C H E M I S T R Y 1 1 2

 An atom is the smallest particle of matter that retains its identity in a chemical reaction
 Democritus was an early Greek philosopher who suggested the existence of atoms
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He believed they were indivisible and indestructible

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From experimentation, Dalton was able to devise a set of ideas supporting the idea of the atom
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Elements are composed of tiny indivisible particles called atoms
Atoms of the same elements are identical and one element’s atom differ from another
Atoms of different elements can come together to form compounds
Atoms of one element cannot change into atoms of another

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We now know that the atom is divisible
It is composed of subatomic particles
Electrons are negatively charged particles and they are found outside the nucleus in regions called orbitals
Protons are positively charged and found in the nucleus of an atom with neutrons, which have no charge
 There are even smaller particles but we do not study them in introductory chemistry

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The nucleus is found at the core of an atom
It is very dense and positively charged
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The region around the nucleus is large and mostly empty space
 Although all three subatomic particles are small, the electron is 1/1840 the size of a proton/neutron
 Table 4.1, page 108

 Elements are different from one another because they contain different numbers of protons
 The atomic number = number of protons
 In atoms, the number of proton = number of electron
 Therefore, an atom is electrically neutral

 The mass number is the number of protons added to the number of neutrons
 The number of neutrons can vary from one atom to the next within one sample of an element
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Such atoms are called isotopes of an element and have differing mass numbers – they are still, however, chemically alike because their p+ and ecount are equivalent to one another

 There is a certain method used for naming and writing isotopes