The changing atom Flashcards
Who developed the first idea of the atom?
The Greek philosopher Democritus
What did Democritus suggest?
He suggested that you could divide a sample of matter only a certain number of times. Eventually, he believed, you would end up with a particle that could not be spilt any further. Democritus called this particle ‘átomos’, which is Greek for ‘indivisible’.
What 4 things did Democritus discover and propose about the concept of atoms? (4)
- Proposed that everything is made of tiny uncuttable building blocks known as atoms.
- Identified the real existence of the cosmos with these permanent and indivisible elements.
- Described invisible atoms as the basis of all matter, which anticipated modern scientific discoveries.
- Developed a theory that atoms are in constant motion and interact with each other to create the observed properties of matter.
In what time period did Dalton develop his atomic theory?
In the early 1800s
What did John Dalton’s atomic theory state? (4)
- Atoms are tiny particles that make up elements.
- Atoms cannot be divided.
- All atoms of a given element are the same.
- Atoms of one element are different from those of every other element.
Apart from his atomic theory, what else did Dalton do? (2)
- Dalton used his own symbols to represent atoms of different elements.
- He developed the first table of atomic masses.
Describe what scientists already knew before J.J. Thomson discovered electrons
Scientists had discovered cathode rays, which were emitted from cathode ray tubes.
Describe what J.J. Thomson discovered and concluded about cathode rays
Thomson discovered that cathode rays were a stream of particles with the following properties:
- They had a negative charge.
- They could be deflected by both a magnet and an electric field.
- They had a very, very small mass.
Cathode rays were, in fact, electrons. Thomson concluded that they must have come from within the atoms of the electrodes themselves.
As J.J. Thomson concluded that electrons came from the atoms of the electrodes, what idea had he disproved?
The idea that an atom could not be split any further, proposed by the ancient Greeks and by Dalton.
Describe what J.J. Thomson proposed
Thompson proposed that atoms are actually made up of negative electrons moving around in a ‘sea’ of positive charge. This model is commonly called the plum-pudding atom. In Thomson’s atom, the overall negative charge is the same as the overall positive charge. this means that the atom is neutral with no overall charge.
Which scientist carried out the gold-leaf experiment?
Ernest Rutherford
When did Rutherford and two of his students carry out the gold-leaf experiment?
1909
Describe the gold-leaf experiment
In 1909, Rutherford and two of his students carried out an experiment where they directed α-particles (alpha particles) towards a sheet of very thin gold foil. They measured any deflection (change in direction) of the particles. Rutherford calculated that a plum-pudding atom would hardly deflect α-particles at all.
What were the results of Rutherford’s gold-leaf experiment? (3)
- Most of the particles, as expected, were not deflected at all.
- However, a small percentage of particles were deflected through large angles.
- Very few particles were actually deflected back towards the source.
What points did Rutherford make about his new version of the atom model?
- The positive charge of an atom and most of its mass are concentrated in a nucleus, at the centre.
- Negative electrons orbit this nucleus, just as the planets orbit the Sun.
- Most of an atom’s volume would be the space between the tiny nucleus and the orbiting electrons.
- The overall positive and negative charges must balance.
