Download SECTION 3-2: THE STRUCTURE OF THE ATOM

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Period «Per» Int. Chemistry
SECTION 3-2: THE STRUCTURE OF THE ATOM
Atom - the smallest part of an element that retains the chemical properties of that
element.
Atoms have Two Regions:
1. Nucleus: * Has at least one positively charged particle called a proton and
generally one or more neutral particles called neutrons. * Very small region
located near the center of the atom
2. Electrons: Surrounds the nucleus (electron cloud) and are negatively charged
Protons, neutrons, and electrons are often referred to as subatomic particles.
A series of experiments carried out by the English physicist Joseph John
Thomson in 1897 showed proof for Daltons Atomic Theory
Observations from the Cathode Ray tube: Proof for Atoms
• An object placed between the cathode and the opposite end of the tube casts a
shadow on the glass.
• A paddle wheel placed on rails between electrodes rolled along the rails from the
cathode towards the anode.
• Cathode rays were deflected by a magnetic field in the same manner as a wire
carrying electric current, which was known to have a negative charge.
• The rays were deflected away from a negatively charged object.
Figure 3-4 and 3-5 Pages 70 & 71 – It is realized that Sub-atomic Particles Exist
– But there needed to be more evidence.
CHARGE AND MASS OF THE ELECTRON
• 1909- Robert A. Millikan showed that the mass of the electron is about 1/2000
the mass of the simplest type of hydrogen atom.
• He confirmed electrons carry a negative charge.
• More-accurate experiments conducted since then indicate that the electron has a
mass of 9.109 x 10−31kg, or 1/1837 the mass of the simplest type of hydrogen
atom.
DISCOVERY OF THE ATOMIC NUCLEUS
3-2-2 Summarize the experiment carried out by Rutherford and his co-workers that
led to the discovery of the nucleus.
In 1911, New Zealander - Ernest Rutherford (and his associates Hans Geiger
and Ernest Marsden) bombarded a thin, gold foil with fast moving alpha particles
which are positively charged particles with about four times the mass of a
hydrogen atom. The rebounded alpha particles must have experienced some
powerful force within the atom. Rutherford called it the Nucleus. Rutherford
had discovered that the volume of a nucleus was very small compared with the
total volume of an atom. Metaphorically the
nucleus is a marble compared to a football field
(the total volume of an atom). Although he had
no supporting evidence, Rutherford suggested
that the electrons surrounded the positively
charged nucleus like planets around the sun.
He could not explain, however, what kept the
electrons in motion around the nucleus.
Figure 3-7 Page 73
«Num»
3-2-3 List the properties of protons, neutrons, and electrons.
Composition of the Atomic Nucleus
Except for the nucleus of hydrogen atom, all atomic nuclei are made of two kinds
of particles, protons and neutrons.
Protons: A particle with a positive charge equal in magnitude to the negative
charge of the electron and a mass of 1.673 × 10−27 kg, which is 1836 times greater
than the mass of an electron
Neutron and nuclear forces
Neutrons have no charge; the mass of a neutron is 1.675 × 10−27 kg, slightly
larger than that of a proton.
Important things about atoms
• Atoms are electronically neutral because they contain equal numbers of protons
and electrons.
• The number of protons in an atoms nucleus determines the identity of the atom.
–
• # of protons = atomic number (Also) = # of electrons (e )
Forces in the Nucleus: There are two Nuclear forces: a short-range very strong
attraction between proton-neutron, proton-proton, and neutron-neutron that hold
the nuclear particles together; and a less strong shorter ranged force that holds
the parts of proton and neutrons together
It is when these forces are released that we get “Atomic Power”
Atomic Shape and Size
• It is convenient to think of the region occupied by the electrons as an
electron cloud – a spherical shaped cloud of negative charge.
• The radius of an atom is the distance from the center of the nucleus to the
outer portion of this electron cloud.
• Unit used for measuring the atom is the picometer.
• The abbreviation for the picometer is pm
Home Work:
Section Review: p. 74. # 1–5.
Chapter Review: p. 87. # 3–6.