Term
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Definition
How can we see inside something that is not visible to us? The early Greek
philosophers pondered this question, and it continues to challenge scientists 2000 years later.
Today, scientists use conceptual and physical models to study phenomena that are not easily observed, and they test those models through experiments. Scientific models predict the behaviour of systems. |
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| The First Scientific Atomic Theory |
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Definition
Dalton envisioned the atom, the smallest piece of any element, as a smooth solid sphere, without an electrical charge.
Dalton’s Theory
• All matter is made of atoms, which are particles that are too small to see.
• Each element has its own kind of atom, with its own particular mass.
• All atoms of any one element are identical.
• Compounds are created when atoms of different elements combine in a specific ratio.
• Atoms cannot be created, destroyed, or subdivided during chemical
changes. |
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Term
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Definition
Scientists now consider a molecule to be two or more non-metallic atoms combined—then smallest particle of most compounds.
Compounds that are formed from metals and
non-metals technically do not have molecules. |
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Term
| Revisions to Dalton’s Theory |
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Definition
• Matter must contain positive and negative charges.
• Opposite charges attract, and like charges repel.
• Atoms combine to form the particles of a compound because of the
electrical attraction between charged atoms. |
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Term
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Definition
Thomson’s “raisin-bun” model of an atom
In the late 1800s, the discovery of a particle that is many times smaller
than the smallest atom required another major change to the atomic theory.
This particle, the negatively-charged electron, was discovered by J.J. Thomson in 1898. |
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Term
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Definition
Rutherford’s “nuclear” model
of the atom
Rutherford reasoned that most of the mass of an atom, and all the positive charge, is concentrated in the centre. This location is called the nucleus, and it contains two types of particles: positively charged protons and uncharged neutrons. |
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Term
| Rutherford’s Revisions to the Atomic Theory: The Nucleus |
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Definition
• The nucleus contains all of the positive charge and most of the mass
of the atom.
• The nucleus contains positively charged protons and uncharged neutrons.
• Neutrons have the same mass as protons.
• The nucleus is very small, compared with the size of the atom.
• The electrons orbit the nucleus, like satellites around a planet.
• The mass of an electron is
1/1800 the mass of a proton.
• The size of the atom is determined by the size of the orbit of the electrons.
• There is only empty space between the electrons and the nucleus.
A well-established theory of electromagnetic waves predicted that Rutherford’s orbiting electrons should continuously emit energy, which they do not. Atomic theory required a further modification. |
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Term
| Thomson’s Revision to the Atomic Theory: Electrons |
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Definition
• Atoms contain electrons.
• The electrons have a negative charge and a very small mass.
• The rest of the atom has a positive charge.
• The electrons are embedded randomly in the positive part of the atom.
• Electrons can be removed from, or added to, atoms to create charged atoms.
The Theory Is Changed |
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