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Chemistry - Elements

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    Study Guide Chemistry – Elements 1. Discovery and Similarity 1.1 How Chemistry Began: Discovering Elements Modern chemistry began in the 18th century , when scientists tried to understand what substances are really made of. They studied how materials break down when heated or when electricity is passed through them. In 1774 , an English scientist named Joseph Priestley made an important discovery. When he heated a red powder called mercuric oxide , it broke apart into: • a liquid metal (mercury), and • a colorless gas that helped things burn. This gas was later named oxygen . Scientists found that many substances can be broken down into simpler parts using heat or electricity. However, some substances cannot be broken down any further , no matter how much energy is used. These simplest substances are called chemical elements . They are the basic building blocks of all matter. 1.2 How Many Elements Are There? In 1789 , the French chemist Antoine Lavoisier published one of the first lists of elements. At that time, only 33 elements were known — and some of them turned out to be incorrect. By 1930 , careful work by many chemists increased the number of naturally occurring elements to 90 . Later, scientists working in high - energy laboratories created about 20 additional elements . These elements are radioactive and unstable , and they do not occur naturally on Earth, although they are likely formed in the hot centers of some stars. Today, the total number of known elements has reached 112 , and the list may continue to grow. The good news for students is that only about 40 elements are important for basic chemistry.

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    Study Guide 1.3 What You Need to Know as a Student Take a look at the periodic table at the end of the chapter and find calcium (element 20) . You should become familiar with: • the symbols and general properties of the first 20 elements (up to calcium), and • about 20 more elements that are especially important in chemistry. Important chemistry ideas are printed in bold the first time they appear in the text. These are key concepts that you will see again and again. Their definitions can be found in the glossary at the end of the book. 1.4 Groups of Elements with Similar Properties Figure 1 Some elements behave in very similar ways. Because of this, they are grouped together on the periodic table.

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    Study Guide Inert (Noble) Gases • Helium, neon, argon, krypton, and xenon • These are colorless gases • They rarely react with other elements • Because they are so unreactive, they are called inert gases (also known as noble gases ). Halogens (Salt Formers) • Fluorine, chlorine, bromine, and iodine • Fluorine and chlorine are greenish, corrosive gases • They react easily with metals to form salts • This is why they are called halogens , which means salt formers . Alkali Metals • Lithium, sodium, and potassium • These metals are very light and can float on water • They are extremely reactive and may burst into flames when they touch water • They form strong alkalis, so they are called alkali metals . 1.5 Where Similar Elements Are Found in Nature Elements with similar properties are often found in the same natural environments . For example: • Halogens are common in seawater • The main salt in ocean water is sodium chloride • Bromine is obtained from salt beds formed after seawater evaporates • Iodine is collected from kelp , a type of seaweed that grows in oceans

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    Study Guide 2. Quiz: Discovery and Similarity 1. Question The modern periodic table is arranged Answer Choices • by increasing atomic mass • by increasing atomic number • alphabetically Correct Answer by increasing atomic number Why This Is Correct The modern periodic table is organized according to increasing atomic number , which means elements are arranged based on the number of protons in their atoms. This arrangement explains repeating (periodic) patterns in the chemical properties of elements more accurately than atomic mass. For example: Hydrogen (atomic number 1) comes before Helium (atomic number 2) because it has one fewer proton. 2. Question The group of most reactive metals that form strong alkaline solutions with water: Answer Choices • inert gases • halogens • alkali metals

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    Study Guide Correct Answer alkali metals Why This Is Correct Alkali metals are extremely reactive metals found in Group 1 of the periodic table. They react vigorously with water to form alkaline (basic) solutions and release hydrogen gas. For example: • Sodium reacts with water to form sodium hydroxide (a strong alkali) and hydrogen gas. In contrast: • Inert gases are very unreactive. • Halogens are reactive nonmetals, not metals, and do not form alkaline solutions with water. 3. Question The group of most reactive nonmetals that readily combine with metals to form salts: Answer Choices • inert gases • halogens • alkali metals Correct Answer halogens Why This Is Correct Halogens are the most reactive nonmetals on the periodic table. They have seven valence electrons and readily gain one electron to form stable negative ions. Because of this, they easily react with metals to form salts. For example: • Sodium (a metal) + Chlorine (a halogen) → Sodium chloride (table salt)

