history of an atom + periodic table development Flashcards
What did the ancient greeks believe a long time ago - democritus
Believed that everything is made of atoms
They believed that atoms are tiny spheres which cannot be divided
- this idea was accepted for hundreds of years
What happened in 1897
Scientists discovered that atoms contain tiny negative particles - which were called electrons
The discovery of electrons told scientists that atoms are not tiny spheres that cannot be divided
Instead, atoms must have an internal structure
New model for the structure of atoms was suggested by scientists - plum pudding model
describe the plum pudding model of atomic structure
In the plum pudding model, an atom is a ball of positive charge with negative electrons embedded in it.
describe the alpha scattering experiment and what it was used for
This experiment was used to see if the plum pudding model was correct - experiment was called alpha scattering experiment
First the scientists took a piece of gold foil
They used gold because we can hammer gold into very thin foil just a few atoms thick
Scientists then fired tiny particles at the gold foil. These particles are called alpha particles
Alpha particles have a positive charge
results of alpha scattering experiment
The scientists found that most of the alpha particles passed straight through the gold foil without changing direction
Sometimes an alpha particles was deflected - it changed direction as it passed through the gold foil
Sometimes an alpha particle bounced straight back off the gold foil
describe how the results of the alpha scattering experiment lead to the nuclear model
The fact that most of the alpha particles went through the gold foil told the scientists that atoms are mainly empty space
- So straight away they knew that the plum pudding model had to be wrong
The fact that some of the alpha particles were deflected, this told the scientists that the centre of the atom must have a positive charge
- remember alpha particles are positive, so any alpha particle that comes close to the positive centre of an atom is repelled and changes direction
The fact that some alpha particles bounced straight back this told scientists that the mass of an atom was concentrated at the centre
(nucleus)
This nuclear model
replaced the plum pudding model
describe the nuclear model of atomic structuer
most of the atom is simply empty space
centre of atom - there is a tiny positive nucleus, containing most of the mass of the atom
around the edge of the atom, there are negative electrons
new experimental evidence led to the plum pudding model being replaced by the nuclear model
describe how the nuclear model was modified by the discoveries of electron energy levels, the proton and the neutron
Electrons are found at the edge of the atom
Niels Bohr proposed that electrons orbit the nucleus at specific distances rather than just in a general area
This idea was based on calculations that Bohr had carried out
Bohrs proposal was accepted because it agreed with the results of experiments by other scientists
These orbits are now called energy levels of shells
Scientists found out that the positive charge in the nucleus is due to tiny positive particles which they called protons
No. of protons determines the amount of positive charge in the nucleus
Around 20 years after nuclear model was proposed, the scientist James Chadwick made a final important discovery
Chadwick discovered that the nucleus also contained neutral particles which he called neutrons
radius of an atom
0.1 nm 1x10^-10 meters
radius of nucleus
1x10^-14m
radius of nucleus in comparison to radius of atom
radius of nucleus is less than 1/10,000th the radius of the atom
however nearly all the mass of the atom is found in the nucleus
PERIODIC TABLE LAY OUT
ARRANGED INTO COLUMNS -CALLED GROUPS
AND ROWS CALLED PERIODS
WHY IS IT CALLED A PERIODIC TABLE
The table is called a periodic table
because similar properties occur at regular intervals.
E.G. GROUP 1 CONSISTS OF HIGHLY REACTIVE METALS, LI, NA, K, RB
All the elements in a group have similar chemical properties
- they react in a similar way
That is because all the elements in a group have the same number of electrons in their outer energy level
describe early attempts to arrange the elements into a periodic table
Newlands - arranged the elements in order of increasing atomic weight
He noticed that every eighth element reacts in a similar way
He called this his law of octaves
Problems - his system had problems
By always sticking to the exact order of atomic weight sometimes elements were grouped together when they had totally different properties
meant that Newland’s law of octaves was not really taken seriously by other scientists
How Mendeleev developed the first modern periodic table
Started by arranging all the elements in order of increasing atomic weight
However he then did two things that no-one had done before
Firstly, if he needed to, Mendeleev would switch the order of specific elements so they fitted the patterns of other elements in the same group
Secondly, Mendeleev realised that some elements had not been discovered. So he simply left gaps in his periodic table where he thought an element was missing
Mendeleev was so confident that his table was correct, that he predicted the properties of the undiscovered elements based on other elements in the same groups
Several years later the missing elements were discovered and their properties matched Mendeleev’s predictions
Because of that, other Scientists no accepted that Mendeleev’s table was correct
