Atoms and the Periodic Table

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Scientific Models of the Atom

  • Atoms thought to be tiny spheres, could not be devided into simpler particles
  • 1898-Thomson discovered electrons
  • Atom is neutral, has no charge
  • Thomson thought atoms contained negative electrons, surrounded by sea of positive charge-Plum pudding model
  • Geiger and Marsden-bombarded thin sheet of gold with alpha particles
  • Most of positively charged particles passed through atoms,tiny number bounced back to source
  • Rutherford looked at results, concluded positive charge in atom must be concentrated in small area-area named Nucleus, replaced plum pudding model
  • Bohr-electrons must orbit nucleus at speicific distances, otherwise would spiral inwards
  • 20 years after nucleus became accepted scientific idea, James Chadwick provided evidence of neutrons within the nucleus
  • Later experiments led to idea that positive charge of nucleus could be subdivided into whole number of smaller particles, each with same amount of poistiive charge
  • These particles are protons
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Subatomic Particles

Atoms very small, have atomic radius of 1 x 10-10m

  • Contain three types of subatomic particle: Proton- Relative Mass of 1 and Relative Charge of +1; Neutron- Relative Mass of 1 and Relative Charge of 0; and Electron- Relative Mass of very small and Relative Charge of -1.
  • Most of mass of atom in nucleus
  • Atoms have no overally charge- contain equal number of protons and electrons
  • All atoms of particular element have same number of protons
  • Atoms of different elements have different number of protons
  • Atomic Number- Number of protons
  • Mass Number- Sum of protons and neutrons in atom
  • Elements arranged in order of increasing atomic number in modern periodic table.
  • number of protons = number of electrons

number of neutrons = mass number - atomic number

Question- How many protons, electrons and neutrons are there in Sodium?Answer- Sodium is Na 23, 11.  11 Protons; 11 Electrons; 12 Neutrons (23-11)

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Isotopes and Ions

  • Isotopes of element have same number of protons, different number of neutrons.
  • Have same atomic number but different mass number
  • Eg: Chlorine has two isotopes-

 Cl-35, 17. 17 Protons, 17 Electrons and 18 Neutrons (35-17).

 Cl-37, 17. 17 Protons, 17 Electrons and 20 Neutrons (37-17)

  • Atoms can gain or lose electrons to become ions:
  • Metal atoms can lose electrons to form positive ions
  • Non-metal atoms gain electrons to form negative ions
  • Ions made up of a metal and non metal
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Electron Configuration

  • Electrons in an atom occupy lowest available shell or energy level
  • Electron configuration of atom shows how the electrons can be arranged around the nucleus in shells

For first 20 elements:

  • First shell can only hold a maximum of two electrons
  • The next two shells can each hold a maximum of eight electrons
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Quick Test-Atoms and the Periodic Table

1. Describe the plum-pudding model of the atom

2. An atom of potassium has an atomic number of 19 and a mass number of 39. State the number of protons, neutrons and electrons in this atom.

3. An ion of potassium-39 has a 1+ charge. State the number of protons, neutrons and electrons in this ion.

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