C1 The History of the Atom (page 103) Flashcards
At the start of the 19th Century, who described atoms as ‘Solid Spheres’?
John Dalton
In 1897 who concluded from his experiments that Atoms weren’t solid spheres?
J J Thomson
How did J J Thomson conclude Atoms wasn’t spheres?
His measurements of charge and mass showed that an atom must contain even smaller negatively charged particles - electrons.
The ‘solid sphere’ idea of atomic structure had to be changed, so the New theory was known as what?
the ‘plum pudding model’
what did The Plum Pudding model show?
it showed the atom as a ball of positive charge with electrons stuck in it (look at page 103 for diagram).
Who showed that the ‘Plum Pudding Model was wrong’ in 1909?
Ernest Rutherford and his student Ernest Marsden
in 1909 Ernest Rutherford and his student Ernest Marsden conducted what famous experiment?
Alpha Particle scattering experiments.
What was the Alpha Particle Scattering Experiment’?
They fired positively charged alpha particles at an extremely thin sheet of gold.
from the plum pudding model, they were expecting the particles to pass straight through the sheet or be slightly deflected at most. This was because the positive charge of each atom was thought to be very spread out through the ‘pudding’ of the atom. But, whilst most of the particles did go straight through the gold sheet, some were deflected more than expected, and a small number were deflected backwaards. So the plum pudding model couldn’t be right.
(deflected means (something to change direction)
After Ernest Rutherford found out the plum pudding model was incorrect, what model did he come up with to expalin this new evidence?
The nuclear model
What was Ernest Rutherford’s ‘Nuclear Model of the atom?
The nuclear model of the atom, In this, there’s a tiny, positvely 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. If they were fired directly at the nucleus, they were deflected backwards. Otherwide, they passed through the empty space.
What are Alpha particles?
They consist of two protons and two nutrons bound together into a particle identical to a hellium - 4 nucleus. They are generally produced in the process of alpha decay, bu may also be produced in other ways
(Also called alpha rays or alpha radiation)
After Ernest Rutherford’s Alpha particle scattering experiments’, scientists realised electrons in a ‘cloud’ around the nucleus of an atom would be attracted to the nucleus, causing the atom to collaspse. Whose nuclear model of the atom suggested that all the electrons were contained in shells?
Niels Bohr
What did Niels Bohr’s Nuclear Model explain about the atoms?
Bohr proposed that electrons orbit the nucleus in fixed shells and aren’t anywhere in between. Each shell is a fixed distance from the nucleus. (see diagram on page 103)
Bohr’s theory of atomic structure was supported by many experiments and it helped to explain lots of other scientists’ observations at the time.
Further experiments showed the existance of Protons, by what scientist?
Ernest Rutherford
plus other scientist. it showed that the nucleus can be divided into smaller particles, which each have the same charge as a hydrogen nucleus.
these particles were names protons
About 20 years after scienists accepted that atoms have a nuclei, what sicentist carried out an experiment which provided evidence for nutral particles in the nucleus (which are now called neutrons)?
James Chadwick
This discovery of neutrons resulted in a model of the atom which was pretty close to the modern day accepted version, known as the nuclear model
