Chemistry 3a Flashcards
In the past, what were the obvious ways to categorise elements?
Physical/chemiscal properties and their relative atomic mass
Why did they categories them this way?
They didn’t know about atomic structure or protons/electrons so they didn’t have atomic numbers
When did they discover these things and arrange the elements in order of atomic numbers?
20th century
What was the first good effort of making a periodic table?
Newlands’ Law of octaves in 1864
What did Newlines discover?
That every eight element had similar properties so he listed the known elements in rows of seven
What was wrong with this organisation?
The pattern broke on third row with transition metals because he left no gaps
Why was his work criticised?
His groups contained elements that didn’t have similar properties (carbon and titanium), He mixed up metals and non-metals (oxygen and iron) and he didn’t leave any gaps for elements that hadn’t been discovered yet
What was the other person to try and make te periodic table?
Dmitri Mendeleev in 1869
What did Mendeleev do?
He put the elements in order of atomic mass like Newlines, but he also left gaps to keep similar propertied elements in the same vertical columns (groups)
Why were the gaps that he left very useful?
They predicted the properties of undiscovered elements, which then fit the pattern (convincing evidence for other scientists)
Why did he have to leave big gaps in the first two rows?
The transition metals
When were protons, neutrons and electrons discovered?
Late 19th century and the periodic table, matched what they discovered so scientists then accepted it as a very important and useful summary of the structure of an atom
What is the modern periodic table based on?
Electronic structure
How can you predict an elements chemical properties?
Using the electron arrangement
What do the shells in an atom correspond to?
To an energy level
Apart from the transition metals, what are similarities between elements in the same group?
They have the same number of electrons in their highest occupied energy level (outer shell)
What is an example of this?
Group 6 all have 6 electrons in their outer shell
What does the positive charge of the nucleus do?
Attracts electrons and holds them in place, the further from the nucleus the electron is, the less the attraction
What also makes attraction of the nucleus less?
When there are a lot of inner electrons as they get in away of the nucleus charge, reducing the attraction
What is this effect known as?
Shielding
What does the combination of increased distance and increased shielding mean?
That an electron in a higher energy level is more easily lost because theres less attraction from the nucleus holding it in place. Thats why group 1 metals get more reactive as you down the group
What does increased distance shielding also mean?
That a higher energy level is less likely to gain an electron as theres less attraction from the nucleus pulling electrons into the atom. Thats why group 7 elements get less reactive going down the group
What elements are group 1?
The alkali metals
What happens as you go down group 1?
The alkali metals become more reactive because the outer electron is more easily lost, because its further from the nucleus, and also the metals as you go down have lower melting and boiling points
Whats a similarity between group 1 alkali metals?
They have low density (the first three even have less density than water)
What are the main group 1 elements?
Lithium, sodium, potassium and also rubidium and caesium
How many electrons do the alkali metals have in their outer shell?
One electron
What do the alkali metals form ionic compounds with?
Non-metals
How do alkali metals form ionic compounds?
They’re keen to lose the one electron to make a 1+ ion, so they ALWAYS form ionic bonds and produce white compounds that dissolve in water to form colourless solutions
What is produced when group 1 metals react with water?
They produce hydrogen gas
What happens when lithium, sodium or potassium are put into water?
They react very vigorously and float/move around the surface whilst fizzing a lot, they produce hydrogen, potassium gets hot enough to ignite it, a lighted splint will indicate hydrogen by producing a pop sound.
What do they form?
Hydroxides that dissolve in water to give alkaline solutions
What is an example of this?
2NA(s) + 2H2O(l) -> 2K)H(aq) + H2(g)
As you go down group 7, the halogens have what properties?
They become less reactive because its harder to gain an extra electron as the other shells further from the nucleus. They have a higher melting point and a higher boiling point
What are the halogens?
Non metals with coloured vapours
What is fluorine?
A very reactive, poisonous yellow gas
What is chlorine?
A fairly reactive, poisonous dense green gas
What is bromine?
A dense, poisonous red-brown volatile liquid
What is iodine?
A dark grey crystalline solid or a purple vapour
How do the halogens form ionic bonds with metals?
The halogens form 1- ions called halides when they bond with metals
What is an example of this?
Na+CL- or Fe3+Br-3
What happens with more reactive halogens?
They will displace less reactive ones
How does this happen?
A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt
What is an example of this?
chlorine can displace bromine and iodine from an aqueous solution of its salt (a bromide of iodide). Bromine will also displace iodine because of the trend in reactivity.
What are transition elements?
Transition elements or transition metals are typical metals and have properties that you would expect of a ‘proper’ metal
What are the properties of transition elements?
Good conductors of heat/electricity, dense, strong, shiny, much less reactive than group 1 metals(don’t react as vigorously with water or oxygen for example), also much denser, stronger and harder than the group 1 metals and have much higher melting points
How many ions do transition metals have?
They usually have more than one ion e.g. Fe2+, Fe3+
What do the different ions usually form?
Different coloured compounds, Fe2+ ions usually give green compounds whereas Fe3+ ions usually form red/brown compounds e.g. rust
Why are the compounds colourful?
Due to the transition metal ion they contain e.g. potassium chromate(VI) is yellow. Potassium manganate(VII) is purple. Copper(II)sulphate is blue.
What do the colours in gemstones and pottery glazes depend on?
The transition metals
What do transition metals and their compounds make?
Good catalysts
What are examples of this?
Iron is the catalyst used in the Haber process for making ammonia. Manganese(IV) oxide is a good catalyst for the decomposition of hydrogen peroxide. And Nickel is useful for turning oils into fats for making margarine
What does soft water produce with soap?
You get a lather
What happens with hard water and soap?
You get a scum
Why does this happen with hard water?
The dissolved calcium ions and magnesium ions in the water react to make the scum which is insoluble. To get a decent lather you need to use more soap-costing more money
What happens when hard water is heated?
It forms furring or scale (mostly calcium carbonate) on the insides of pipes, boilers and kettles. Badly scaled pipes and boilers reduce efficiency of heating systems, and may need to be replaced and scale can even eventually block pipes
What does it mean by ‘scale can be a slight thermal insulator’?
A kettle with scale on the heating element takes longer to boil than a clean non scaled kettle-it becomes less efficient
How can hard water form?
Rain falling on some types of rocks e.g. limestone, chalk and gypsum, can dissolve compounds like magnesium sulphate which is soluble, and calcium sulphate which is only a bit soluble
What are the two types of water hardness?
Temporary and permanent
What is temporary hardness caused by?
The hydrogen carbonate ion, HCO-3 in Ca(HCO3)2
What is permanent hardness caused by?
Dissolved calcium sulphate (among other things as well)
How is temporary hardness removed?
By boiling, when heated, the calcium hydrogen carbonate decomposes to form calcium carbonate which is insoluble. This solid is the ‘limescale’ in kettles