Atomic Physics What is the ATOM???

MATTER = ATOM All matter is composed of atoms. Atoms are the smallest part of an element that keeps that element’s properties. If an atom were the size of a football field, the nucleus would be the size of a marble and nearly all of the mass of the atom is in the nucleus. The space around the nucleus and the electron have very, very little mass. This means that matter is mostly empty!

The Atom and the nucleus The nucleus contains most of the mass of the atom (protons + neutrons). The rest is mostly empty space!

Democritus The Greek Understanding Democritus (BC 400) and the “Atomists” debate the “four” elements (fire, water, earth, and air). Democritus concluded that matter could not be divided into smaller and smaller pieces forever. Eventually, the smallest piece of matter would be found. He used the word “atomos” to describe the smallest possible piece of matter.

The Dalton Model All elements are composed of indivisible particles. Atoms of the same element are exactly alike (isotopes not known yet). Atoms of different elements are different. Compounds are formed by joining atoms of two or more elements.

Thompson’s “Plum Pudding” Model Discovered negatively charged particles coming off a gas known to be neutral. Called these “early” electrons, corpuscles. Predicted that there must be positive particles (protons) present in equal number to balance negative charges. Proposed atoms were made of pudding like positive charges, with negative electrons inside scattered like plums.

Robert A. Millikan Experiments showed that the mass of an electron is smaller than the simplest type of hydrogen atom. Mass of electron = x kg Electron has a negative charge.

Based on what was learned about electrons, Two other inferences were made about atomic structure: 1. Because atoms are electrically neutral, they must contain a positive charge to balance the negative electrons. 2. Because electrons have so much less mass than atoms, atoms must contain other particles that account for most of their mass.

Rutherford’s Gold Foil Experiment

Rutherford’s Model Fired tiny positive particles at gold foil…most went through without change of course. What did this prove? Some particles bounced back as if they hit something solid. Reasoned that they were repelled by similar charged particles. Discovered and proved the existence of the proton. Reasoned that atoms are mostly “empty” space, but with a small dense center with positive charges.

These deflections were not consistent with Thomson's model. Rutherford was forced to discard the Plum Pudding model. He reasoned that the only way the alpha particles (positively charged) could be deflected backwards was if most of the mass in an atom was concentrated in a nucleus. He thus developed the “Planetary model” of the atom which put all the protons in the nucleus and the electrons orbited around the nucleus like planets around the sun.

ATOMIC NUCLEUS Atoms are electrically neutral because they contain equal numbers of protons and electrons. Protons have a positive charge. Mass of Proton = x kg 1 amu = 1 atomic mass unit The number of protons present determines the identity of the atom. Neutrons are electrically neutral. Mass of Neutron = x kg 1 amu

Mosely Determined the number of PROTONS in elements

Nuclear Forces You would expect a nucleus with like charged protons to repel one another and be very unstable. However, when two protons are extremely close to each other, there is a strong attraction between them. The short-range proton-neutron, proton- proton, and neutron-neutron forces hold the nuclear particles together.

The Bohr Model Amended Rutherford's “planetary” model in Brought the model in line with the regular patterns (spectral series) of light emitted by real hydrogen atoms. Bohr’s model worked well explaining simple atoms like hydrogen, but didn’t work for more complex atoms.

Orbiting electrons of circular orbits had discrete radii (specific distances from the nucleus). They each had discrete wavelengths in the emission spectrum. Light, he proposed, radiated from atoms only when an electron made a transition from an outer orbit to one closer to the nucleus.

Electrons travel in defined circular orbits around the nucleus. The orbits are labeled by an integer, the quantum number n. Electrons can jump from one orbit to another by emitting or absorbing energy. The inset shows an electron jumping from orbit n=3 to orbit n=2, emitting a photon of red light with an energy of 1.89 eV.

The Modern Atomic Model Based on wave mechanics, this model proposes that electrons have NO definite path in an atom (unlike Bohr’s model). The probable location of an electron is based on how much energy it has. The more energy an electron has, the farther from the nucleus. The small, positively charged nucleus is surrounded by a large space in which there are enough electrons to make the atom neutral.

Based on Energy Levels Draw this Picture

EXTRA CREDIT Where does the word “electron” come from and what does it mean? Besides being a profound influence in the world of science, Bohr was a talented athlete. In what sport did he excel? Who did Democritus study under? What does atomism mean?

Works Cited atomic-theory/rutherford-model.html atomic-theory/rutherford-model.html requency.html requency.html rpoint/chap5/sld001.htm rpoint/chap5/sld001.htm htm 2.htm d/ d/