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Chapter 6 Introduction to Atoms

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Section 1 Development of the Atomic Theory

Section 2 The Atom

Concept Map


Section 1 Development of the Atomic TheoryChapter 6

Bellringer

In your Science Journal, draw a picture that shows what you think an atom looks like. On your drawing, include labels for the locations of protons, neutrons, and electrons.



What You Will Learn

• There have been different models of the atom over time.

• The atomic theory has changed as scientists have experimented and discovered new information about the atom.



The Beginning of Atomic Theory

• Matter is made of particles that we call atoms.

• An atom is the smallest particle into which an element can be divided and still have the properties of that element.


Introduction to AtomsChapter 6

Atom



Dalton’s Atomic Theory Based on Experiments

• By the late 1700s, scientists had learned that elements combine in certain proportions based on mass to form compounds.

• Dalton’s atomic theory, published in 1803, suggested that elements combine in certain proportions because they are made of atoms.



Thomson’s Discovery of Electrons

• In 1897, Thomson’s experiments led him to conclude that there must be negatively charged particles inside the atom. These particles are now called electrons.

• An electron is a subatomic particle that has a negative charge.





Rutherford’s Atomic “Shooting Gallery”

• In 1909, Rutherford designed an experiment to study the parts of an atom.

• The results of Rutherford’s gold-foil experiment suggested that atoms are not all the same throughout. He concluded that atoms have different parts.





The Nucleus and the Electrons

• Rutherford’s results led him to conclude that most matter in an atom is found in one very small area.

• In 1911, Rutherford revised the atomic theory and proposed that in the center of the atom is a tiny, extremely dense, positively charged area called the nucleus.



The Nucleus and the Electrons, continued

• In physical science, the nucleus is an atom’s central region which is made of protons and neutrons.

• From Rutherford’s results, the important idea emerged that atoms are mostly empty space with a tiny, massive nucleus at the center and electrons surrounding the nucleus.




• In 1913, Bohr’s results led him to propose that electrons move around the nucleus in definite paths.

• According to modern atomic theory, electrons are likely to be found moving in certain regions around the nucleus of an atom. These regions are called electron clouds, or orbitals.




• Electron clouds are regions around the nucleus of an atom where electrons are likely to be found. Each electron cloud has a definite energy level.

• Therefore, each electron in an atom has a definite energy based on which electron cloud the electron is moving in.



Electron Cloud



The Size of an Atom

• Atoms are so small that light waves are too large to be used to observe them. Scientists use scanning tunneling electron microscopes to provide images of atoms.

• However, these images are not an actual picture of the atom. They show an image of the surface of a material at the atomic level.


Section 2 The AtomChapter 6

Bellringer

An atom is the smallest particle of an element.

Atoms are made of protons, neutrons, and electrons.

Write a paragraph in your Science Journal explaining how the two sentences above relate to one another. Are they both true? If so, how is that possible?



What You Will Learn

• Protons, neutrons, and electrons make up atoms.• All atoms of a given element have the same number

of protons in the nucleus.• Isotopes of an element differ by the number of

neutrons in the nucleus.• Atomic mass is an average of the masses of all of the

naturally occurring isotopes of an element.• Four forces are at work in atoms.



The Parts of an Atom

• Almost all kinds of atoms are made of the same three particles. These particles are protons, neutrons, and electrons.

• Protons, neutrons, and electrons are called subatomic particles because they are each much smaller than an atom.



The Parts of an Atom, continued

• The number of subatomic particles that are in an atom and the way the particles interact determine the properties of an atom.

• A proton is a subatomic particle that has a positive charge and that is located in the nucleus of an atom.




• A neutron is a subatomic particle that has no charge and that is located in the nucleus of an atom.

• An electron is a subatomic particle that has a negative charge and that is found outside the nucleus in electron clouds.




• The charges of protons and electrons are opposite but equal, so the charges cancel out. If the numbers of electrons and protons become unequal, the atom becomes a charged particle called an ion.

• The SI unit that is used to express the mass of a particle in an atom is the atomic mass unit (amu).





Atoms and Elements

• There are more than 110 different elements. The atoms of each of these elements are different from the atoms of all other elements.

• Electrons and protons are found in all atoms.

• An atom does not have to have equal numbers of neutrons and protons.



Atoms and Elements, continued

• The atomic number is the number of protons in the nucleus of an atom.

• All atoms of the same element have the same atomic number.

• The atomic number of each element is listed on the periodic table.



Atomic Number



Isotopes

• An isotope is an atom that has the same number of protons as other atoms of the same element do, but has a different number of neutrons.

• Atoms that are isotopes of each other are always the same element, because isotopes of the same element always have the same number of protons.



Isotopes, continued

• Some isotopes of an element have special properties because they are unstable. An unstable atom is an atom with a nucleus that will change over time. This type of atom is radioactive.

• Isotopes of an element share most of the same chemical and physical properties.



Isotopes, continued

• Each isotope of an element can be identified by its mass number. The mass number is the sum of the protons and neutrons in the nucleus of an atom.

• To identify a specific isotope of an element, write the name of the element followed by a hyphen and the mass number of the isotope.



Mass Number





Isotopes, continued

• Most elements contain a mixture of two or more isotopes.

• The atomic mass of an element is the weighted average of the masses of all the naturally occurring isotopes of that element.



Forces in Atoms

• There are four basic forces that are at work everywhere in nature, even within the atom. These forces are gravitational force, electromagnetic force, strong force, and weak force.

• These four forces work together to give an atom its structure and properties.




Chapter 6 Introduction to Atoms

Use the terms below to complete the concept map on the next slide.

nucleus

mass number

isotopes

protons

atoms

electrons

atomic number

Concept Map



Concept Map



Concept Map