Developing the Atomic Theory 2 Lesson 6 October 4th, 2010

The Discovery of the Atomic Nucleus

Ernest Rutherford (1891–1937) Tested Thomson’s model of the atom. He conducted an experiment in which he shot positively charged particles at a very thin foil of pure gold

Ernest Rutherford (1891–1937) Rutherford obtained the stream of positive particles from a radioactive substance, which he placed in a lead block with a tiny hole. Out of the hole escaped a stream of these particles, which Rutherford directed at the gold foil.

Ernest Rutherford (1891–1937) In the experiment, most of the high speed positive particles went right through the foil. However, about 1 in 10 000 positive particles bounced back from the foil as if it had been deflected by something very massive and positively charged.

Ernest Rutherford (1891–1937) Rutherford had discovered the nucleus, the centre of the atom. This tiny positively charged part of the atom also contains most of the atom’s mass.

Ernest Rutherford (1891–1937) He calculated that the size of the nucleus compared to the rest of the atom was like the size of a single green pea compared to that of an entire football field!

Ernest Rutherford (1891–1937) Based on his gold foil experiment, Rutherford revised the atomic model using his prediction that all atoms everywhere contain a nucleus His model was like Thomson’s except that all of the atom’s positive charge and most of the atom’s mass were concentrated at a tiny point in the centre. The electrons surrounded the nucleus and occupied most of the atom’s volume, but they contained only a small fraction of the atom’s total mass.

Ernest Rutherford (1891–1937)

Inside the Nucleus Jams Chadwick (1891–1974), Rutherford’s student, refined the concept of the nucleus. Discovered that the nucleus contains neutral particles as well as positively charged particles. The neutral particles in the nucleus of the atom are called neutrons.

The positively charged particles in the atom are called protons. Each neutron in an atom has about the same mass as each proton in the same atom, but the neutron carries no electrical charge

Atoms are composed of three subatomic particles Protons- Heavy positively charged found in the nucleus Neutrons -are neutral particles that have the same mass as protons and are located in the nucleus Electrons- Negatively charged particles with almost no mass. They circle the nucleus at different energy levels.

Atoms are composed of three subatomic particles Atoms are electronically neutral so the number of electrons = the number of protons

Electrons Exist in Energy Levels Niels Bohr (1885–1962), Studied the properties of electrons in atoms and transformed Rutherford’s model into one of the models that are used today. Although some of the features shown here, such as the pairing up of electrons, were discovered after Bohr did his work, this kind of illustration has come to be known as a Bohr model, or Bohr diagram.

Niels Bohr (1885–1962), Bohr suggested that electrons surround the nucleus in specific energy levels, called shells. He discovered that electrons jump between these shells by gaining or losing energy. Each shell can contain only a specific number of electrons. The maximum number of electrons that can exist in each of the first three shells is two, eight, and eight.

Niels Bohr (1885–1962),

The Quantum Mechanical Model The most advanced and accurate model of the atom, and the one in use today by physicists and chemists, is called the quantum mechanical model. In this model, electrons do not exist as tiny points inside an atom. Electrons exist in specific energy levels, but they surround the positively charged nucleus in a form resembling a cloud.

The Quantum Mechanical Model

A Summary of the Atom All elements are composed of atoms, and one atom is the smallest unit of any element. The atoms themselves are made of different kinds of smaller particles, called subatomic particles. Three subatomic particles are protons, neutrons, and electrons, and they have different properties. One such property is relative mass.

A Summary of the Atom Relative mass compares the mass of an object to the mass of another object. An electron is the least massive subatomic particle of the three subatomic particles, so it is assigned a relative mass of 1. Compared to it, a proton has a relative mass of 1836, meaning that it is 1836 times heavier than an electron. Compared to an electron, a neutron is 1837 times heavier.

In energy levels surround nucleus Name Symbol Relative Mass Electric Charge Location Proton p 1836 1+ Nucleus Neutron n Electron e 1 1- In energy levels surround nucleus

Questions- Homework Check 1. (a) What is similar about the ancient and modern definitions of “elements”? (b) What is different about the ancient and modern definitions of “elements”? 2. What evidence led J.J. Thomson to believe that atoms of all elements contain electrons? 3. On what information did J.J. Thomson base his hypothesis that atoms contain positive particles? 4. Describe the experiment that showed that the atom has a dense, positively charged nucleus. 5. Describe three ways in which protons are different from electrons. 6. What are three features of a Bohr diagram?

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