Atoms

What did Dalton do?

He studied the ratios in which elements combine in chemical reactions

Scanning Electron microscopes

can make atoms visible can move around atoms a beam of electrons is focused on the sample the ability to move individual atoms holds future promise for the creation of atomic-sized electronic devices, such as circuits and computer chips. A nanocar is a device made from individual atoms

In 1913, Niels Bohr (1885-1962), a young Danish physicist, developed a new atomic model.

he included evidence from newer experiments about how the energy of an atom changes when an atom absorbs or emits light. He based his model on the simplest atom, the hydrogen atom.

what is an atom?

the smallest particle of an element that retains its identity in a chemical reaction

A student of Rutherford's came up with a model to help explain the behavior of electrons in an atom that would account for the chemical property of electrons.

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Rutherford's atomic model is known as the nuclear atom theory

1) because most of the alpha particles showed no deflection, Rutherford hypothesized that most of the atom is empty space 2) because a few of the alpha particles showed large angles of deflection, he hypothesized that all the positive charge and almost all the mass must be concentrated in a small region of the atom. He called this small positively charged region the nucleus. Today we know that the nucleus is a tiny central core of an atom that is composed of protons and neutrons.

Daltons Atomic Theory

All elements are composed of tiny indivisible particles called atoms. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds. Chemical reactions occur when atoms are separated from each other, joined, or rearranged in a different combination. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.

In 1886 Eugen Goldstein (1850-1930) found evidence of a positively charged particle. Was this really 11 years before Thomson's discovery?

He observed a cathode ray tube and found rays traveling in the opposite direction to that of the cathode rays. He called these rays canal rays and concluded that they were composed of positively charged particles, called protons. Each proton has a mass about 1840 times greater than the mass of an electron.

Electrons JJ Thomson (1856-1940) discovered the electron.

In 1897, the English physicist JJ Thomson (1856-1940) discovered the electron. Nobel Prize in 1906, "in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases. Electrons are negatively charged subatomic particles. Thomson's experiments: He passed electric currents through gases at low pressure. He sealed the gases in glass tubes fitted at both ends with metal disks called electrodes. The electrodes were connected to a source of energy. One electrode, the anode became positively charged; the other electrode, the cathode became negatively charged. The result was a glowing beam, or cathode ray, that traveled from the cathode to the anode. Thomson found that a cathode ray is deflected by electrically charged metal plates. The cathode ray is attracted to positively charged metal plates. The cathode ray is repelled by negatively charged plates. Thomson hypothesized that a cathode ray is a stream of tiny negatively charged particles moving at high speed. Cathode rays are the electrons given off by atoms. Why did he make this hypothesis? Because like charges repel and opposite charges attract. Thomson called the particles corpuscles; later they were named electrons. To test his hypothesis that a cathode ray is a stream of tiny negatively charged particles moving at high speed he measured the ratio of an electron's charge to its mass. 1) The ratio was constant across several masses. 2) The charge-to-mass ratio of electrons was not dependent on a) the kind of gas in the cathode-ray tube or b) the type of metal used in the electrodes. Thomson concluded that electrons are a component of the atoms of all elements.

How can one describe the structure of the nuclear atom?

JJ Thomson believed that electrons were evenly distributed throughout an atom filled uniformly with positively charged material. The PLUM-Pudding Model. electrons were stuck into a lump of positive charge similar to raisins stuck in dough. One of JJ Thomson's former students proved that the plum pudding model of atomic structure was wrong.

Protons and Neutrons

Protons and Neutrons are what is left after an atom has given off its electrons in a cathode ray. 4 things to think about: 1) atoms have no net electric charge, they are electrically neutral. (this explains why we do not get shocked every time we touch something) 2) electric charges are carried by particles of matter. 3) Electric charges always exist in whole number multiples of a single basic unit, that is there are no fraction of charges. 4) When a given number of negatively charged particles combines with an equal number of positively charged particles, an electrically neutral particle is formed.

But Rutherford's atomic model could not explain the chemical property of elements.