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    Study Guide • Why the Other Options Are Incorrect • Inert gases: Very unreactive due to full valence shells • Alkali metals: Highly reactive, but they are metals, not nonmetals • Therefore, halogens is the correct answer. 4. Question The group of nonreactive elements in which only two of the elements form compounds: Answer Choices • inert gases • halogens • alkali metals Correct Answer inert gases Why This Is Correct Inert gases (also called noble gases) are mostly nonreactive because they have full valence electron shells , making them very stable. Most inert gases do not form compounds at all. Only two heavier inert gases — xenon and krypton — are known to form a small number of compounds , under special conditions. For example: • Xenon can form compounds like xenon difluoride (XeF ₂ ) • Most others (helium, neon, argon) do not react Why the Other Options Are Incorrect • Halogens : Highly reactive nonmetals • Alkali metals : Very reactive metals • Therefore, inert gases is the correct answer.

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    Study Guide 5. Question The symbol of an element often contains: Answer Choices • the first two letters of the element name • an arbitrary selection of letters • the first letter of the element name Correct Answer the first letter of the element name Why This Is Correct An element’s symbol usually begins with the first letter of its name . If a second letter is used, it is typically another letter from the element’s name (not an arbitrary choice) and is written in lowercase. For example: • H for Hydrogen • C for Carbon • Ca for Calcium (C is the first letter, a is another letter from the name) So, the most common and consistent feature is that the symbol contains the first letter of the element’s name . 6. Question The halogens are often called: Answer Choices • salt formers • lanthanides • noble gases

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    Study Guide Correct Answer salt formers Why This Is Correct Halogens readily react with metals to form salts , which is why they are known as salt formers . The name halogen itself comes from Greek words meaning “salt - producing.” For example: • Sodium + Chlorine → Sodium chloride (table salt) In contrast: • Lanthanides are a series of rare earth metals. • Noble gases are very unreactive elements. 7. Question The symbol for chlorine is: Answer Choices • C • Ch • Cl Correct Answer Cl Why This Is Correct Chemical symbols use one or two letters . • The first letter is always capitalized . • If there is a second letter, it is lowercase .

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    Study Guide For chlorine: • Cl is correct • C represents carbon • Ch is not a valid chemical symbol So, the correct symbol for chlorine is Cl . 8. Question The inert elements are also known as: Answer Choices • metals • noble gases • actinides Correct Answer noble gases Why This Is Correct Inert elements are called noble gases because they are very unreactive. Their outer electron shells are completely filled, which makes them chemically stable. Examples include: • Helium • Neon • Argon In contrast: • Metals include many reactive elements. • Actinides are radioactive elements found at the bottom of the periodic table.

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    Study Guide 9. Question Elements with atomic number 93 and greater are all: Answer Choices • naturally occurring • man - made • known as noble gases Correct Answer man - made Why This Is Correct Elements with atomic numbers 93 and higher are called transuranium elements . They do not occur naturally in significant amounts and are created artificially in laboratories or nuclear reactors through nuclear reactions. In contrast: • Naturally occurring elements are mostly found with atomic numbers 92 or lower. • Noble gases are nonreactive elements in Group 18 and are not defined by high atomic numbers. 3. Atomic Masses 3.1 Understanding Why Atomic Mass Matters In the early 1800s, chemists were trying to make sense of the growing number of chemical elements. One important thing they noticed was that elements combine with each other in fixed mass ratios when they form compounds. These mass relationships were not random — they followed clear patterns.

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