100364
Developing Hypotheses About Matter
- The Irish scientist Robert Boyle (1627-1691) measured relationships between volume and pressure of gases. From his experiments, he concluded that gases are made up of tiny particles that group together to make different
substances
- The French scientist Antoine Lavoisier (1743-1794) measured the masses of the substances that reacted together and the substances produced in many chemical reactions. He discovered that mass is neither produced nor
lost during a chemical reaction. He called this the Law of Conservation of Mass.
- During this early period, many scientists were investigating matter, and many models of atoms were proposed. However, four classic models are always discussed because they are examples of the scientific process at its best.
- English chemist and physicist John Dalton (1766-1844) imagined that all atoms were like small
spheres, but that they could have different properties. They varied in size, mass, or color. Dalton
used the following model to explain matter:
- - All matter is made of small, indivisible particles called atoms
- - All the atoms of an element are identical in properties such as size and mass
- - Atoms of different elements have different properties
- - Atoms of different elements can combine in specific fixed ratios to form new substances
- The English physicist Joseph John Thomson (1856-1940) discovered the Electron. In 1890. Thomson
was experimenting with beams of particles produced in a vacuum tube. His experiments showed that
the beam was made of negative charges. By testing many different elements, he showed that they all
produce the same type of beam. This suggested that atoms of different elements contained smaller
particles that were identical.
- J. J. Thomsons model stated that all atoms are made of smaller subatomic particles put together in
different combinations to make different elements. He suggested that an atom was a sphere of positive
charge in which negative particles are embedded. The negatively charged particles were called Electrons.
- The Japanese scientist H. Nagaoka proposed a different model in 1904. He placed the electrons on the
outside of the sphere. There they traveled around the central sphere in a pattern like the rings around the
planet Saturn.
- Ernest Rutherford (1871-1937) did research at McGill University in Montreal, where he performed
an experiment that led to the discovery of the Nucleus of the Atom.
- He had a radioactive material encased in lead with one small opening. This material released
positively charged particles which he aimed at a thin sheet of gold foil. Using Thomsons model of
the atom, he predicted that all the high-speed particles would pass right through the foil. The gold
atoms would either have no effect on the particles or would deflect them slightly. This is exactly
what happened to most of the particles. However, a few - about 1 in 10,000 - bounced back, and a
few others were sharply deflected. This was entirely unexpected. Rutherford knew that even though
the unexpected results happened rarely, they still meant that Thomsons model was wrong.
- Rutherford developed her own model. He suggested that an atom is mainly empty space through
which the positive particles could pass, but that each atom had a tiny, positively charged core.
This dense core of positive charge was so strong that it was causing some of the positively charged
high-speed particles to bounce back. Electrons move through the rest of the atoms volume.
Rutherford called the small dense center the Nucleus. He calculated the size of the nucleus to be
about 1 ∕ 10,000 of the size of the atom. Rutherford received the Nobel Prize in chemistry in 1908
for his work on radioactivity.
- The Danish physicist Neils Bohr (1885-1962) proposed that electrons surrounded the nucleus in specific
energy levels.
- He found evidence for these energy levels by examining the light released by hydrogen atoms when
they are made to glow in a tube. The individual bands of light correspond to gaps between the energy
levels of the electrons. When electrons fall from higher energy levels to lower energy levels, they
release a particular color of light. From these colors, it is possible to identify the energy levels in atoms
of all the elements in the periodic table.
- Bohrs experiments also partly explained why the negatively charged electrons do not merge with the
positively charged nucleus. The reason is that electrons cannot fall below the lowest energy level.
Thus an electron cannot fall into a nucleus under normal circumstances.
- The Quantum Mechanical Model of the Atoms
- Todays model of the atom is based on a theory called Quantum Mechanics. This abstract model is difficult to visualize. It uses mathematical probability to describe how electrons exist in atoms. Each electron can be
thought of as a "cloud" of negative charge, instead of a tiny negative particle. Rather than thinking of the electron as a small particle moving quickly through a space, as Bohr originally did, the whole idea of electron
movement is abandoned. Instead, electrons "occupy" the whole space all at once at different energy levels.
- The electron cloud surrounds a nucleus containing two types of particles called Nucleons: Protons and Neutrons. Protons have a positive electrical charge, and neutrons have no electrical charge. This model of the
atom could change in the future as scientists learn more about the atom and subatomic particles.
