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Chemistry - Atomic Structure

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    Study Guide Chemistry – Atomic Structure 1. Structure of the Atom 1.1 What Is an Atom? Everything around you is made of atoms. An atom is the smallest unit of an element that still keeps the properties of that element. For example, a block of iron contains an enormous number of tiny iron atoms packed closely together. 1.2 Inside an Atom Even though atoms are very small, they are made of even smaller particles: • Protons – positively charged particles • Neutrons – particles with no charge • Electrons – negatively charged particles Protons and neutrons are tightly packed in the center of the atom, in a tiny region called the nucleus . Electrons do not sit in one fixed spot. Instead, they move around the nucleus in a cloud - like region that surrounds it. 1.3 Atoms and Isotopes All atoms of the same element have the same number of protons in their nuclei. This number defines the element. However, atoms of the same element can have different numbers of neutrons . These different forms are called isotopes . • Most elements in nature exist as a mixture of isotopes . • Each isotope has a slightly different atomic mass . • The atomic mass listed on the periodic table is a weighted average of the masses of all naturally occurring isotopes of that element.

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    Study Guide 1.4 Ions: When Atoms Gain or Lose Electrons Atoms can also change by gaining or losing electrons: • If an atom loses electrons , it becomes positively charged. • If an atom gains electrons , it becomes negatively charged. Charged atoms are called ions . 1.5 Radioactive Nuclei Sometimes, a nucleus becomes unstable . This usually happens when there are too many protons and neutrons packed together. When this happens, the atom becomes radioactive and can break apart. During this process: • The atom may change into a new element . • It releases particles called alpha or beta radiation . • It also releases energy in the form of gamma radiation . Summary Atoms are tiny but complex. They are made of smaller particles, can exist in different forms called isotopes, can become charged ions, and sometimes even break apart to release energy. Understanding atoms helps explain how matter behaves in everyday life — a nd in science as a whole. 2. Subatomic Particles 2.1 Understanding Atomic Structure As scientists studied the periodic table, they noticed something interesting. As you move from one element to the next, atomic mass usually increases in a steady way . This pattern became clear when scientists discovered that atoms are made of smaller particles .

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    Study Guide 2.2 Discovery of the Nucleus In 1911 , the study of atomic structure took a big step forward. A scientist named Ernest Rutherford discovered that atoms have a small, dense center called the nucleus . He found that: • The nucleus contains positively charged particles , called protons • Most of the atom is empty space • Tiny negatively charged electrons move around the nucleus Table 1. Parts of an Atom • The mass , charge , and location of protons, neutrons, and electrons 2.3 Protons and Atomic Number Soon after Rutherford’s discovery, scientists realized something very important: Each element is defined by the number of protons in its nucleus. This number is called the atomic number . For example: • Hydrogen has 1 proton • Helium has 2 protons • Lithium has 3 protons As you move through the periodic table, the number of protons increases by one each time.

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    Study Guide 2.4 Neutrons and Atomic Mass Except for the simplest hydrogen atom (which has only one proton ), all atoms contain neutrons . Neutrons: • Have no electric charge • Are found in the nucleus • Add to the mass of the atom For most light elements , the number of protons and neutrons is almost equal . Table 2. Nuclear Structure of the Lightest Elements • Atomic mass (rounded) is the sum of protons and neutrons • This total is called the mass number Mass number = protons + neutrons 2.5 Mass Number Made Simple A n example: • Carbon has 6 protons and 6 neutrons • Its mass number is 12 This pattern appears clearly in the table and explains why atomic masses increase across the periodic table.

