Atom - the smallest part of an element that retains the chemical properties of that element.
SECTION 3-2: THE STRUCTURE OF THE ATOM
Subatomic Particles:
Nucleus: positively
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 charged protons and neutral particles neutrons. Give the atom its mass
2. Electrons: Surrounds the nucleus (electron cloud) and are negatively charged
Protons, neutrons, and electrons are often referred to as subatomic particles .
Electrons - negatively charged surround the
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 nucleus – Give the atom its size
J. J. Thomson in 1897 realized that
• 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
−31 kg, or 1/1837 the mass of the simplest type of hydrogen atom.
Subatomic Particles
Exist; discovered that there is a charged section of an atom
It takes about 2000 electrons to equal the mass of 1 proton or 1 neutron.
Electrons are negatively charged
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
The nucleolus gives the atom its mass; the electrons give it its volume
Rutherford believed 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
Figure 3-7 Page 73 occasional 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. that the electrons orbited the nucleus like planets around the sun

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 .
Hydrogen is the only
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 larger than that of a proton.
Important
• things about atoms
−27
kg , slightly
Atoms are electronically neutral because they contain equal numbers of protons element that has an isotope without a neutron
Proton

Positive
Neutrons

Neutral
Electrons

Negative
Masses of the proton and neutron are very close to equal
•
• and electrons.
The number of protons in an atoms nucleus determines the
# of protons = atomic number (Also) = # of electrons (e
–
) identity of the atom.
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
See vocabulary for more information 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 ” about Nuclear Forces
•
•
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
Electrons are “Clouds of Charge”
The atomic radius is the distance from the nucleus to the edge of the electron cloud
1 pm = 1 x10
– 12
m
(1 000 000 000 000 pm = 1 m)
Home Work:
Section Review: p. 74. # 1–5.
Chapter Review: p. 87. # 3–6.