how is radioactivity related to atomic structure?

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How is radioactivity related to atomic structure?

Following the discovery of radioactivity by Henri Becquerel in 1896, many scientists were keen to find out more about it and understand where it came from.

In a radioactive atom, the nucleus is unstable and so it emits particles or waves of radiation to form a more stable atom.

This process is called radioactivity or radioactive decay.

This is why early experiments with radioactivity lead to important discoveries about the structure of the atom.

As a result of this work, we now know that radiation comes from radioactive atoms.


What are atoms?

It is now known that all matter is made of atoms.

In some substances, all the atoms are the same, which means that the substance is called an element.

For example, gold is an element made up of only gold atoms.

It is only relatively recently that we have had microscopes powerful enough to ‘see’ individual atoms. Before that, the idea that atoms existed was only a theory.

The first person to suggest the idea of atoms was the Greek philosopher Democritus, in 450 BC.


What did Dalton think atoms were like?

Ideas about atomic structure have changed over time.

In 1803, John Dalton reintroduced the idea that everything is made of atoms. He said atoms were solid spheres of matter that could not be split.

Dalton also suggested that each element contained identical atoms.


How did electrons spoil Dalton’s model?

In 1897, whilst studying cathode rays, JJ Thomson discovered tiny particles with a negative charge.

His discovery did not fit with Dalton’s model of the atom, and so Thomson had to propose a new model.

These negative particles were given out by atoms and were much smaller than atoms.

Thomson had discovered the existence of electrons.


What is the plum pudding model?

Thomson’s model became known as the plum pudding model, because the electrons in the atom were thought to be like raisins in a plum pudding.

Based on his discovery, Thomson adapted Dalton’s model of the atom.

Electrons had been proved to exist but there were doubts about this model.

He suggested that an atom is a positively-charged sphere with negative electrons distributed throughout it.


What was Rutherford’s involvement?


• http://www.youtube.com/watch?v=wzALbzTdnc8


What did Geiger and Marsden do?


The results of Geiger and Marsden’s experiment were:

What were Geiger and Marsden’s results?

The experiment was carried out in a vacuum, so deflection of the alpha particles must have been due to the gold foil.

1. Most alpha particles went straight through the gold foil, without any deflection.

2. Some alpha particles were slightly deflected by the gold foil.

3. A few alpha particles were bounced back from the gold foil.

How can these results be explained in terms of atoms?


Rutherford had expected all the alpha radiation to pass through the gold foil. He was surprised that some alpha particles were deflected slightly or bounced back.

The ‘plum pudding’ model could not explain these results, so Rutherford proposed his ‘nuclear’ model of the atom.

How did Rutherford interpret the results?

He suggested that an atom is mostly empty space with its positive charge and most of its mass in a tiny central nucleus.

Electrons orbited this nucleus at a distance, like planets around the Sun.


How did Rutherford explain the results?


Which model of the atom?


The electrons orbit the nucleus in layers called shells.

The nucleus is where most of the mass of the atom is found. It contains protons and neutrons.

Experiments showed that Rutherford’s atomic model (a tiny, positively-charged nucleus orbited by electrons) was correct.

What is the modern model of the atom?

Further developments in understanding about atomic structure followed, but Rutherford’s nuclear model still forms the basis of the modern model of the atom.


Atoms are made of three basic building blocks called protons, neutrons and electrons.

In any atom, the number of electrons is equal to the number of protons and so the overall charge of an atom is zero.

What are atoms made of?

There are two properties of protons, neutrons and electrons that are especially important: mass and charge.

-1almost 0

01

+11

electron

neutron

proton

ChargeMassParticle


Particles in the modern model


What makes a carbon atom carbon?

The atoms of any particular element always contains the same number of protons.

In the periodic table, there are two numbers found with each element. What do these numbers represent?

Carbon atoms always have six protons. Atoms with different numbers of protons must be other elements. For example:

all atoms with 1 proton are hydrogen atoms;

all atoms with 17 protons are chlorine atoms.

Atomic number (or proton number) is the number of protons

Mass number is the number of protons + the number of neutrons.


All carbon atoms have the same number of protons, but not all carbon atoms are identical.

mass number is different

atomic number is the same

Although atoms of the same element always have the same number of protons, they can have different numbers of neutrons. Atoms that differ in this way are called isotopes.

What are isotopes?

For example, carbon exists as three different isotopes: carbon-12, carbon-13 and carbon-14:

Potassium is another element that exists as three different isotopes: potassium-39, potassium-40 and potassium-41.


Atomic structure – key words

how is radioactivity related to atomic structure?

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