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Atom Structure

Atoms are very small, having a radius of about 1 × 10-10 metres. 

The basic structure of an atom is a positively charged nucleus  composed of both protons and neutrons surrounded by negatively  charged electrons

The radius of a nucleus is less than 1/10 000 of the radius of an atom.  Most of the mass of an atom is concentrated in the nucleus. 

Absorbing EM Radiation the electron moves further from the nucleus

The electrons are arranged at different distances from the nucleus  (different energy levels).

The electron arrangements may change:

  • With the  absorption of electromagnetic radiation (move further from the nucleus;  a higher energy level)
  • By the emission of electromagnetic radiation  (move closer to the nucleus; a lower energy level) 
Emitting EM Radiation the electron moved towards the nucleus and emits EM radiation

In an atom the number of electrons is equal to the number of protons in the nucleus. Atoms have no overall electrical charge

All atoms of a particular element have the same number of protons. The  number of protons in an atom of an element is called its atomic number

The total number of protons and neutrons in an atom is called its mass  number

An isotope is a version of the element with the same number of protons and a different number of neutrons

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Atoms turn into positive ions if they lose one or more outer electron(s). 

Question: What are isotopes? (2 marks)

Question: In a street light the sodium atoms emit light.
What would cause light to be emitted from a sodium atom? (1 mark)

Question: A helium atom is much larger than an alpha particle.
Give one other difference between a helium atom and an alpha particle. (1 mark)

Question: Atoms have different atomic numbers and mass numbers.
In terms of sub-atomic particles, describe the difference between an atom’s atomic number and its mass number.

Discovery of the Atom

New experimental evidence may lead to a scientific model being  changed or replaced. 

Before the discovery of the electron, atoms were thought to be tiny  spheres that could not be divided. 

Greek Idea about Atoms

The discovery of the electron led to the plum pudding model of the  atom. The plum pudding model suggested that the atom is a ball of  positive charge with negative electrons embedded in it. 

Plum Pudding Model

The results from the alpha particle scattering experiment led to the  conclusion that the mass of an atom was concentrated at the centre  (nucleus) and that the nucleus was charged. This nuclear model  replaced the plum pudding model. 

Nuclear Model

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. 

Bohr’s Model (electrons in orbits)

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 proton  was given to these particles. 

Atom with only protons in the nucleus

The experimental work of James Chadwick provided the evidence to  show the existence of neutrons within the nucleus. This was about  20 years after the nucleus became an accepted scientific idea. 

Chadwick’s Model (Protons & Neutrons in nucleus)

Question: In the early part of the 20th century, scientists used the ‘plum pudding’ model to explain the structure of the atom.
Following work by Rutherford and Marsden, a new model of the atom, called the ‘nuclear’ model, was suggested.
Describe the differences between the two models of the atom. (4 marks)