Same outer electrons make group elements react similarly
Reactivity
How readily an element takes part in chemical reactions.
- Chemical reactions involve the outer electrons of atoms, which are lost, gained or shared.
- Atoms in the same group have the same number of outer electrons, so they react in similar ways.
- Lithium, sodium and potassium each have one outer electron, and all react with water to give hydrogen and an alkaline solution.
- The atomic number fixes the number of electrons, so it also fixes the electron arrangement and the way an element reacts.
- Group 0 elements have full outer shells, so they have no tendency to lose, gain or share electrons and are very unreactive.
Fluorine, 2.72.72.7, and chlorine, 2.8.72.8.72.8.7, each need one more electron to fill the outer shell, so both react with sodium to form compounds with the same ratio, NaF and NaCl.
Elements near period ends lose or gain electrons
- Atoms with one or two outer electrons, in Groups 1 and 2, react by losing them to form positive ions.
- Atoms with six or seven outer electrons, in Groups 6 and 7, react by gaining electrons to form negative ions.
- This is why elements from opposite sides of the table, such as sodium and chlorine, react vigorously with each other.
- Elements with four outer electrons, such as carbon, tend to share electrons instead.
An element's position gives its number of outer electrons, which predicts whether it will lose, gain or share electrons in a reaction.
Mendeleev ordered by atomic mass but grouped by properties
Relative atomic mass
The average mass of an atom of an element compared with one-twelfth of the mass of a carbon-12 atom.
- In 1869, Dmitri Mendeleev arranged the 60 or so elements then known in order of increasing atomic mass.
- He placed elements with similar properties in the same columns, even when this broke the mass order.
- He put iodine after tellurium so that it sat with the other halogens, although iodine has the lower atomic mass.
- He left gaps for elements that had not yet been discovered.
- He predicted the properties of the missing elements, and when gallium and germanium were found their properties closely matched his predictions.
Mendeleev predicted that an element below silicon would have an atomic mass of about 727272 and a white oxide, and germanium, discovered in 1886, has an atomic mass of 72.672.672.6 and forms a white oxide.
Ordering by atomic number explains Mendeleev's swaps
Periodic Table
A chart of the elements in which elements with similar properties are placed in the same column.
- In 1913, Henry Moseley showed that each element has its own atomic number, which was later explained as the number of protons in its nucleus.
- Ordering by atomic number puts tellurium, 525252, before iodine, 535353, with no swap needed.
- Atomic masses can run out of order because of isotopes, since an element with many heavier isotopes has a higher average mass.
- Atomic number also sets the electron arrangement, which explains why elements in a group behave alike.
- The noble gases, unknown to Mendeleev in 1869, fitted in as a whole new group, Group 0.
When a question asks how Mendeleev's table was refined, link the change to atomic number and give a pair such as tellurium and iodine as evidence.
- Why do elements in the same group react in similar ways?
- Why are Group 0 elements unreactive?
- How did Mendeleev order the elements, and why did he leave gaps?
- Why did Mendeleev place iodine after tellurium even though iodine has the lower atomic mass?
- What property is the modern Periodic Table ordered by?
