UWorld General Chemistry

Matter can be classified into what two broad categories?

Pure substances and mixtures.

What is a pure substance?

A material that contains only one chemical species (either an element or a compound) and has a distinct identity.

What is an element?

A substance that cannot be separated into simpler substances by chemical interactions.

What is a compound?

A substance made of two or more chemically bonded elements that forms a new substance with a new identity.

How can a compound be separated?

By chemical means (not physical methods).

What is a mixture?

A combination of two or more types of substances with variable composition.

How can mixtures be separated?

By physical methods.

What are the two types of mixtures?

Heterogeneous mixtures and homogeneous mixtures.

What is a heterogeneous mixture?

A mixture with varying particle distribution throughout.

What is a homogeneous mixture?

A mixture with uniform particle distribution throughout.

What are the characteristics of a pure substance?

• one type of substance

• fixed composition

• cannot be separated by physical methods

What are the characteristics of a mixture?

• two or more substances

• variable composition

• can be separated by physical processes

What are elements composed of?

Atoms.

What is an atom?

The smallest unit into which matter can be divided without changing its identity or properties.

What is the limit of chemical separation of matter?

The elemental level.

What do all atoms have in common structurally?

• a central nucleus

• protons (positively charged)

• neutrons (no charge)

• electrons (negatively charged) surrounding the nucleus

Where are protons and neutrons located?

In the nucleus of the atom.

Where are electrons located?

Surrounding the nucleus.

What determines the identity of an element?

The number of protons in its nucleus.

What is the atomic number of an atom?

The number of protons in the nucleus.

What does the atomic number determine?

The elemental identity of the atom.

Do all atoms of the same element have the same atomic number?

Yes, because they have the same number of protons.

If two atoms have different number of protons, what does that mean?

They are different elements with different properties.

What is the mass number of an atom?

The total number of protons and neutrons in the nucleus.

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

# of neutrons = mass number - atomic number

Why can you subtract atomic number from mass number to find neutrons?

Because mass number = protons + neutrons, and atomic number = protons.

Why is an atom in its elemental state electrically neutral?

Because the number of electrons equals the number of protons.

What is an atomic ion?

An atom that has gained or lost electrons and therefore has a net charge

What happens when an atom loses one electron?

It gains a +1 net charge.

What happens when an atom gains one electron?

It gains a -1 net charge.

Why does gaining or losing electrons create a charge?

Because the number of negatively charged electrons no longer equals the number of positively charged protons.

Does the number of neutrons determine the elemental identity of an atom?

No. Elemental identity is determined by the number of protons.

What do neutrons contribute to in an atom?

The mass of the atom.

What are isotopes?

Atoms of the same element (same number of protons) that have different number of neutrons.

Why are isotopes identified by their mass number?

Because isotopes differ in the number of neutrons, which changes the mass number.

How can isotopes be written in name format?

Element name — mass number (e.g., zinc-65).

How can isotopes be written in nuclear symbol format?

With the mass number as a superscript to the upper left of the element symbol.

Where is the atomic number written in full nuclear notation?

As a subscript below the mass number.

Why is the atomic number often omitted in nuclear notation?

Because all atoms of the same element have the same number of protons.

How can you determine the number of protons in an element?

By looking up its atomic number on the periodic table.

Do most naturally occurring elements exist as a single isotope?

No. Most exist as a mixture of different isotopes.

What is natural abundance?

The relative percentage of each isotope naturally present on Earth.

How can the distribution of isotopes be determined experimentally?

By analyzing a pure sample using mass spectrometry.

What primarily determine the chemical behavior of an atom?

Its electron configuration.

Why do isotopes of the same element have nearly identical chemical properties?

Because they have the same electron configuration.

How do isotopes of the same element differ from each other?

They differ in physical properties such as mass and density.

What unit is used to measure and compare atomic masses?

The atomic mass unit (amu).

How is 1 atomic mass unit (amu) defined?

As one twelfth (1/12) the mass of a carbon-12 atom.

Why is carbon-12 used to define the amu?

It serves as a standard reference for comparing atomic masses.

Why can’t the atomic mass of a single isotope be used for most lab calculations?

Because most samples contain a mixture of isotopes, not just one isotope.

What is atomic mass (atomic weight)?

A weighted average of the masses of an element’s isotopes based on their natural abundance?

What does “weighted average” mean in terms of atomic mass?

