Chemistry Unit 3 - Atoms

What does the Latin root word for "composition" mean?

Comes from the Latin compositio (from com- "together" + ponere "to put"), meaning "putting together" or "how something is put together/arranged."

What makes a pure substance different from a mixture?

A pure substance has a constant, uniform chemical composition and set properties throughout, while a mixture is a physical combination of two or more substances that retain their individual properties and can vary in proportion.

What is an element?

A pure substance composed of only one type of atom that cannot be broken down into simpler substances by ordinary chemical or physical means.

How many known elements are there?

There are 118 known elements on the periodic table (92 occur naturally, and the rest are synthetic).

What is an atom, and can two different elements have the same type of atom?

The fundamental, smallest unit of an element that retains the chemical identity of that element. No, two different elements cannot have the same type of atom (each element has a unique number of protons).

Name two elements that are liquids at room temperature.

Mercury (Hg) and Bromine (Br).

Name two elements that are solids and two that are gases at room temperature.

Solids: Iron (Fe) and Gold (Au) [or Carbon, Copper]. Gases: Oxygen (O2) and Nitrogen (N2) [or Helium, Hydrogen].

What are the capitalization rules for writing an element's chemical symbol?

The first letter of a chemical symbol is ALWAYS capitalized. If there is a second letter, it is ALWAYS lowercase (e.g., Cu, Zn, Na).

Give the correct chemical symbol for Copper and Zinc.

Copper = Cu | Zinc = Zn

Define a chemical compound.

A pure substance formed when two or more different elements chemically combine in a fixed ratio.

How do the properties of a compound compare to the properties of the elements that form it?

The physical and chemical properties of a compound are completely different from the properties of the individual elements that make it up.

Provide an example showing how water's properties differ from hydrogen and oxygen.

Hydrogen (H2) and Oxygen (O2) are both highly flammable/reactive gases at room temperature, but when combined chemically into water (H2O), they form a stable, non-flammable liquid used to put out fires.

What does it mean for a compound to have a "fixed proportion"?

A compound always contains the exact same elements combined in the exact same numerical or mass ratio, regardless of the size or source of the sample.

Can elements be broken down into simpler substances? Can compounds?

Elements CANNOT be broken down into simpler substances. Compounds CAN be broken down into their constituent elements.

What is required to break a compound apart into its elements?

A chemical reaction/change (chemical means) involving energy input (such as heat or electricity) is required to break a compound apart.

What is electrolysis?

A process that uses an electric current to drive a non-spontaneous chemical reaction, such as breaking water down into hydrogen and oxygen gas.

What does the Law of Definite Proportions state?

States that a given chemical compound always contains its component elements in fixed ratio by mass, regardless of the sample size or source.

Write the formula used to calculate the percent by mass of an element in a compound.

Percent by Mass = (Mass of Element / Total Mass of Compound) * 100

A 10 g sample of a compound contains 2.5 g of oxygen. What is the percent by mass of oxygen in the sample?

25%. Calculation: (2.5 g / 10 g) * 100 = 25%

If you double the size of a water sample from 5 g to 10 g, does the percent by mass of hydrogen change? Explain why

No, doubling the sample size from 5 g to 10 g does NOT change the percent by mass. Because compounds have a fixed proportion (Law of Definite Proportions), both the element mass and compound mass double, keeping the percentage identical.

Where are protons located in an atom, and what is their electrical charge?

Protons are located inside the atomic nucleus and have a positive electrical charge (+1).

What is the atomic number of an element, and why is it important?

The atomic number is the number of protons in the nucleus of an atom. It is important because it identifies the element (every element has a unique atomic number).

Which subatomic particle has no electrical charge, and where is it located?

The neutron has no electrical charge (neutral, 0) and is located inside the atomic nucleus.

How does the mass of an electron compare to the mass of a proton or neutron?

An electron is extremely small—about 1/1,840 the mass of a proton or neutron (its mass is considered negligible).

If an atom is electrically neutral and has an atomic number of 8, how many protons and electrons does it have?

It has 8 protons and 8 electrons (in a neutral atom, protons equal electrons).