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    Study Guide 2.6 Dalton’s Early Idea and Modern Understanding Earlier, John Dalton suggested that atomic masses were simple multiples of hydrogen’s mass. While this idea was not completely correct , it was close to the truth . Today we know that: • Atomic mass increases because atoms gain nucleons • Nucleons are the particles found in the nucleus: o Protons o Neutrons By adding these nucleons, nature creates the series of elements with increasing atomic masses. Summary • Atoms have a nucleus with protons and neutrons • Atomic number = number of protons • Mass number = protons + neutrons • Atomic masses increase by adding nucleons 3. Quiz: Subatomic Particles 1. Question Ions are Answer Choices • atoms of an element that have different numbers of protons • atoms of an element that have different numbers of neutrons • atoms that have a charge

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    Study Guide Correct Answer atoms that have a charge Why This Is Correct Ions are formed when atoms gain or lose electrons , which gives them a net electrical charge . • If an atom loses electrons , it becomes a positively charged ion (cation) . • If an atom gains electrons , it becomes a negatively charged ion (anion) . Changing the number of protons would change the element itself, and changing the number of neutrons creates isotopes — not ions. 2. Question What particles are found in the nucleus of an atom? Answer Choices • protons and neutrons • protons and electrons • neutrons and electrons Correct Answer protons and neutrons Why This Is Correct The nucleus is the dense center of an atom and contains: • Protons → positively charged particles • Neutrons → neutral (no charge) particles These particles make up nearly all of an atom’s mass.

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    Study Guide Electrons are not found in the nucleus. Instead, they move around the nucleus in regions called electron clouds. Example: In a carbon atom, the nucleus contains 6 protons and 6 neutrons , while 6 electrons are found outside the nucleus. 3 . Question Which subatomic particle has the smallest mass? Answer Choices • proton • neutron • electron Correct Answer electron Why This Is Correct Among the three main subatomic particles, the electron has the smallest mass. • Electrons have a mass about 1/1836 that of a proton. • Protons and neutrons are much heavier and have nearly the same mass. Because of its very small mass, the electron contributes almost nothing to an atom’s total mass, even though it plays a major role in chemical behavior. For example: Even though every hydrogen atom contains one electron , almost all of hydrogen’s mass comes from its proton .

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    Study Guide 4 . Question Which subatomic particle has a charge of zero? Answer Choices • proton • neutron • electron Correct Answer neutron Why This Is Correct A neutron has no electric charge , which is why it is considered neutral. • Protons have a positive (+) charge • Electrons have a negative ( – ) charge • Neutrons have zero charge Neutrons are found in the nucleus of an atom and help stabilize it by reducing repulsion between positively charged protons. 5. Question Which subatomic particle has a charge of +1? Answer Choices • proton • neutron • electron

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    Study Guide Correct Answer proton Why This Is Correct A proton carries a positive electric charge of +1 . Protons are found in the nucleus of an atom and determine the atom’s identity (atomic number). • Neutrons have no charge (0) • Electrons have a negative charge ( – 1) For example, hydrogen has one proton , giving it a +1 nuclear charge, which defines it as hydrogen. 6 . Question Which section of an atom has the highest density? Answer Choices • electron cloud • nucleus • all sections have the same density Correct Answer nucleus Why This Is Correct The nucleus has the highest density because it contains almost all of an atom’s mass packed into a very small volume. Protons and neutrons are located in the nucleus, while electrons in the electron cloud have very little mass and occupy most of the atom’s space. For example: Even though the nucleus is tiny compared to the whole atom, it contains nearly 99.9% of the atom’s mass , making it extremely dense.

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    Study Guide 7. Question Which section of an atom has the lowest density? Answer Choices • electron cloud • nucleus • all sections have the same density Correct Answer electron cloud Why This Is Correct The electron cloud has the lowest density because it occupies a very large volume of space around the nucleus, but contains extremely little mass (only electrons). In contrast, the nucleus is extremely dense because it contains protons and neutrons packed into a very small space, holding almost all of the atom’s mass. So, even though the electron cloud makes up most of the atom’s size, it has the lowest density . 8. Question The atomic number tells the Answer Choices • number of electrons in the atom • number of protons in the atom • number of neutrons in the atom Correct Answer number of protons in the atom

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