- The electron cloud surrounds a nucleus containing two types of particles called Nucleons: Protons and Neutrons. Protons have a positive electrical charge, and neutrons have no electrical charge. This model of the
atom could change in the future as scientists learn more about the atom and subatomic particles.
- Todays model of the atom is based on a theory called Quantum Mechanics. This abstract model is difficult to visualize. It uses mathematical probability to describe how electrons exist in atoms. Each electron can be
thought of as a "cloud" of negative charge, instead of a tiny negative particle. Rather than thinking of the electron as a small particle moving quickly through a space, as Bohr originally did, the whole idea of electron
movement is abandoned. Instead, electrons "occupy" the whole space all at once at different energy levels.
- The Quantum Mechanical Model of the Atoms
- Bohrs experiments also partly explained why the negatively charged electrons do not merge with the
positively charged nucleus. The reason is that electrons cannot fall below the lowest energy level.
Thus an electron cannot fall into a nucleus under normal circumstances.
- He found evidence for these energy levels by examining the light released by hydrogen atoms when
they are made to glow in a tube. The individual bands of light correspond to gaps between the energy
levels of the electrons. When electrons fall from higher energy levels to lower energy levels, they
release a particular color of light. From these colors, it is possible to identify the energy levels in atoms
of all the elements in the periodic table.
- The Danish physicist Neils Bohr (1885-1962) proposed that electrons surrounded the nucleus in specific
energy levels.
- Rutherford developed her own model. He suggested that an atom is mainly empty space through
which the positive particles could pass, but that each atom had a tiny, positively charged core.
This dense core of positive charge was so strong that it was causing some of the positively charged
high-speed particles to bounce back. Electrons move through the rest of the atoms volume.
Rutherford called the small dense center the Nucleus. He calculated the size of the nucleus to be
about 1 ∕ 10,000 of the size of the atom. Rutherford received the Nobel Prize in chemistry in 1908
for his work on radioactivity.
- He had a radioactive material encased in lead with one small opening. This material released
positively charged particles which he aimed at a thin sheet of gold foil. Using Thomsons model of
the atom, he predicted that all the high-speed particles would pass right through the foil. The gold
atoms would either have no effect on the particles or would deflect them slightly. This is exactly
what happened to most of the particles. However, a few - about 1 in 10,000 - bounced back, and a
few others were sharply deflected. This was entirely unexpected. Rutherford knew that even though
the unexpected results happened rarely, they still meant that Thomsons model was wrong.
- Ernest Rutherford (1871-1937) did research at McGill University in Montreal, where he performed
an experiment that led to the discovery of the Nucleus of the Atom.
- The Japanese scientist H. Nagaoka proposed a different model in 1904. He placed the electrons on the
outside of the sphere. There they traveled around the central sphere in a pattern like the rings around the
planet Saturn.
- J. J. Thomsons model stated that all atoms are made of smaller subatomic particles put together in
different combinations to make different elements. He suggested that an atom was a sphere of positive
charge in which negative particles are embedded. The negatively charged particles were called Electrons.
- The English physicist Joseph John Thomson (1856-1940) discovered the Electron. In 1890. Thomson
was experimenting with beams of particles produced in a vacuum tube. His experiments showed that
the beam was made of negative charges. By testing many different elements, he showed that they all
produce the same type of beam. This suggested that atoms of different elements contained smaller
particles that were identical.
- - Atoms of different elements can combine in specific fixed ratios to form new substances
- - Atoms of different elements have different properties
- - All the atoms of an element are identical in properties such as size and mass
- - All matter is made of small, indivisible particles called atoms
- English chemist and physicist John Dalton (1766-1844) imagined that all atoms were like small
spheres, but that they could have different properties. They varied in size, mass, or color. Dalton
used the following model to explain matter:
- During this early period, many scientists were investigating matter, and many models of atoms were proposed. However, four classic models are always discussed because they are examples of the scientific process at its best.
- The French scientist Antoine Lavoisier (1743-1794) measured the masses of the substances that reacted together and the substances produced in many chemical reactions. He discovered that mass is neither produced nor
lost during a chemical reaction. He called this the Law of Conservation of Mass.
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