The early periodic table.
In 1863 Newlands came up with the Law of Octaves:
- he put the 56 known elements in order according to their atomic weights (relative atomic mass)
- he stated that similar properties were repeated every eighth element
- however, the properties did not match well within the groups so other scientists did not accept his ideas
In 1869 Mendeleev produced a better table by leaving gaps for undiscovered elements:
- when new elements were discovered they had the same properties the Mendeleev predicted so scientists accepted his ideas
- this table by Mendeleev became that basis for the modern periodic table
The modern periodic table.
At the start of the 20th century scientist found out about protons and electrons.
The modern periodic table is based on atomic numbers and lined up in groups.
Groups contain similar chemical properties because their atoms have the same number of electrons in their highest occupied energy level (outer shell).
Most reactive metals are at the bottom of their group.
Most reactive non-metals are at the top of their group.
The reactivity of an element:
1) depends on the total number of electrons
2) the more occupied energy levels, the lower in the group an element is
3) the electrons that are in the outer shells are less attracted to the nucleus
Group 1 - the alkali metals.
Group 1 elements are reactive metals, they:
- are soft solids at room temperature (we can cut them with a knife)
- are shiny when we first cut them, but become dull as they form a layer of oxide (when reacting with the air)
- have to be stored in oil to stop them reacting with the air
- have low melting and boiling points (that decrease going down the group)
- have low densities
- all react readily with air and water (with water they produce hydrogen gas and a metal hydroxide)
eg. lithium + water => lithium hydroxide + hydrogen
The modern periodic table cont.
When metals react they lose electrons so the lower in the group an element is the easier it is to lose electrons as the attraction to the nucleus is weak.
When non-metals react they gain electrons so the higher in the group an element is the easier it is to gain electrons as the attraction to the nucleus is strong.
Group 7 - the halogens.
Group 7 elements are non-metals, they:
- exist as small molecules made up of pairs of atoms joined by covalent bonds.
- have low melting and boiling points (that increase going down the group)
- have 7 electrons in their outer shell (so they need just one more electron to acheive stability)
- form ionic compounds with metals in which their ions have a negative charge. eg. F-
- bond covalently with other non-metals to form molecules
Reactivity decreases going down the group as the number of occupied shells increases causing the attraction to the nucleus to weaken meaning that it is harder to gain electrons.
A more reactive halogen can displace a less reactive halogen from its compunds in a solution. eg. Chlorine will displace bromine if we bubble the gas through a solution of potassium bromide..
The Transition elements.
These elements are found between Group 2 and Group 3.
They have typical metallic structure, the atoms are in a large structure held together by metallic bonds, and the outer electrons of each atom are delocalised (they can move about freely, meaning they can conduct things)
They have higher melting and boiling points than the alkali metals, except mercury (which is liquid at room temperature).
Most of them are strong and dense (used in buliding or as alloys). eg. brass = copper + zinc and cupro-nickel = copper + nickel, very hard.
They are malleable and ductile.
Good conductors of heat and electricity.
React slowly, or not at all, with H2O or O2 at ordinary temperatures.
They form positive ions with various charges. eg. Fe2+, Fe3+.
Group 7 - the halogens. cont.
Cl2 + 2KBr => 2KCl + Br2
At room temperature:
- Fluorine = pale yellow gas
- Chlorine = green gas
- Bromine =red brown liquid
- Iodine = grey solid (which easily vapourises to violet gas when heated)
Fluorine reacts Furiously - its the most reactive halogen !
Group 1 - the alkali metals. cont.
- lithium and sodium float on water, potassium reacts vigorously and the H gas produced catches fire, producing a purple flame.
- react with Group 7 elements to form ionic salts (metal halides) that are white or colourless crystals.
eg. sodium + chloride => sodium chloride
- have one electron in their outer shell, they lose this electron when they react with substances and they form ionic compounds and their ions have a single positive charge. eg. Na+
Reactivity increases going down the group as the outer electrons are less attracted to the nucleus, due the increasing number of occupied shells, meaning it is easier for electrons to be lost.
Transition metals. cont.
The name of the transition metal compound tells us what charge the ions in the compound are, iron(II) chloride = Fe2+.
Compounds of transition metals are often brightly coloured.
They can be used as catalysts for chemical reactions.
Most of their properties are due to transition elements having a small number of electrons in their outer shell and an incomplete inner shell. This inner shells can hold up to ten more electrons for elements beyond calcium, and so there are then transition elements in each period.