Notes 4-1

Intro to AtomsNotes 4-1Whats an atom?Smallest particle of an element

Atomic TheoriesThere have been many theories of proposed models of the atom.Some were completely wrong!And some were partially correct or close to correct.We will look at some of the most known theories and models.Daltons Atomic TheoryDalton thought that atoms were like smooth, hard balls that could not be broken into smaller pieces.He was credited with coming up with the earliest model of the atomMost of this theory is still accepted today

4#2Thomsons ModelThomson suggested that atoms had negatively charged electrons embedded in a positive sphere.This is the first model that included smaller particles of the atom His model is similar to blueberries in a muffin

5Rutherfords ModelDeveloped the first model that included a nucleusSimilar to a peach with a pit, the pit = the heavy mass of protons in the nucleus, the fruit = the empty space made up of electrons

Rutherfords Gold Foil ExperimentTurn to page 126-127 in your text bookLets read about Rutherfords model/ experimentWho wants to read first?Everyone else, follow along.Rutherfords Gold Foil ExperimentRutherford was surprised that a few particles were deflected strongly. This led him to propose an atomic model with a positively charged nucleus.

8#5Bohrs ModelSaid electrons could only have a certain amount of energy, so they must move in certain orbitsSimilar to planets orbiting the sun (sun = nucleus), or the rings/layers of an onion

Cloud ModelSays Bohrs model is wrong, electrons do not orbit the nucleus like planets do the sunInstead, electrons can be found anywhere in a cloud-like region around the nucleus, rapidly orbiting in every direction

Later Atomic ModelsThrough the first part of the twentieth century, atomic models continued to change.

11Modern Atomic ModelAccepted since the 1930sAt the center of the atom is a massive nucleus, containing positively charged protons and neutrally charged neutrons (no charge)Surrounding the nucleus is a cloud-like region of moving negatively charged electrons# protons in atom = # electrons in atom# neutrons can changeParticles in an AtomAn atom is composed of positively charged protons, neutral neutrons, and negatively charged electrons. Protons and neutrons are about equal in mass and are measured in atomic mass units (amu). An electron has about 1/2,000 the mass of a proton or neutron.

13#18Atomic NumberEvery atom of a given element has the same number of protons in its nucleusThis is known as the elements atomic numberFor example, Heliums atomic number is 2, so every atom of helium has 2 protons in its nucleus. If the element has more than 2 protons, it is NOT helium!Carbons atomic number is 6. How many protons does an atom of carbon have?Atomic NumberThe atomic number is usually found above the letter on the periodic tableLook at helium belowThe atomic number tells the number of protons in the nucleus.

#715Isotopes: Atoms of elements have same number of protons, but different numbers of neutronsAtoms of all isotopes of carbon contain six protons and six electrons, but they differ in their number of neutrons. Carbon-12 is the most common isotope.

16Mass numberAn isotope is identified by its mass numberMass number = #protons + #neutronsExample: Carbon-12, the mass number is 12. We know carbon has 6 protons by looking at its atomic number on the periodic table. How many neutrons does carbon-12 have?6How do you know?Because if the mass number is 12, 12 - 6protons = 6neutronsProblem:The atomic number of nitrogen is 7. How many protons, neutrons, and electrons make up Nitrogen-15?Protons = 7Neutrons = 8Electrons = 7