NAS2 Part 1 Review

What is matter?

Matter is any substance that occupies space and has mass.

What is an element?

An element is a unique form of matter with specific chemical and physical properties that cannot be broken down into smaller substances by ordinary chemical reactions.

How many elements are known, and how many occur naturally?

There are 118 known elements; 98 occur naturally. The remaining elements are unstable and must be synthesized in laboratories.

Why is oxygen considered matter?

Oxygen occupies space and has mass, so it meets the definition of matter.

How do an element and a compound differ?

An element contains one type of atom and cannot be chemically broken down by ordinary reactions; a compound contains atoms of two or more different elements chemically bonded together.

How are chemical symbols written?

A chemical symbol is either one capital letter or a capital letter followed by a lowercase letter.

True or false: The symbol CA correctly represents calcium.

False. Calcium is Ca. Chemical symbols are case-sensitive.

Why is sodium represented by Na rather than S?

Na comes from natrium, the Latin name for sodium; S is already the symbol for sulfur.

Which four elements are common to all living organisms?

Oxygen, carbon, hydrogen, and nitrogen (O, C, H, and N).

According to Table 2.1, which element makes up the largest percentage of the human body?

Oxygen, at approximately 65%.

According to Table 2.1, which element dominates Earth’s atmosphere?

Nitrogen, at approximately 78%.

According to Table 2.1, which element is most abundant in Earth’s crust?

Oxygen, at approximately 46%.

How does carbon abundance differ among humans, the atmosphere, and Earth’s crust?

Carbon is about 18% of the human body but occurs only in trace amounts in the atmosphere and Earth’s crust.

True or false: Different chemical laws apply to elements in living and nonliving matter.

False. All elements and their reactions obey the same chemical and physical laws in living and nonliving systems.

Place these levels in order from smallest to largest: organism, molecule, tissue, cell, organ system.

Molecule → cell → tissue → organ system → organism.

How do naturally occurring elements contribute to the organization of life?

Elements combine to form molecules; molecules form cells; cells form tissues; tissues form organ systems; organ systems make up organisms.

What is an atom?

The smallest unit of matter that retains all the chemical properties of an element.

Why is a single gold atom still considered gold?

It retains the chemical properties and reactivity characteristic of the element gold.

In Figure 2.2, where are protons and neutrons located?

Inside the nucleus at the center of the atom.

In Figure 2.2, where are electrons located?

In orbitals surrounding the nucleus.

What is unusual about the most common isotope of hydrogen?

It has one proton and one electron but no neutron.

What are the charge, approximate mass, and location of a proton?

+1 charge, about 1 amu, in the nucleus.

What are the charge, approximate mass, and location of a neutron?

0 charge, about 1 amu, in the nucleus.

What are the charge, approximate mass, and location of an electron?

−1 charge, approximately 0 amu for mass-number calculations, in orbitals.

How are protons and neutrons alike and different?

Both are in the nucleus and have masses of about 1 amu; protons are positively charged, whereas neutrons are uncharged.

How do electrons differ from protons in mass and charge?

Electrons have a negative charge equal in magnitude to a proton’s positive charge, but an electron has only about 1/1800 the mass of a proton.

What is one atomic mass unit (amu), also called one Dalton?

Approximately 1.67 × 10⁻²⁴ grams, roughly the mass of one proton or neutron.

Why are electrons usually ignored when calculating an atom’s mass number?

Their mass is extremely small compared with the masses of protons and neutrons.

What condition makes an atom electrically neutral?

It has equal numbers of protons and electrons, so positive and negative charges cancel.

True or false: Neutrons contribute to an atom’s charge because they are in the nucleus.

False. Neutrons contribute to mass but have no electric charge.

Approximately how much of an atom’s volume is empty space?

More than 99%.

Why do solid objects not pass through one another even though atoms are mostly empty space?

The negatively charged electron regions of their atoms repel one another.

What is an element’s atomic number?

The number of protons in the nucleus.

What determines the identity of an element?

Its number of protons, which is its atomic number.

What is a mass number?

The total number of protons plus neutrons in an atom.

How do you calculate the number of neutrons in an atom?

Mass number − atomic number.

An atom has atomic number 8 and mass number 16. How many protons, neutrons, and electrons does a neutral atom have?

8 protons, 8 neutrons, and 8 electrons.

True or false: The atomic number can change among isotopes of the same element.

False. Isotopes have the same atomic number because they have the same number of protons.

What is atomic weight (relative atomic mass)?

The weighted mean of the masses of an element’s naturally occurring isotopes.

Why is chlorine’s atomic weight about 35.45 rather than a whole number?

Natural chlorine is a mixture of isotopes, mainly chlorine-35 and chlorine-37, so its listed mass is a weighted average.

What are isotopes?

Different forms of the same element that have the same number of protons but different numbers of neutrons.

Compare carbon-12 and carbon-14.

Both have 6 protons and, when neutral, 6 electrons. Carbon-12 has 6 neutrons; carbon-14 has 8 neutrons.

Figure 2.3 shows carbon-12 and carbon-13. How many neutrons do they have, respectively?

Carbon-12 has 6 neutrons; carbon-13 has 7.

True or false: Isotopes of an element differ in their number of electrons.

False. Isotopes differ in neutron number; neutral isotopes of the same element have the same number of electrons.

What is a radioisotope?

An unstable isotope that emits particles or radiation as it moves toward a more stable, lower-energy nuclear configuration.

What is radioactive decay?

The energy-releasing process in which an unstable atomic nucleus emits radiation and becomes more stable.

