AS Chemistry Unit 1 Module 1 (Atoms and Reactions) incomplete up to titrations

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  • Created by: tazzxo
  • Created on: 30-03-13 17:50


Protons, Neutrons and Electrons

Chemists usually use a model of a nuclear atom first put forward around 100 years ago. Each atom is made up of protons, neutrons and electrons.

Essential features of this model :-

  • Protons and neutrons make up the nucleus which is at the centre of the atom.
  • Electrons orbit the nucleus in shells (Scientists now believe that electrons exist as clouds of electrical charge in ATOMIC ORBITALS).
  • Nucleus = tiny compared with the total volume of an atom.
  • Nucleus  = extremely dense and makes up almost all of the atom's mass (but only makes up a tiny portion of the atom's volume - E.G. Nucleus = grape, then outer electrons = 1.5 km away).
  • Most of an atom = empty space between the tiny nucleus and the elctron shells.

#Protons have a +ve charge and electrons have a -ve charge

#Atoms = electrically neutral (same no. of protons and electrons)

#Proton = virtually the same mass as a neutron

#Protons + Neutrons make up almost all of the atom's mass

Particle                      Relative Mass                  Relative Charge

proton                              1.0                                   1+

neutron                            1.0                                    0

electron                           1/2000                               1-


Most elements are made up of a mixture of ISOTOPES.

Isotopes of the same element:   1) diff. masses

                                               2) same no. of protons and electrons

                                               3) diff. no. of neutrons in nucleus

In Greek, isotope means 'at the same place' ----> the word isotope is used for different atoms of the same element that occupy the same position in the periodic table.

We describe an isotope using 2 numbers: > Atomic Number (proton number) - P

                                                             > Mass Number (nucleon number) - P+N([Symbol for an isotope]

Isotopes are atoms of the same element with different numbers of neutrons.

Atomic (proton) number is the number of protons in the nucleus of an atom.

Mass (nucleon) number is the number of particles (protons and neutrons)


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