Aim: What is the basic structure of an atom?
Objectives: Student will be able to:
Model the atom and explain how theories regarding the atom have changed throughout history by taking notes, watching videos, drawing models, and analyzing models and creating a concept map of the atom.
Do Now/Motivation: Draw what you think an atom looks like. 3min
Dalton’s Atomic Theory
An atom is a complex unit of particles which vary. Like the cell is the basic unit for life, the atom is considered the basic unit of matter. One thing that everyone should know is that the modern structure of the atom was based upon the work of many scientists over a period of time.
One of those scientists was Dalton. He stated four basic ideas about atoms that were suspected from previous experiments.
Dalton did many experiments that provided evidence for the existence of atoms. For example:
He investigated pressure and other properties of gases, from which he inferred that gases must consist of tiny, individual particles that are in constant, random motion.
He researched the properties of compounds, which are substances that consist of more than one element. He showed that a given compound is always comprised of the same elements in the same whole-number ratio and that different compounds consist of different elements or ratios. This can happen, Dalton reasoned, only if elements are made of separate, discrete particles that cannot be subdivided.
He concluded that:
1. All elements are composed of atoms (Greek, “atomos” – indivisible). Are they really indestructible?
2. All atoms of the same element are exactly alike; they have the same mass. (Or do they?)
3. Atoms of different elements are different.
4. Compounds are formed by the joining of atoms in different ratios.
Rutherford’s Discovery of the Nucleus
The structure of the atom was the result of many experiments. One very important discovery was made by Ernest Rutherford (1871 –1937, Nobel Prize 1908, also worked with British to detect submarines by hydrophone and helped to design the Geiger counter) and his Gold Foil Experiment.
This Video Here describes the experiment (shot alpha particles and most went through the foil).
How did the alpha particles that have mass get through the solid foil?
The experimental setup for Rutherford’s gold foil experiment:
A radioactive element that emitted alpha particles was directed toward a thin sheet of gold foil that was surrounded by a screen which would allow detection of the deflected particles.
Rutherford found that a small percentage of alpha particles were deflected at large angles, which could be explained by an atom with a very small, dense, positively-charged nucleus at its center
The conclusions of this experiment were:
1. The atom is mainly empty space.
2. Size of the nucleus is small compared to the rest of the atom.
3. Mass of an atom is almost entirely concentrated in the nucleus. Therefore, the nucleus is dense.
4. Nucleus has a positive charge. Why?
Niels Bohr was a Danish physicist. He proposed a model of the atom that is similar to the model of the solar system. The electrons go around the nucleus like planets orbit around the sun.The Bohr model is outdated, but it depicts the three basic subatomic particles in a comprehensible way. Electron clouds are more accurate representations of where electrons are found. Electrons are negatively charged and are located in the orbits or shells surrounding the nucleus. Protons are positively charged and are located in the nucleus. The neutrons have no charge and are located in the nucleus. The mass of an atom is concentrated in the nucleus.
How can a nucleus remain together if there are protons repelling each other?
Atomic Structure and the Periodic Table
The atomic number of the atom tells us the number of protons in the nucleus. The mass number tells us the number of protons and neutrons in the nucleus of the atom. In a neutral atom the number of protons is equal to the number of electrons
Mass # = # of protons + # of neutrons
Atomic # = # of protons = # of electrons (neutral atoms) or also the nuclear charge. Atomic
number can be found on the Periodic Table.
Find the number of protons, electrons and neutrons in the atom represented by the diagram below.
Questions 7 min
1. The “gold-foil” experiment had the following results: ____________ of the
alpha particles were deflected, ______________ of the alpha particles
were reflected, and _____________ of the alpha particles passed through
the gold foil.
2. Match the word or phrase with the correct definition
A. Nucleus B. Electron C. Proton D. Neutron E. John Dalton F. Ernest Rutherford G. Niels Bohr H. Orbits I. Mass number
1. _____Uncharged particle in an atom
2. _____ He proposed that electrons orbited the nucleus like the path of planets.
3. _____ Gives the mass of the total number of protons and neutrons in an element.
4. _____ He developed an elaborate experiment that showed that the majority of an atom is empty space, with most of its mass concentrated in a tiny nucleus.
5. _____ He first suggested the presence of atoms and named them atomos (“indivisible”).
6. _____ Balances the positive charge of the nucleus
7. _____He proposed that electrons travel in unpredictable paths around the nucleus.
8. _____ Gives the nucleus its electrical charge
9. _____ is a path where electrons move around
Class Activity: 10 min
Using each of the following words, construct a concept map that links the words using verbs to help explain your understanding of the atom
Below is an exemplar from one of your peers.