Ions

When an atom gains a charge, its called an ion. Negative ions have more electrons than protons, they are an anion. Positive ions have more protons than electrons, they are cations.

Isotopes of an element are atoms with the same number of protons but a different number of neutrons. As the number of the electrons are the same, they have the same chemical properties as it is the number and arrangement of electrons that determines this. The physical properties differ slightly as they are more dependent on the mass of the atom.

Isotopes with more neutrons have:

Higher mass
Higher density
Higher melting and boiling points
Slower rates of reaction

The relative atomic mass calculation is:

Ar = ((mass of each isotope x abundance of that isotope) + (the next isotope and abundance) + etc) / total abundance

Sometimes total abundance is a percentage, other times its relative abundance.

The relative atomic mass is the weighted mean mass of an element, compared to 1/12th of the mass of an atom of carbon-12.

The relative isotopic mass is the mass of an atom of an isotope compared to 1/12th the mass of an atom of carbon-12

Brief History of the Model of the Atom

A Greek philosopher Democritus gave us the first model of the atom around 460BC.
John Dalton received the idea that an atom was the smallest piece of matter. He developed the billiard ball theory. That they were solid spheres that differed slightly.
JJ Thomson declared that atoms were not solid. He measured change and mass, discovering that they had charges
Rutherford conducted the gold foil experiment and developed the nuclear model. This stated that there was a tiny positive nucleus at the centre of the atom, surrounded by a cloud of electrons. The issue with this model was that the electrons cant be in a cloud otherwise they will be attracted to the nucleus.
Bohr finally developed a model stating :
Electrons exist in fixed orbits
Each shell has fixed energy
Electrons that move between shells, em radiation is emitted/absorbed
The energy of the shells decrease with increased distance

	Creado por Sarah . hace casi 8 años

A level Chemistry Apunte sobre OCR A - A level Chemistry Module 2, creado por Sarah . el 04/02/2017.