second a liquid and the third a solid.

The whyFurther down the group the molecules have a higher relative molecular mass, and the melting and boiling points are higher. That is why chlorine is a gas at room temperature, bromine a liquid and iodine a solid.

A halide ion

HintChloride, bromide and iodide are the three you will meet most; the family word shares their ending.

The whyWhen chlorine, bromine or iodine reacts with a metal, each halogen atom takes one electron. The product is an ionic salt, such as sodium chloride or potassium bromide: white or colourless crystals.

Made of small covalent molecules

HintNeither atom is a metal, so neither gives an electron away outright.

The whyWith non-metals the halogens share electrons instead of taking them, forming covalent bonds. The compounds, such as hydrogen chloride, HCl, are made of small molecules, and many are gases at room temperature.

Chlorine

HintReactivity in this group runs the opposite way to Group 1, where the lowest metal is the liveliest.

The whyA more reactive halogen displaces a less reactive one from a solution of its salt. Reactivity falls going down Group 7, so the top one of these three displaces the other two, bromine displaces only iodine, and iodine displaces neither.

No — iodine is less reactive than bromine

HintCompare where the two halogens sit in Group 7.

The whyDisplacement only works downwards: a halogen can push out one that is below it in the group, never one above. The bromide ions stay in the solution and nothing new is formed.

potassium chloride; iodine

HintThe more reactive halogen takes the other's place in the salt and turns it out as the element.

The whyThe halogen added is more reactive than the one in the salt, so it ends up in the compound and the other is set free. The metal is unchanged throughout; it simply swaps partners.

Br2 + 2KI → 2KBr + I2

HintBoth halogens are two-atom molecules, so you will need two units of each potassium compound.

The whyThe formulae are Br2, KI, KBr and I2. One Br2 molecule supplies two bromine atoms, enough for two KBr, and two KI supply the two iodine atoms in I2. Check each element: Br 2 and 2, K 2 and 2, I 2 and 2.

Bromine

HintPlace it in the order of reactivity: above the element it pushes out, and not above the others.

The whyIt displaces iodine, so it is more reactive than iodine. It does not displace chlorine, so it is less reactive than chlorine. A halogen cannot displace itself, so no change with the middle solution fits as well.