T1- History of the atom and periodic table Flashcards
What did John Dalton think?
He described atoms as solid spheres, and said that the different spheres made up the different elements.
What did JJ Thompson think?
He concluded from experiments that atoms weren’t solid spheres. His measurements of charge and mass showed that an atom must contain even smaller, negatively charged particles (electrons).
This was called the plum pudding model.
The plum pudding model showed the atom as a ball of positive charge with electrons stuck in it.
What did Ernest Rutherford think?
Ernest Rutherford fired positively charged alpha particles at an extremely thin sheet of gold.
They found that most particles went straight through the gold sheet and some were deflected sideways however a few were deflected backwards. This proved the plum pudding model wrong.
Rutherford’s new idea was that there is a tiny positively charged nucleus at the centre where most of the mass is concentrated. A ‘cloud’ of negative electrons surrounds this nucleus so most of the atom is empty space.
When alpha particles came near the concentrated positive charge of the nucleus they were deflected.
What did Niels Bohr think?
Scientists realised that Rutherford’s idea of electrons being in a ‘cloud’ was not possible as they would be attracted to the nucleus causing the atom to collapse.
Niels Bohr’s nuclear model of the atom suggested that all the electrons were contained in shells. The electrons orbit the nucleus in fixed shells and a fixed distance away from the nucleus.
This theory was supported by many experiments and other scientists at the time.
Who discovered that the nucleus could be divided into smaller particles?
Rutherford found that the nucleus could be divided into smaller particles, each of which has the same charge as a hydrogen nucleus.
Who discovered neutrons?
James Chadwick found neutral particles in the nucleus called neutrons.
What is the order of the history of the atom?
1) John Dalton - solid spheres
2) JJ Thompson - Plum pudding model
3) Ernest Rutherford - Alpha particle scattering experiment
4) Niels Bohr - Electron shells
5) Ernest Rutherford - Nucleus could be divided into smaller particles.
6) James Chadwick - Neutrons
What are the rows on the periodic table called?
Periods. The periods is the number of electron shells.
Example: carbon with an atomic number of 6 = 2, 4 = 2 shells, period 2
What are the columns on the periodic table called?
Groups. The group number tells you how many electrons there are in the outer shell. Elements in the same group react in a similar way.
What is group 0?
Group 0 is called the noble gases. It is made up of elements with full outer shells. They are unreactive as they are complete.
What are the trends of reactivity?
In group 1 the elements react more vigorously as you go down the group. In group 7 the reactivity decreases as you go down the group.
What was Dmitri Mendeleev’s periodic table?
Dmitri Mendeleev mainly put the elements in order of atomic weight but made exceptions for some elements if they had similar chemical properties. He left gaps for undiscovered elements. The discovery of isotopes supported Mendeleev’s decision to not place elements in order of atomic weight.
