1.1 Atomic structure Flashcards
History of an atom - 1 - John Dalton
1800’s - John Dalton suggests that each of the elements are made from just one type of atom: tiny spheres that could not be divided.
History of an atom - 2 - JJ Thomson
1897 - J Thomson discovers the electron and proposes the plum pudding model: the atom is a ball of positive charge and the negative charge are embedded in it
History of an atom - 3 - Ernest Rutherford
1911 - Ernest Rutherford fired alpha particles at a piece of very thin gold foil(about 10,000 atoms thick).
Thomson’s plum pudding model predicted that…
All alpha particles… passed straight through
However what was observed was:
Most alpha particles… passed straight through
A very few alpha particles… were deflected by more than 90 degrees
Most of the atom is empty space
All the positive charge and most of the mass is concentrated in a small volume (the nucleus)
History of an atom - 4 - Niels Bohr
1913 - Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances The theoretical calculations of Bohr agreed with experimental observations.
History of an atom - 5 - James Chadwick and others
1920 - Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge. The name protons was given to these particles.
About 20 years after scientists had accepted that atoms have nuclei, in 1932 James Chadwick carried out an experiment which provided evidence for neutral particles in the nucleus. These became known as neutrons. The discovery of neutrons resulted in a model of the atom which was pretty close to the modern-day version.
Structure of the atom
The mass of an atom is concentrated in the nucleus, because the nucleus contains the heaviest subatomic particles (the neutrons and protons).
Electrons orbit the nucleus of the atom, contributing very little to its overall mass, but creating a ‘cloud’ of negative charge.
The electrostatic attraction between the positive nucleus and negatively charged electrons orbiting around it is what holds an atom together.
Sub atomic particles
Protons - +1 charge, 1 relative atomic mass
Neutrons - 0 charge, 1 relative atomic mass
Electrons - 1 charge, 1/1840 relative atomic mass (negligible)
Atom and Nucleus size
Atoms are extremely small with a radius of about 1 x 10-10 metres.
The radius of the nucleus is about 10 000 times smaller than that of the atom, so it is an extremely small region of space compared to the overall size of the atom.
This means that rather than being evenly spread out throughout the atom, virtually all of the atom’s mass is concentrated inside the nucleus.
Isotopes
Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.
Relative atomic mass
Ar = (% of isotope A x mass of isotope A) + (% of isotope B x mass of isotope B) + (% of isotope C x mass of isotope C)… / 100