Specifically it could not explain why metals or compounds of metals give off characteristic colors when heated in a flame. For example it could not explain why iron goes from a dull red, then yellow, and then white as it gets hotter and hotter.

nuclear atom theory

The nuclear atom consists of 1) protons and neutrons located in a positively charged nucleus and 2) electrons distributed around the nucleus and occupying almost all the volume of an atom. He hypothesized that electrons move around the nucleus like planets move around the sun. The nucleus is tiny and densely packed. The ratio of a nucleus to an atom is similar to the ratio of a marble to a football stadium.

Robert A. Millikan (1868-1953) determined the quantity of an electron's charge

US physicist Robert A. Millikan (1868-1953) performed experiments to determine the quantity of an electron's charge He did oil-drop experiments. He suspended negatively charged oil droplets between two charged plates. He then changed the voltage on the plates to see how this affected the droplets' rate of fall. His data showed that the charge on each oil droplet was a multitude of 1.6 * 10(exp-19) coulomb, meaning this must be the charge of an individual electron. Using his charge value (1.6 * 10(exp-19) coulomb) and Thomson's charge-to-mass ratio of an electron, Millikan calculated the mass of an election. Millikan's values for electron charge and mass are similar to those accepted today. An electron has one unit of negative charge and its mass is 1/1840 the mass of a hydrogen atom.

Atoms can be broken down into subatomic particles

most of Dalton's theory is accepted today. However one error was the theory that atoms were not divisible. Today we know that atoms are divisible. 3 types of subatomic particles are protons, electrons and neutrons

Ernest Rutherford (1871-1937) won the nobel prize in chemistry in 1908.

In 1911, Rutherford and his co-workers at the University of Manchester in England, used alpha particles and a piece of gold-foil to test the plum-pudding model of atomic structure. Alpha particles are helium atoms that have lost their 2 electrons and thus have a double positive charge because of the 2 remaining protons. They shot a narrow beam of alpha particles at a very thin sheet of gold foil. The plum-pudding theory predicts that the alpha particles should have easily passed through the foil, with a minimal amount of deflection because of the positive charge that was believed to be spread out throughout the gold atoms. Rutherford did not get the expected results! Instead 1) most alpha particles passed straight through the gold foil, or were only slightly deflected. suggesting that most of the foil was void of charge. 2) But a small percentage of alpha particles bounced off the gold foil at very large angles. Some even bounced back at the source, at an angle of 180 degrees. Based on these results Rutherford developed a new theory of atomic structure.

In 1932 english physicist James Chadwick (1891-1974) found evidence for neutrons.

Neutrons have no electrical charge. A neutron has a mass similar to a proton.

The Quantum Mechanical Model

New experiments showed that the Bohr model was wrong about electron behavior. In 1926, Austrian physicist, Erwin Schrodinger (1887-1961) developed and solved a mathematical equation to show how electrons might behave in a Hydrogen atom. The Quantum Mechanical Model came from the solutions to Schrodinger's equations. This model, like Bohr's model, restricts the energy of electrons to certain values. But it does not define a path that the electron takes around the nucleus. The location of electrons is based on probability. It examines how likely it is to find an electron in various locations around the nucleus. The probability of finding an elecron in a certain volume of space surrounding a nucleus is represented as a fuzzy cloudlike region. The cloud is more dense where the probability of finding an electron is high and it is less dense where the probability of finding an electron is low. There is no boundary to the cloud, because there is a slight chance that an electron will be seen at a considerable distance from the nucleus. In pictures, only areas where there is probability of finding an electrode 90% of the time.

Quarks

Physicists believe that protons and neutrons are composed of even smaller particles called quarks.

Bohr proposed that an electron is found only in specific circular paths, or orbits, around the nucleus. Each orbit has its own, fixed energy level. The orbit farthest from the nucleus has the least energy level and the orbit closest to the nucleus has the highest energy level. An electron jumps from one orbit to another by gaining or losing energy. A quantum of energy is the amount of energy required to move an electron from one energy level to another energy level. But the Bohr model could not explain behavior for atoms with more than one electron

The Bohr model

Using data, Dalton transformed Classical greek ideas into a scientific theory

The modern process of discovery regarding atoms began with John Dalton 1766-1844 an English chemist and school teacher