Atomic Structure
Structure of the atom:
• Atoms have a radius of about 1 × 10!"# 𝑚.
• Atoms contains a positively charged nucleus made up of protons and neutrons, which is surrounded by
negatively charged electrons.
• The energy level of an electron may change when the atoms emits or absorbs electromagnetic radiation
o Absorbing electromagnetic radiation moves electrons to a higher energy level, further from the
nucleus.
o Electromagnetic radiation is emitted when an electron drops to a lower energy level.
• Isotopes are different forms of a particular element. Isotopes have the same number of protons but
different numbers of neutrons.
Developing the model of the atom:
• 1904 J. J. Thompson - The plum pudding model suggested that the atom is a ball of positive charge
with negative electrons embedded in it.
• In 1909 Geiger and Marsden directed a beam of alpha particles at a thin sheet of gold foil.
o Based on the plum pudding model atom we would expect that the alpha particles would pass
straight through the atom as charge is evenly spread out through the atom and also overall
neutral. But this did not happen
o Most of the alpha particles passed straight through the metal foil.
o The number of alpha particles deflected per minute decreased as the angle of deflection
increased.
o About 1 in 1000 alpha particles were deflected by more than 90°
• In 1911 Rutherford produced the nuclear model – the experiment showed that the mass of an atom
was concentrated at the centre and that the nucleus was charged.
• Bohr’s model – electrons in an atom orbit the nucleus at specific distances and specific energy levels.
Also, that electrons can change to another energy level by absorbing or emitting electromagnetic
radiation.
• Further alpha scattering experiments showed:
o The hydrogen nucleus has the least amount of charge
o Any nucleus consists of a smaller bunch of particles each with an equal positive charge.
• 1932 James Chadwick discovered the neutron
Radioactive Decay:
• Radiation is given out when an unstable nuclei decay. Decay is a random spontaneous process.
• You cannot say when decay is going to happen, you cannot say which nuclei is going to decay, you
cannot do anything to make decay happen – it is not affected by temperature or chemical bonding.
• A Geiger counter is a device used to measure the amount of radioactivity given out by a material. It
measures radioactivity in Becquerels. 1Bq = 1 decay per second.
• Some atomic isotopes are unstable and emit radiation, so they become more stable, this is called
radioactive decay.
• There are 3 types of radiation
Structure of the atom:
• Atoms have a radius of about 1 × 10!"# 𝑚.
• Atoms contains a positively charged nucleus made up of protons and neutrons, which is surrounded by
negatively charged electrons.
• The energy level of an electron may change when the atoms emits or absorbs electromagnetic radiation
o Absorbing electromagnetic radiation moves electrons to a higher energy level, further from the
nucleus.
o Electromagnetic radiation is emitted when an electron drops to a lower energy level.
• Isotopes are different forms of a particular element. Isotopes have the same number of protons but
different numbers of neutrons.
Developing the model of the atom:
• 1904 J. J. Thompson - The plum pudding model suggested that the atom is a ball of positive charge
with negative electrons embedded in it.
• In 1909 Geiger and Marsden directed a beam of alpha particles at a thin sheet of gold foil.
o Based on the plum pudding model atom we would expect that the alpha particles would pass
straight through the atom as charge is evenly spread out through the atom and also overall
neutral. But this did not happen
o Most of the alpha particles passed straight through the metal foil.
o The number of alpha particles deflected per minute decreased as the angle of deflection
increased.
o About 1 in 1000 alpha particles were deflected by more than 90°
• In 1911 Rutherford produced the nuclear model – the experiment showed that the mass of an atom
was concentrated at the centre and that the nucleus was charged.
• Bohr’s model – electrons in an atom orbit the nucleus at specific distances and specific energy levels.
Also, that electrons can change to another energy level by absorbing or emitting electromagnetic
radiation.
• Further alpha scattering experiments showed:
o The hydrogen nucleus has the least amount of charge
o Any nucleus consists of a smaller bunch of particles each with an equal positive charge.
• 1932 James Chadwick discovered the neutron
Radioactive Decay:
• Radiation is given out when an unstable nuclei decay. Decay is a random spontaneous process.
• You cannot say when decay is going to happen, you cannot say which nuclei is going to decay, you
cannot do anything to make decay happen – it is not affected by temperature or chemical bonding.
• A Geiger counter is a device used to measure the amount of radioactivity given out by a material. It
measures radioactivity in Becquerels. 1Bq = 1 decay per second.
• Some atomic isotopes are unstable and emit radiation, so they become more stable, this is called
radioactive decay.
• There are 3 types of radiation