Into which element does carbon-14 eventually decay?

Nitrogen-14.

What atmospheric process creates carbon-14?

Cosmic rays cause nitrogen-14 to gain a neutron and lose a proton, producing carbon-14.

Why does a living organism have a carbon-14-to-carbon-12 ratio similar to the atmosphere?

It continually takes in carbon through processes such as photosynthesis and feeding while alive.

Why does the carbon-14-to-carbon-12 ratio begin to decrease after an organism dies?

The organism stops taking in carbon-14, while its existing carbon-14 continues to decay.

What is half-life?

The time required for half of the original amount of a radioactive isotope to decay into a more stable form.

What is the approximate half-life of carbon-14?

5,730 years.

A sample begins with 100 units of carbon-14. Approximately how much remains after two half-lives?

25 units.

What types of materials can carbon dating be used to date?

Formerly living, carbon-containing materials such as bone or wood.

What limitation of carbon dating is emphasized by Figure 2.4?

It is most accurate for carbon-containing remains less than about 50,000 years old.

Why are potassium-40 and uranium-235 useful for dating older materials than carbon-14?

They have much longer half-lives—about 1.25 billion years for potassium-40 and about 700 million years for uranium-235.

How does radiometric dating contribute to evolutionary biology?

It helps determine the ages of fossils and remains, allowing scientists to reconstruct the history of evolutionary change.

What does the periodic table organize?

The chemical elements according to atomic number and shared chemical and physical properties.

Who developed the periodic table described in the section, and when?

Dmitri Mendeleev in 1869.

What four pieces of information are typically shown in an element’s square in Figure 2.5?

Atomic number, chemical symbol, element name, and atomic mass.

Where are atomic number and approximate atomic mass shown relative to the element symbol?

Atomic number appears above the symbol; approximate atomic mass appears below it.

How do atomic number and atomic mass differ on the periodic table?

Atomic number is a whole number equal to proton number; atomic mass is usually a decimal representing the average mass of naturally occurring isotopes.

How many groups and periods are displayed in the standard periodic table?

18 groups and 7 main periods.

What are the three broad element categories shown in Figure 2.5?

Metals, nonmetals, and metalloids.

Why do elements in the same periodic-table column often have similar chemical properties?

They have similar valence-electron arrangements and therefore similar bonding tendencies.

What is chemical reactivity?

An element’s ability to combine and chemically bond with other elements.

What is a molecule?

Two or more atoms chemically bonded together.

Why do atoms’ electrons interact first when atoms approach to form a bond?

Electrons occupy the outermost regions of atoms.

What does the Bohr model depict?

A central nucleus containing protons and neutrons, with electrons occupying specific electron shells or energy levels around it.

What does the notation 1n represent?

The first principal energy level, closest to the nucleus.

According to Figure 2.6, where does an electron normally exist?

In the lowest-energy shell available, closest to the nucleus.

What happens when an electron absorbs energy from a photon and then returns to its ground state?

It moves to a higher-energy shell, becomes unstable, returns to the lower-energy ground state, and releases a photon.

What is the ground state of an electron?

Its lowest available energy state.

In what general order do electrons fill orbitals?

They fill lower-energy orbitals closest to the nucleus first; equal-energy orbitals receive one electron each before pairing.

What are valence electrons?

Electrons in an atom’s outermost occupied energy level.

Which electrons most strongly determine an atom’s stability and bonding behavior?

Its valence electrons.

What is the octet rule?

Except for the first shell, atoms are generally most stable when their valence shell contains eight electrons.

True or false: Every stable atom must have eight electrons in its outer shell.

False. The first shell is full with two electrons; helium is stable with two.

What is the maximum number of electrons in the first principal shell?

2.

How many electrons can the second principal shell hold?

8.

How many electrons can the third principal shell hold according to the orbital model?

18.

How many electrons can the fourth principal shell hold?

32.

Which Figure 2.7 elements have full valence shells?

Helium, neon, and argon.

How many valence electrons do group 1 elements shown in Figure 2.7 have?

One.

How many electrons must group 1 elements lose to reach a stable configuration?

One.

How many electrons would group 14 and group 17 atoms need to gain to complete an octet, if gain alone were considered?

Group 14 would need 4; group 17 would need 1.

Why are group 18 elements called noble or inert gases?

Their outer electron shells are full, so they are highly stable and usually nonreactive.

Compare the bonding tendencies of group 1 and group 17 elements.

Group 1 atoms tend to lose one electron and become positive ions; group 17 atoms tend to gain one electron and become negative ions.

Why can carbon form several covalent bonds?

It has four valence electrons and can share electrons to complete an outer shell of eight.

True or false: Electrons orbit the nucleus in fixed circular paths exactly like planets orbit the Sun.

False. Electrons occupy probability regions called orbitals; the Bohr model is a simplified representation.

What is an electron orbital?

A three-dimensional region where an electron is most likely to be found.

What are wave functions used to predict?

The probability of finding an electron in a particular region around the nucleus.

Which letters designate electron subshells?

s, p, d, and f.

What is the shape of an s orbital in Figure 2.8?

Spherical.

What is the shape of a p orbital?

Dumbbell-shaped.

How many orbitals are in an s subshell, and how many electrons can it hold?

One orbital, holding up to two electrons.

How many orbitals are in a p subshell, and how many electrons can the entire p subshell hold?

Three orbitals, holding up to six electrons total.

How many orbitals are in d and f subshells?

d has 5 orbitals; f has 7.

Which orbital fills first in every atom?

The 1s orbital.

What is hydrogen’s electron configuration?

1s¹.