Understand the TEKS It is important to understand these terms as you read this lesson about the structure of an atom. An element is a substance that cannot break down into a simpler substance. An atom is the smallest, individual particle of an element. Elements are made up of only one kind of atom. An electron is a subatomic particle with a mass of almost zero and has a negative electric charge. A proton is a subatomic particle with a mass of 1 amu and has a positive electric charge. A neutron is a subatomic particle of about the same mass as a proton but without an electric charge. An atom that has lost or gained electrons is called an ion. An isotope consists of two or more atoms of the same element with a different number of neutrons but the same number of protons. Guided Instruction Read the following information and answer the questions. The idea that matter is made of very small particles dates back to the ancient Greeks. Since then, theories about the structure of matter and of atoms have changed as scientists have conducted more refined experiments. The results of these experiments have shown that atoms can be separated into even smaller particles. Each of these particles has unique properties that allow scientists to tell one kind of particle from the others. Scientists also discovered that the simplest form of each substance in the universe is made of a different combination of these particles. The ancient Greeks were the first people in history known to have thought about the nature of matter. Today we know that an element is a substance that cannot be broken down into a simpler substance. The Greeks believed that there were only four different elements: fire, air, earth, and water. Democritus and Leucippus concluded that when an element is divided, the eventual result is an indivisible particle that they called the atom. About two thousand years later, John Dalton further refined the theory of the atom to include many more elements. John Dalton proposed that atoms of different elements can have similarities but are not completely identical. J. J. Thomson later conducted experiments that proved the existence of a particle, called an electron, which was smaller than an atom and had a negative electric charge. The particle had less mass than a hydrogen atom, which scientists had thought to be the smallest particle. Thomson hypothesized that the particle came from inside the atom. He knew atoms had no charge, so they are electrically neutral. He proposed a “plum pudding model” in which atoms are made up of small negatively charged “plums” mixed with a “pudding” of positive charge. Words to Know element atom electron proton neutron ion isotope R TEKS 8.5(A) Describe the structure of atoms, including the masses, electrical charges, and locations, of protons and neutrons in the nucleus and electrons in the electron cloud. TEKS 8.3(D) Relate the impact of research on scientific thought and society, including the history of science and contributions of scientists as related to the content. What Is the Structure of an Atom? Lesson 1 MasteryEducation.com Copying is illegal. Unit 1 • How Do Atoms Affect Chemistry? 1 SAMPLE
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Understand the TEKSIt is important to understand these terms as you read this lesson about the structure of an atom.
An element is a substance that cannot break down into a simpler substance.
An atom is the smallest, individual particle of an element. Elements are made up of only one kind of atom.
An electron is a subatomic particle with a mass of almost zero and has a negative electric charge.
A proton is a subatomic particle with a mass of 1 amu and has a positive electric charge.
A neutron is a subatomic particle of about the same mass as a proton but without an electric charge.
An atom that has lost or gained electrons is called an ion.
An isotope consists of two or more atoms of the same element with a different number of neutrons but the same number of protons.
Guided Instruction
Read the following information and answer the questions.The idea that matter is made of very small particles dates back to the ancient Greeks. Since then,
theories about the structure of matter and of atoms have changed as scientists have conducted more refined experiments. The results of these experiments have shown that atoms can be separated into even smaller particles. Each of these particles has unique properties that allow scientists to tell one kind of particle from the others. Scientists also discovered that the simplest form of each substance in the universe is made of a different combination of these particles.
The ancient Greeks were the first people in history known to have thought about the nature of matter. Today we know that an element is a substance that cannot be broken down into a simpler substance. The Greeks believed that there were only four different elements: fire, air, earth, and water. Democritus and Leucippus concluded that when an element is divided, the eventual result is an indivisible particle that they called the atom. About two thousand years later, John Dalton further refined the theory of the atom to include many more elements. John Dalton proposed that atoms of different elements can have similarities but are not completely identical.
J. J. Thomson later conducted experiments that proved the existence of a particle, called an electron, which was smaller than an atom and had a negative electric charge. The particle had less mass than a hydrogen atom, which scientists had thought to be the smallest particle. Thomson hypothesized that the particle came from inside the atom. He knew atoms had no charge, so they are electrically neutral. He proposed a “plum pudding model” in which atoms are made up of small negatively charged “plums” mixed with a “pudding” of positive charge.
Words to Knowelementatomelectronprotonneutronionisotope
R TEKS 8.5(A) Describe the structure of atoms, including the masses, electrical charges, and locations, of protons and neutrons in the nucleus and electrons in the electron cloud.
TEKS 8.3(D) Relate the impact of research on scientific thought and society, including the history of science and contributions of scientists as related to the content.
