atomic models

Question 1

which model of the atom do we need to know?

Answer 1

• the currently accepted ones

Question 2

why were completely different models used in the past?

Answer 2

• they fitted evidence available at the time
• as scientists did more experiments, new evidence was found and the models were modified to fit it

Question 3

what happened at the start of the 19th century?

Answer 3

• John Dalton described atoms as solid spheres and said that different spheres made up the different elements

Question 4

what happened in 1897?

Answer 4

• J.J tompson concluded from his experiments that an atom must contain even smaller, negatively charged particles, electrons
• the solid sphere idea of atomic structure had been changed
• the new model was known as the plum pudding model

Question 5

what happened in 1909?

Answer 5

• Earnest Rutherford and his students Hans Geiger and Ernest Marsden conducted the famous gold foil experiment
• they fired alpha particles (which are positively charged) at a very thin sheet of gold
• from the plum pudding model, they were expecting most of the alpha particles to be deflected slightly by the positive pudding that made up most of the atom
• in fact, most of the alpha particles passed straight through the gold atoms and a very small number were deflected backwards
• so the plum pudding model could not be right
• so rutherford came up with an idea that could explain this new evidence, the nuclear model of the atom
• in this, there’s a tiny positively charged nucleus at the centre, surrounded by a cloud of negative electrons, most of the atom is empty space

Question 6

describe bohr’s model

Answer 6

• scientists realised that electrons in a cloud around the nucleus of an atom, as rutherford described, would quickly spiral down into the nucleus, causing the atom to collapse
• niels bohr proposed a new model of the atom with 4 basic principles:
  -> electrons only exist in fixed orbitals (shells) and not anywhere in between
  -> each cell has a fixed energy
  -> when an electron moves between shells, electromagnetic radiation is emitted or absorbed
  -> because the energy of shells is fixed,the radiation will have a fixed frequency
• the frequencies of radiation emitted and absorbed by atoms were already known from experiments
• the bohr model fitted these observations

Question 7

describe the refined bohr model

Answer 7

• scientists later discovered that not all the electrons in a shell had the same energy
• this meant the bohr model wasn’t quite right, so they refined it to include subshells
• the refined model fitted observations even better than bohr’s original model
• we know that the refined model is not perfect either but it is still widely used to describe atoms because it is simple and explains many observations from experiments, like bonding and ionisation energy trends
• this model explained why some of the elements are inert
• bohr believed that the shells of an atom can only hold a fixed number of electrons
• in addition to this, an element’s reactivity is due to its electrons, so when the shell is full it is inert and does not react

Question 8

what is the most accurate model we have today based on?

Answer 8

• quantum mechanics

Question 9

what does the quantum model explain?

Answer 9

• some observations that can’t be accounted for by the bohr model, but it is harder to understand and visualise

Question 10

which model does scientists use?

Answer 10

• whichever model is most relevant to what they’re investigating