ATOMIC STRUCTURE

In 1808 John Dalton started thinking . . . He said that matter was made up of tiny particles and called them ATOMS (thanks Democritus) He said a few things that turned out to be wrong. . . like that all atoms of an element were identical (they’re not) and that atoms couldn’t be divided (they can, that’s how we make atomic bombs) But, he got some things right. . . he said that atoms can combine to make compounds (NaCl) and that atoms can separate, combine & rearrange in chemical reactions

90 years later, J.J. Thomson got a new toy. . . … a cathode ray tube and he found out that beams bent toward a positive plate. We know that opposite charges attract (remember that from playing with magnets) . . . so the beam must be negative! Thomson thinks that atoms must look like Plum Pudding (WHAT?!). The Plum Pudding model

Then Ernest Rutherford comes along. . . He fired Helium at a really thin piece of gold. Most of the Helium passed straight through, but every now and then, a little bit would bounce off to the side of shoot straight back. Rutherford used this evidence to say atoms have a nucleus and it was positive (opposite of Thomson’s electrons). He also concluded that atoms are made of most empty space because so many passed directly through the gold foil.

Niels Bohr was one of Rutherford’s students. Bohr figured out that electrons can get excited. He said that they had to orbit the nucleus in energy levels, and that each level could only hold a set number of electrons.

Parts of an atom: PROTONS (p+): POSITIVE CHARGE HAVE MASS IN THE NUCLEUS NEUTRONS (n0): NO CHARGE (NEUTRAL) HAVE MASS IN THE NUCLEUS ELECTRONS (e-): NEGATIVE CHARGE HARDLY ANY MASS OUTSIDE THE NUCLEUS

He 2 4 This would be called Helium - 4 HOW WE WRITE IT…. Atomic number: the number of protons AND the electrons in an atom (they are the same) 4 Atomic mass: the number of protons PLUS the number of neutrons in an atom This would be called Helium - 4

Use your periodic table 1.) How many protons are in Carbon? 2.) How many electrons are in Carbon? 3.) What is the mass of Carbon? 4.) How many neutrons are in Carbon?

Isotopes are atoms of the same element having different masses because they have a different number of neutrons. ELEMENT PROTONS NEUTRONS ELECTRONS MASS Aluminum - 31 Nitrogen - 16 Sodium - 24 The atomic mass on the periodic table is the AVERAGE MASS of the naturally occurring isotopes of that element.

When the electrons get excited, they more up an energy level When the electrons get excited, they more up an energy level. When they fall back down (to their “ground state”) they release energy in the form of a photon (a little packet of energy). Each energy level gives off its own light and makes a line spectrum!