What Is the Structure of an Atom?Lesson1
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Thomson’s Model of the Atom
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Later, scientist Ernest Rutherford proposed that the positive charges of an atom are contained in smaller subatomic particles. These subatomic particles, called protons, are in a small nucleus at the center of an atom. The nucleus contains most of the mass of the atom and also includes other subatomic particles, called neutrons, which do not have electric charges. Neutrons and protons have about the same mass. In this model, the electrons are in orbit around the nucleus and have much less mass than a proton or neutron. Another scientist, Niels Bohr, further described electrons as orbiting the nucleus at fixed energy levels. He said that an electron could move from one energy level to another in an atom when the atom gains or loses energy. Today’s model of the atom, developed by Erwin Schrödinger, shows the electrons orbiting the nucleus in an electron cloud.
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Bohr’s Model of the Atom Electron Cloud Atom Model
All atoms have mass. The model atom of each different element has a different mass and an electric charge of zero, meaning that it has an equal number of negatively charged electrons and positively charged protons. The number of protons in an atom of a particular element is the element’s atomic number. In nature, atoms of a given element can gain or lose electrons, which can make them have a net positive or negative charge. An atom that has lost or gained electrons is called an ion. In nature, atoms of a given element also can have more or fewer neutrons, which causes them to have more or less mass. Atoms of an element with different numbers of neutrons are called isotopes of each other.
Name of Particle Mass (amu = Atomic Mass Unit) Charge Location
Electron 0.000548597 amu or 1.1 � 10�31 kg negative cloud around nucleusNeutron 1.0086654 amu or 1.6749 � 10�27 kg none nucleusProton 1.0072766 amu or 1.6726 � 10�27 kg positive nucleus
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Lesson 1 What Is the Structure of an Atom?
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1. What is an atom?
2. What small charged particle did Thomson discover?
3. Why do you think Thomson hypothesized that electrons must come from inside an atom?
4. What comprises an atom?
5. How did Bohr’s model of an atom differ from Schrödinger’s?
6. What is the charge of a model atom of an element?
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What Is the Structure of an Atom? Lesson 1
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Critical Thinking
Answer the following questions.
1. Why did Thomson hypothesize that something positively charged existed within an atom?
2. Describe today’s accepted model of the atom.
3. Differentiate between an ion and an isotope.
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Lesson 1 What Is the Structure of an Atom?
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★ Practice
DIRECTIONS Read each question. Then circle the letter for the correct answer.
1 Which of the following cannot be broken down into a smaller substance?
A Proton
B Neutron
C Element
D Ion
2 What is the smallest, indivisible particle of an element?
F Proton
G Neutron
H Ion
J Atom
3 Why might scientists’ theories have changed over the years?
A Science changes over the years.
B Their research is more refined.
C Modern scientists are smarter.
D The structures of matter and atoms changed.
4 Which is smaller than an atom and has a negative electric charge?
F Proton
G Neutron
H Isotope
J Electron
5 What is the impact of scientific thought on society?
A Scientists do not always know what they are doing.
B All scientists are basically the same.
C Empirical evidence results as scientists refine research and experimentation.
D Modern scientists are smarter than the ones from past centuries.
6 Why is it not possible to determine the type of atom by the number of neutrons it has?
F All isotopes are different but have the same number of neutrons.
G The number of neutrons changes all the time, so the type of atom always changes.
H All elements have the same number of atoms and neutrons.
J Different isotopes of an element can have different numbers of neutrons.
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What Is the Structure of an Atom? Lesson 1
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1 What does today’s model of the atom show?
A Electrons orbit the nucleus at fixed energy levels.
B Electrons orbit the nucleus in a cloud.
C Protons and neutrons orbit the nucleus.
D Ions and isotopes orbit the nucleus.
2 How many electrons are in a nickel ion that has an atomic number of 28 and a positive charge of 2?
Record your answer and fill in the bubbles in the grid. Be sure to use the correct place value.
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3 How many electrons are in an oxygen atom that has an atomic number of 8?
A 12
B 7
C 14
D 8
4 Using your knowledge of atomic theory, how many protons are contained in a neutral palladium atom that has 46 electrons?
F 46 H 64
G 32 J 18
5 Which particles are in the center of the atom, or the nucleus?
A Ions and isotopes
B Protons and isotopes
C Protons and neutrons
D Electrons and protons
6 Look at the table.
Name of Particle
Mass (amu = Atomic Mass Unit) Charge Location
Electron 0.000548597 amu or 1.1 × 10–31 kg
negative cloud around nucleus
Neutron 1.0086654 amu or 1.6749 × 10–27 kg
none nucleus
Proton 1.0072766 amu or 1.6726 × 10–27 kg
positive nucleus
Which subatomic particle has the most mass, no charge, and is located in the nucleus?
F Electron
G Neutron
H Ion
J Proton
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★ Assessment
DIRECTIONS Read each question. Then circle the letter for the correct answer.
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