6 1 the atom
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6.1 The Atom. Atomic Structure Nuclear Structure. 6.1.1. Describe a model of the atom that features a small nucleus surrounded by electrons. This is a simplified view of the atom known as the Rutherford model. History. Atom – the term atomos comes from Greek and means “indivisable”

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6.1 The Atom

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6.1 The Atom

Atomic Structure

Nuclear Structure


6.1.1

  • Describe a model of the atom that features a small nucleus surrounded by electrons.

  • This is a simplified view of the atom known as the Rutherford model


History

  • Atom – the term atomos comes from Greek and means “indivisable”

  • Originally thought to be the smallest parts of matter

  • The spheres to the right are the top layer of a gold sheet

  • Each sphere is one atom


History

  • 1803 – John Dalton proposed that atoms reacted with each other

  • 1897 – JJ Thompson discovered the electron with the invention of the cathod ray tube

  • Thompson's model was the plumb pudding version, where negative "corpuscles" were distributed though out positive “pudding”


History

  • The Rutherford model had a positively charged nucleus, with the negatively charge electron orbiting the nucleus

  • In 1913 Niels Bohr added Quantum energy levels saying the electrons could only exist in certain orbits with definite energy

  • These models didn’t adequately explain the atom so in 1926 Schrödinger, citing DeBroglie’s wave model for mass, created his Model using the uncertainty principle


The Schrödinger Model

  • The nucleus contain the nucleons (Protons and Neutrons) with are made up of quarks

  • The electrons exist in electron clouds around the nucleus

  • All the components of the atom are made from elementary particles

  • Try this Atom Builder


6.1.2

  • Outline the evidence that supports a nuclear model of the atom.

    • A qualitative explanation of the Geiger–Marsden experiment and its results is all that is required.

  • Also know as the Gold foil experiment, It was performed by Hans Geiger and Ernest Marsden in 1909 under the supervision of Rutherford


Evidence of the Nuclear model

  • The expected path for the “Plum pudding model” would be for an Alpha particle (He nucleus) to travel straight through or get absorbed

  • In reality some a-particles where reflected back

  • Rutherford said of the results "It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you".

  • Click here for a picture


6.1.3

  • Outline evidence for the existence of atomic energy levels.

    • Students should be familiar with emission and absorption spectra, but the details of atomic models are not required.

  • Take a trip back to the Bohr model


If I only had an orbit

  • Imagine you are at the science museum and you see that yellow funnel thing, the spiral wishing well.

  • You know, the one you drop the penny in and watch it swirl around until in goes into the hole at the center

  • That is the fundamental problem with the Rutherford model of the atom, when the electron give off energy the loose momentum and thus should spiral into the nucleus


Bohr’s Solution

  • Bohr solved this by “allowing” the electrons to have discrete energies instead of continuous energies

  • The electron loses energy when it “drops” to a lower energy state

  • The emitted energy is the difference between the two states

  • This is evident in the emission and absorption spectra


Energy Levels

  • When an electron goes down in energy, the excess energy is released as a photon

  • In order to go up in energy, a photon must be absorbed

  • The energy associated with the transition is E = hf

  • h = 6.63 x 10-34 Js

Photon out

Photon in


Emission and absorption

  • Since certain wavelengths with be absorbed or emitted, a given atom will only produce or take in certain frequencies of light

  • Here are both spectrums for Hydrogen


6.1.4 Isotopes and neutrons

  • We already know there are protons and neutrons (nucleons) in the nucleus of the atom, but how do we know this?

  • The discovery of isotopes, and some very ingenious experiments brought the existence of the neutron to light


6.1.5 Explanation of terms

  • Nuclide - This name is given to a nucleus with a specific number of protons and neutrons

  • Here, A is the total number of nucleons, Z is the number of proton, and X is the atomic symbol

  • A nuclide is represented the following way:


Notation

  • This notation can be applied to proton, neutron, and electron as well

  • Isotopes are easy to identify using this notation

  • Common isotopes would be Carbon 14, Hydrogen 3, and Oxygen 18


Isotopes

Hydrogen

Deuterium

Tritium

  • An isotope will have the same number of protons and a different number of neutrons

  • Isotopes will have the same chemical properties, but their physical properties will be slightly different


6.1.6 Atomic mass & number

  • Z denotes the number of protons in a nucleus, also called the atomic number

  • A denotes the total number of nucleons for an atom, so (Z + N = A)

  • N is the number of neutrons

  • If the atomic mass number changes the element changes


6.1.7 More forces

  • Obviously something has to hold together the nucleus

  • We learn in 5.1.7 positive charges repel each other, yet the protons in the nucleus are “glued” together

  • This “glue” comes from the strong nuclear force, which works at very short distances (10-15 m or less)

  • The atomic radius is written as R = 1.2 x A1/3 x 10-15, where A is the mass number.


Weak nuclear force

  • Since Neutrons and protons are made up of quarks there must be a force holding the quarks together as well

  • This force is the Weak nuclear force

  • A neutron will decay into a proton, an electron, and an anti-neutrino

  • This is called Beta decay

  • Click here to see more


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