Developing the model of the atom Flashcards
John dalton
agree w Democritus that
Atoms are solid spheres, different spheres made up the different elements
in 1897 what did JJ Thompson conclude ?
Atoms are not solid spheres
His measurements of charge and mass showed atoms must contain smaller - charged particles = electrons
How is JJ Thompsons theory shown ?
A
Plum pudding model
an atom was a sphere of positive charge, with negatively-charged
electrons
embedded in it.
What occurred in 1909 ?
What is the alpha scattering experiment ?
Ernest Rutherford and his student Ernst Marsden conducted the famous alpha scattering experiment.
They fired positively charged particles at a thin sheet of gold. They expected the particles to pass through the sheet because it was believed that the positive charge of each atom was thought to be spread out through the ‘pudding’ of the atom.
How did Rutherford explain this new evidence
nuclear model of the atom
Tiny + charged nucleus at the centre where most of mass is most concentrated .
Cloud of negatively charged electrons surround this nucleus - most of atom is empty space . When alpha particles came near the concentrated + charge of the nucleus, they were deflected. Fired directly at nucleus = deflect back .
Other wise pass through empty space
Niels Bohr’s nuclear model of the atom
electrons contained in shells
electrons orbit the nucleus in fixed shells - energy levels
Each shell in fixed distance from nucleus
His theoretical calculations agreed w experimental data
Further experiments
1) Rutherford and other showed that the nucleus can be divided into smaller particles which had same charge as hydrogen nucleus = proton and make up overall charge of nucleus
2) James Chadwick carried out an experiment which provided evidence for neutral particles in nucleus = neutrons, this explained the imbalance between atomic and mass number
Current model of the atom
1-5
1) nucleus is tiny and its where mass is most concentrated, it contains protons which as +1 relative charge and neutrons rc of 0 = which gives it an overall + charge. The radius is about 10,000 x smaller than the radius of an atom.
2) Rest of atom mostly empty space. negative electron whizz round outside the nucleus , which gives atoms its overall size - radius of an atom is 1x10^-10
3) Number of protons = number of electrons as they are equal but opposite charge and atoms have over all no charge
4) electrons in energy levels can move within the atom. If they gain energy by absorbing EM radiation, they move to higher energy level , further from nucleus. If one or more outer electrons leaves the atom, the atoms becomes + charged too.
