The Periodic Table Flashcards
Why did scientists in the 19th century use atmoic weights rather an proton numbers to classify the elements?
Becuase they did not know about the structure of atoms
What was the law of octaves?
- Propsed by Newlands in 1863
- States that similar properties are repeated every 8the element
- He put the 62 known elements into seven groups according to their atomic weights
- However, after calcium their properties did not match very well within the groups so other scientists did not accept his ideas
Why was Medeleev’s table better than Newlands’?
- Made in 1869
- Left gaps for undiscovered elements
- This mean that the groups of known elements did have similar properties
- Also predicted the properties of the missing elements
- When the missing elements were discovered, they matched Medeleev’s preditctions
- This made other scientists accept his ideas
- His table became the basis for the modern periodic table
Why do elements in a group have similar chemical properties?
Because their atoms have the same number of electrons in their highest occupied energy level (outer shell)
Within a group, what does the reactivity of elements depend upon?
The total number of electrons
What happens as you go down a group and what effect does this have on reactivity?
- Going down a group, there are more occupied energy levels and the atoms get larger
- As the atoms get larger, the electrons in the outer shell are less strongly attracted to the nucleus
- Metals lose electrons when they react, so the reactivity of metals increases as you go down the group
- Non-metals gain electrons when they react, so the reactivity decreases as you go down the group
Describe the properties of the alkali metals
- Soft solids at room temperature with low melting and boiling points that decrease going down the group
- They have low densities. Li, Na, and K float on water
- They react with water to produce hydrogen gas and a metal hydroxide that is an alkali
- They all have one electron in their outer shell. They lose this in reactions to form ionic compounds in which all their ions have a single positive charge
- They react with halogens to form salts that are white or colourless crystals
- Their compounds dissolve in water, forming solutions that are usually colourless
Why does reactivity increase as you go down the alkali metals?
Because the outer electron is less strongly attracted to the nucleus as the number of occupied energy levels increases and the atoms get larger
What are the properties of the transition metals?
- Except for mercury they have higher melting and boiling points than the alkali metals
- They are mallebale + ductile and are good conductors of heat + electricity
- They react slowly, or not at all, with oxygen and water at ordinary temperatures
- Most are strong + dense and are useful as building materials, often as alloys
- They form positive ions with various charges
- Their compounds are often brightly coloured
- Many transition metals or their compounds are catalysts
Describe the properties of the halogens
- They exist in molecules made up of pairs of atoms
- They have low melting and boiling points that increase going down the group
- At room temperature: fluorine is a pale yellow gas; chlorine is a green gas; bromine is a red-brown liquid; iodine is a grey solid. Iodine easily vaporises to a violet gas
- They all have seven electrons in their highest occupied energy level
- Their reactivity decreases going down the group. A more reactive halogen is able to displace a less reactive halogen from an aqueous solution of a halide compound
Why does reactivity decrease as you go down the halogens?
Because the attraction of the outer electrons to the nucleus decreases as the number of occupied shells increases
