Ch 3 Section 2 Flashcards
Atom
Smallest particle of an element that retains the chemical properties of that element
All atoms consist of
2 regions
Nucleus
Very small region at the center of an atom
Nucleus is made up of at least one
Positively charged particle called a proton and usually one or more neutral particles called neutrons
Surrounding the nucleus is a region occupied by
Negatively charged particles called electrons
Electrons region is very
Large compared with nucleus
Subatomic particles
Proton
Neutron
Election
First discovery of a subatomic particle resulted from investigations into
Relationship between electricity and matter
Many experiments performed in which electric current was passed
Through various gases at low pressures
As scientific advances allowed a deeper exploration of matter it became clear that atoms are actually composed of several basic types of
Smaller particles and that the number and arrangement of these particles determine that atoms chemical properties
Those experiments carried out in
Cathode ray tubes
When current was passed through a cathode ray tube surface of tube
Directly opposite the cathode glowed
Hypothesized that glow was caused by a
Stream of particles, a cathode ray
Ray traveled from cathode to
Anode when current was passed through the tube
Cathode Rays were deflected by a
Magnetic field in the same manner as a wire carrying electric current which is know to have negative charge
Rays were deflected
Away from a negatively charged object
Observations led to hypothesis that particles that commode cathode Rays are
Negatively charged
Cathode ray hypothesis supported by
JJ thomsons experiments
Thomson was able to measure the ratio of the charge of
Cathode Ray particles to their mass and found that ratio was always the same
Thomson concluded that all cathode Rays are composed of
Identical negatively charged particles called electrons
Cathode Rays have identical properties regardless of the
Element used to produce them
Concluded that electrons are present in
Atoms of all elements
Cathode ray experiments provided evidence that atoms are
Divisible and that one of an atoms basic constituents is the electron
Thomsons experiment revealed that the electron has a very large
Charge to mass ratio
Robert a Millikan measured
The charge of the electron
Scientists used the charge of the electron and the charge to mass ratio of the electron to determine that an electrons mass is about
1/2000 the mass of the simplest type of hydrogen atom
Simplest type of hydrogen atom
Smallest atom known
Electron actually has a mass of
1/1837 the mass of the simplest hydrogen atom
Because atoms are electrically neutral they must contain a
Positive charge to balance the negative electrons
Because electrons have so much less mass than atoms atoms must contain
Other particles that account for most of their mass
Thomson proposed the plum pudding model-> negative electrons were spread evenly throughout the
Positive charge of the rest of the atom
Ernest Rutherford, Hans greiger, and Ernest mars den bombarded a thin piece of
Gold foil with fast moving alpha particles
Alpha particles are positively charged particles with about
4x the mass of a hydrogen atom
Greiger and marsden assumed that mass and charge were
Uniformly districts throughout atoms of gold foil
Expected alpha particles to pass through with only a slight
Deflection and for vast majority of particles that was the case
When they checked for the possibility of wide angle deflections they found that approx
1 in 8000 alpha particles were deflected back toward the source
Rutherford reasoned that deflected alpha particles must have experienced some
Powerful force within the atom
Rutherford concluded that the force must be caused by a very densely packed
Bundle of matter with a positive electric charge, the nucleus
Rutherford discovered that volume of nucleus was very
Small compared with total volume of an atom
Niels Bohr proposed a model in which electrons
Surrounded the positively charged nucleus
Except for the nucleus of the simplest type of hydrogen atom all atomic nuclei are made of
Protons and neutrons
A proton has a positive charge equal in
Magnitude to the negative charge of an electron
Atoms are electrically neutral because they contain
Equal numbers of protons and electrons
Neutrons are
Electrically neutral
Simplest hydrogen atom consists of a single
Proton nucleus with a single electron moving about it
A proton has a mass 1836 times greater than
That of an electron
The nuclei of atoms of different elements differ in their number of
Protons and thus in the amount of positive charge they possess
Number of protons determines
Atoms identity
Particles other than electrons protons and neutrons have little
Effect on the chemical properties of matter
Generally particles that have the same electric charge
Repel one another
When 2 protons are extremely close to each other there is a strong
Attraction between them
As many as —- protons can exist
83 protons can exist close together to help form a stable nucleus
Nuclear forces: short Range
Proton-neutron, proton-proton, and neutron-neutron forces that hold nuclear particles together
Electron cloud: region occupied by
Electrons, cloud of negative charge
Radius of an atom is the distance from
Center of the nucleus to the outer portion of the electron cloud
Because atomic radii are so small they are expressed using
The pico meter (pm)
Atomic radio range from about
49 to 270 pm
Nuclei of atoms have radiu about
.001 pm and incredibly high densities (2 x 10^8 metric tons per cm^3)