Each isotope’s mass contributes proportionally according to how abundant it is in nature.

Where is atomic mass (atomic weight) found on the periodic table?

Below each element’s symbol.

Why is atomic weight more useful that isotope mass for lab measurements?

Because it reflects the average composition of naturally occurring samples.

What are nucleons?

Protons and neutrons in the atomic nucleus.

What force holds nucleons together in the nucleus?

The strong nuclear force.

What is the strong nuclear force?

A close-range attractive force between protons and neutrons in the nucleus.

Why do protons repel each other in the nucleus?

Because they are positively charged and repel according to Coulomb’s law.

How does the nucleus remain intact despite proton repulsion?

By achieving a balance between attractive (strong nuclear force) and repulsive (electrostatic) forces.

How do neutrons contribute to nuclear stability?

They help separate and distribute protons, reducing repulsive forces.

What factors influence nuclear stability?

• total number of nucleons

• the neutron-to-proton (n/p) ratio

What neutron-to-proton (n/p) ratio results in the most stable nuclei?

Approximately 1 (especially in smaller nuclei).

Are large nuclei most stable at an n/p ratio of exactly 1?

No. Larger nuclei require slightly more neutrons than protons for stability.

What happens when the balance between nuclear force is disrupted?

The nucleus becomes unstable.

What can an unstable nucleus do if repulsive forces are not sufficiently balanced?

Eject nuclear particles in a process called radioactive decay.

What is radioactive decay?

The process by which an unstable nucleus emits particles and/or energy to become more stable.

What is emitted during radioactive decay?

Nuclear particles and/or high-energy photons (radiation).

Why does radioactive decay increase nuclear stability?

It releases energy and forms smaller nuclei with more favorable neutron-to-proton ratios.

What are the three most common types of radioactive decay?

• alpha decay

• beta decay

• gamma emission

What happens during alpha decay?

An unstable nucleus ejects an alpha (α) particle.

What is an alpha particle made of?

2 protons and 2 neutrons.

What is an alpha particle equivalent to?

A helium-4 nucleus without its electrons.

After alpha decay, how does the atomic number change?

It decreases by 4 units.

After alpha decay, how does the atomic number change?

It decreases by 2 units.

Why does the atomic number decrease by 2 during alpha decay?

Because 2 protons are ejected from the nucleus.

Why does the mass number decrease by 4 during alpha decay?

Because 2 protons and 2 neutrons are lost.

Compared to other common radioactive emissions, alpha particles are:

The most massive and least energetic.

How easily can alpha particles penetrate matter?

They are easily stopped by collisions with matter.

What eventually happens to alpha particles after emission?

They gain electrons and become helium atom.

How are natural underground helium deposits formed?

From alpha decay of radioactive elements (such as uranium) over time.

What are the three forms of beta (β) decay?

• β- decay (electron emission)

• β+ decay (positron emission)

• Electron capture

During all forms of beta decay, what happens to the mass number?

It remains unchanged.

During beta decay, how does the atomic number change?

It either increases or decreases by 1, depending on the type of decay.

What happens during β- decay?

A neutron converts into a proton.

What particles are emitted during β- decay?

• A high-speed electron (β- particle)

• An antineutrino

How does β- decay affect the atomic number?

The atomic number increases by 1.

Why does the atomic number increase during β- decay?

Because a neutron becomes a proton.

What is another name for β+ decay?

Positron emission.

What happens during β+ decay?

A proton converts into a neutron.

What particles are emitted during β+ decay?

• a positron (β+ particle)

• A neutrino

How does β+ decay affect the atomic number?

The atomic number decreases by 1.

Why does the atomic number decrease during β+ decay?

Because a proton becomes a neutron.

What happens during electron capture?

A proton captures an inner electron and converts into a neutron.

Is a particle emitted during electron capture?

No visible particle is emitted.

How does electron capture affect the atomic number?

The atomic number decreases by 1.

Why does electron capture decrease the atomic number?

Because a proton is converted into a neutron.

Which beta decay involves an antineutrino?

β- decay.

Which beta decay involves a neutrino?

β+ decay

Are neutrinos and antineutrinos always written in nuclear equations?

No. They are often omitted in chemistry contexts.

Why are neutrinos sometimes omitted from nuclear equations?

Because they are nearly massless and less relevant to typical chemistry calculations.