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Heep Woh College
F.3 Chemistry Reading Assignment 1
Group Name: ______________________________
Class: 3 (
)
Group Member: 1. _______________________ (
) 2. _______________________ (
)
3. _______________________ (
) 4. _______________________ (
)
Read the following passage and answer the questions below.
How ideas of the atom developed
All chemistry depends on one big idea: that everything is made of atoms. But how
did chemists find out about them?
The atomic theory
In 1807, John Dalton put forward an atomic theory to explain the structure of matter
and certain aspects of chemical reactions. He suggested that all matter was made
of tiny particles, which he called atoms.
Discovery of electrons and protons
In 1897, J.J. Thomson passed high voltage electricity through a gas in a tube at low
pressure. He found that a stream of rays, called cathode rays, moved from the
negative electrode to the positive electrode. Besides, cathode rays were also
deflected strongly towards the positive plate.
Thomson suggested that the rays were composed of particles carrying negative
charges. He called them electrons.
Later Thomson discovered positive particles, he named them protons.
The nucleus – Rutherford’s scattering experiment
In 1911, E. Rutherford carried out a number of crucial experiments.
Thin sheets of metal foil were bombarded with positively charged particles alpha
particles. Most of the alpha particles went straight through the metal foils. Some
were deflected. A few of the alpha particles bounced straight back.
Rutherford concluded that most of the atom was empty space with a small,
positively charged nucleus at its centre.
In 1932, I. Chadwick discovered uncharged particles, which he called neutrons. The
proposals of Rutherford and Chadwick led to a model of the atom which was
composed of protons, neutrons and electrons. Protons and neutrons were packed
closely together in the nucleus. Electrons moved around the nucleus at a
considerable distance.
S.Mo & C.K.Lau
Form 3 / Chemistry / Reading Assignment 1 / 2
Question:
1.
Dalton suggested that all matter was made up of tiny particles.
What did he call these particles?
____________________________________________________________________________
2.
Which scientist discovers electrons?
____________________________________________________________________________
3.
What did Rutherford conclude based on his scattering experiment?
____________________________________________________________________________
____________________________________________________________________________
4.
Dalton though that atoms were solid balls.
(a) Which piece of evidence disproved Dalton’s idea?
________________________________________________________________________
(b) How did this evidence prove that atoms are not solid balls?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
5.
Why were neutrons more difficult to discover than electrons and protons?
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
The End
S.Mo & C.K.Lau