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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.
Section 1 Development of the Atomic Theory
Chapter 6
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.
Section 1 Development of the Atomic Theory
Chapter 6
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.
Section 1 Development of the Atomic Theory
Chapter 6
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.
Section 1 Development of the Atomic Theory
Chapter 6
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.
Section 1 Development of the Atomic Theory
Chapter 6
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.
Section 1 Development of the Atomic Theory
Chapter 6
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.
Section 1 Development of the Atomic Theory
Chapter 6
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.
Section 1 Development of the Atomic Theory
Chapter 6
The Nucleus and the Electrons, continued
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.
Section 1 Development of the Atomic Theory
Chapter 6
The Nucleus and the Electrons, continued
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.
Section 1 Development of the Atomic Theory
Chapter 6
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 1 Development of the Atomic Theory
Chapter 6
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?
Section 2 The Atom
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.
Section 2 The Atom
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.
Section 2 The 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.
Section 2 The Atom
The Parts of an Atom, continued
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.
Section 2 The Atom
The Parts of an Atom, continued
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).
Section 2 The Atom
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.
Section 2 The Atom
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.
Section 2 The Atom
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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.
Section 2 The Atom
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.
Section 2 The Atom
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.
Section 2 The Atom
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.
Section 2 The Atom
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.
Section 2 The Atom
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