Development of the periodic table Flashcards
Plum pudding model
Ball of positive charge
Negative electrons embedded into it
No shells
No nucleus
No protons
Alpha scattering experiment
Take a piece of gold foil only a few atoms thick
Fire alpha particles (positive charge) at this sheet
Most passed through
Some was deflected
Some completely bounced back
What did the alpha scattering experiment tell us?
Went straight through = atoms have empty space
Deflected = they must come close to a positive charge in the centre
Bounced back = positive centre has a great deal of mass and positively charged (nucleus)
The nuclear model
Empty space with a positive nucleus where mass is concentrated
Electrons don’t orbit at specific distances
Bohr’s model
Electrons orbit around the nucleus in specific distances in shells
Accepted because it agreed with other scientists results
Discovery of protons and neutrons
What made up most of the nucleus
What made up the atom’s mass
Isotopes
Atoms of the same element (same number of protons) with a different number of neutrons
Relative atomic mass
Average of the mass numbers of different isotopes weighted based on its abundance
Who discovers protons?
Rutherford
Who discovered neutrons?
Chadwick
Döberiner’s triads
Elements with similar properties occur in 3s
Started the question of arranging elements in a logical order
Newland’s octaves
Arrange elements with increasing mass number
Every 8 elements have similar properties
Problem with newland’s octaves?
Grouping by atomic weight means elements with different properties grouped together
Not taken seriously
Mendeleev’s table
Arranged in increasing atomic weight
But also
switched order of some to fit properties into their groups
Left gaps for undiscovered elements
Why was Mendeleev’s table accepted?
Predicted properties for the undiscovered elements correctly
Since they were discovered later
How is the modern periodic table different?
Discovery of protons therefore the atomic number
So ordered by atomic number
And contains the noble gases
What determines the reactivity?
Elements in the same group react similarly
Because they have the same number of electrons in its outer shell
Radius of an atom
0.1nm
Or 1x10^-10 m
Radius of the nucleus of an atom?
1/1000 the size of a radius of the atom
Or 1x10^-14m
Group 0
Noble gases
Group 0 properties
Very unreactive
Boiling points increase as you go down
Why is group 0 unreactive?
Because they have a full outer shell of electrons
Why do boiling points of noble gases increase going down?
Because the atoms get bigger going down
So they have stronger intermolecular forces due to having a shorter distance
So require more energy to overcome them
Categories of metals
Group 1 and 2
Transition metals
Post transition metal
Group 1 metals
Alkali metals that are in group 1
So have 1 electron in outer shell
Group 1 physical properties
Very soft = cut with a knife
Low density
Low melting points
Group 1 chemical properties
React rapidly with oxygen = metal oxide
React rapidly with halogen
React rapidly with water = alkali hydroxide and hydrogen
Group 1 reaction with oxygen eg lithium
4Li + O₂ —> 2Li₂O
Same for ALL group 1
Group 1 reaction with a halogen eg lithium and chlorine
2Li + Cl₂ —> 2LiCl
Same for ALL group 1 and halogens
Group 1 reaction with water eg lithium
2Li + 2H₂O —> 2LiOH +H₂
Forms a metal hydroxide and hydrogen (effervescence)
Why does group 1 reactivity increase going down?
In order to react group 1 metals must lose outer shell electron
Going down means an increased number of internal shells
Meaning less electrostatic attraction between outer electron and positive nucleus
And greater distance
So going down loses electrons easier = increased reactivity
Transition metals physical properties
Hard and strong
High melting point
High density
Transition metals chemical properties
Less reactive than group 1
Forms ions of different charges
Forms coloured compounds
Used as catalysts
Group 7
Halogens
Non metals
Have 7 electrons in outer shell
Boiling and melting point trends with halogens
Increase as you go down
Meaning more energy required to melt and boil it
Fluorine and chlorine physical properties
Gas at room temperature
Since it’s boiling point to become a gas is very low
Bromine physical property
Liquid at room temperature
Because it’s boiling point to become a gas is above 20°c but it’s melting point is below that
Iodine physical property
Solid at room temperature
Because it’s melting and boiling point to become a liquid or a gas is above room temperature
Halogens chemical properties
Bond with each other covalently to form simple molecule
Forms covalent compounds with other non metal elements
Forms a negative ion when reacted with a metal
Reactivity decreases going down
Why do halogens decrease in reactivity going down?
Because in order to react they must gain an electron
Atoms increase in size going down the group
Meaning more shielding due to internal shells
And a greater distance between the positive nucleus + electron
So less electrostatic attraction between them so less reactive going down
