How the model of the atom changed from Dalton to Bohr, the alpha scattering experiment, subatomic particles, mass number and atomic number, and isotopes.
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1.What did John Dalton think atoms were?
Solid spheres that could not be divided, with different spheres making up the different elements.
2.What did JJ Thomson discover?
The electron, which showed that atoms must contain smaller particles and so are divisible.
3.What was the plum pudding model?
The idea that an atom is a ball of positive charge with negative electrons embedded in it.
4.Describe the alpha particle scattering experiment.
Positively charged alpha particles were fired at a very thin sheet of gold foil, and where they went was recorded.
5.What were the results of the alpha scattering experiment?
Most particles passed straight through, some were deflected, and a very small number bounced back.
6.What did most particles passing straight through show?
That the atom is mostly empty space.
7.What did the deflected and rebounded particles show?
That the mass and the positive charge of an atom are concentrated in a tiny central nucleus.
8.What replaced the plum pudding model?
The nuclear model, with a small positive nucleus surrounded by electrons.
9.What did Niels Bohr add to the model?
That electrons orbit the nucleus at fixed distances, in shells.
10.What did later experiments show about the nucleus?
That the positive charge is made up of individual particles, which were named protons.
11.What did James Chadwick discover, and roughly when?
The neutron, about twenty years after the nuclear model was accepted.
12.Why did the model of the atom change over time?
New experimental evidence was found that the existing model could not explain, so the model was modified or replaced.
13.Name the three subatomic particles.
Protons, neutrons and electrons.
14.What are the relative charges of the three particles?
Proton +1, neutron 0, electron -1.
15.What are the relative masses of the three particles?
Proton 1, neutron 1, electron very small, taken as almost zero.
16.Where is each particle found?
Protons and neutrons are in the nucleus. Electrons are in shells around the nucleus.
17.Why does an atom have no overall charge?
It contains equal numbers of protons and electrons, so the positive and negative charges cancel out.
18.What is the atomic number?
The number of protons in an atom. It is what defines the element.
19.What is the mass number?
The total number of protons and neutrons in an atom.
20.How do you calculate the number of neutrons?
Mass number minus atomic number.
21.An atom has mass number 23 and atomic number 11. How many of each particle?
11 protons, 11 electrons, and 23 minus 11 = 12 neutrons.
22.What is the radius of an atom, roughly?
About 0.1 nanometres, which is 1 x 10 to the power minus 10 metres.
23.How does the radius of the nucleus compare with the atom?
The nucleus is less than one ten thousandth of the radius of the atom, but it contains almost all the mass.
24.What are isotopes?
Atoms of the same element with the same number of protons but different numbers of neutrons.
25.Do isotopes of an element react differently?
No. Chemical reactions depend on electrons, and isotopes have the same electronic structure.
26.What is relative atomic mass?
The average mass of the atoms of an element, taking into account the abundance of each isotope.
27.How do you calculate relative atomic mass from isotope abundances?
Multiply each mass number by its percentage abundance, add the results, then divide by 100.
28.Chlorine is 75 per cent mass 35 and 25 per cent mass 37. Calculate its relative atomic mass.
(35 x 75) + (37 x 25) = 2625 + 925 = 3550. 3550 divided by 100 = 35.5.
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