Are protons and neutrons the smallest known pieces of matter? If not, what makes them up?

No, protons and neutrons are not the smallest pieces of matter. They are made up of smaller fundamental particles called quarks.

What holds electrons in orbit around the atomic nucleus?

The electrostatic attraction between the positively charged nucleus and the negatively charged electrons holds electrons around the nucleus.

How is an Atomic Mass Unit (amu) defined?

Defined as exactly 1/12th the mass of a single Carbon-12 atom.

Why do we ignore electrons when calculating the mass number of an atom?

Electrons are ignored because their mass is so tiny compared to protons and neutrons that it does not significantly change the total mass.

Write out the formula used to calculate the mass number of an atom.

Mass Number = Protons + Neutrons

If an atom has 6 protons and 8 neutrons, what is its mass number?

Mass Number = 14 (6 protons + 8 neutrons = 14).

What is an isotope?

Atoms of the same element (same number of protons) that have different numbers of neutrons, resulting in different mass numbers.

How are isotope names written? Give an example using Carbon.

Written as the element name followed by a hyphen and its mass number. Example: Carbon-12 or Carbon-14.

Why is the atomic mass listed on the periodic table usually a decimal number instead of a whole number?

The atomic mass listed is a weighted average of the atomic masses of all naturally occurring isotopes of that element, based on their relative abundances.

What force keeps protons in the nucleus from pushing each other apart?

The strong nuclear force holds protons and neutrons together in the nucleus, overcoming the electrostatic repulsion between positively charged protons.

What determines whether an atomic nucleus is stable or unstable?

Determined by the neutron-to-proton ratio (N/Z ratio) in the nucleus.

How does a nuclear reaction differ from a chemical reaction?

Chemical reactions involve rearranging electrons around the nucleus without changing the elements involved; nuclear reactions involve changes to the atom's nucleus itself and can convert one element into another.

What is radioactive decay?

The spontaneous process by which an unstable atomic nucleus loses energy by emitting radiation to reach a more stable state.

Name the three main types of radiation and state their electrical charges.

1) Alpha: Positive charge (+2), 2) Beta: Negative charge (-1), 3) Gamma: Neutral / No charge (0)

Why are gamma rays not deflected when passed between charged electric plates?

Gamma rays are high-energy photons with no electrical charge, so they are not attracted to or repelled by positive or negative electric plates.

What is a physical property?

A characteristic of a substance that can be observed or measured without changing the sample's chemical identity or composition.

What is viscosity, and what happens to a liquid's viscosity when you heat it up?

Viscosity is a liquid's resistance to flow. Heating a liquid decreases its viscosity (causes it to flow faster and more easily).

How is heat conductivity related to electrical conductivity in metals?

Metals that are good conductors of heat are typically also good conductors of electricity because both rely on the free movement of delocalized electrons.

What does it mean if a material is malleable? Name one item that is NOT malleable.

Malleability is the ability of a material to be hammered, rolled, or pressed into thin sheets without shattering. Non-malleable item example: Glass (or ceramic/dry pasta, which shatters under pressure).

How can you test which of two materials is harder?

Perform a scratch test: drag one material across another to see which one leaves a scratch (the harder material will scratch the softer material).

Which mineral has a score of 10 on the Mohs hardness scale, and what materials are even harder than it?

Diamond has a rating of 10. Materials harder than natural diamond include aggregated diamond nanorods, wurtzite boron nitride, and lonsdaleite.

What are the melting point and boiling point of water in degrees Celsius?

Melting point = 0°C | Boiling point = 100°C

What is the formula for density, and how can density help you tell if a metal sample is pure?

Formula: Density = Mass / Volume (D = m/V). Because pure metals have a specific, constant density, comparing a measured sample's density to the known theoretical value determines if it is pure or an alloy/adulterated sample.

What is solubility?

The maximum amount of a solute that can dissolve in a given amount of solvent at a specific temperature and pressure.

What are three reasons scientists use physical properties?

1) To identify unknown substances, 2) to choose appropriate materials for specific uses/products, and 3) to separate mixtures.

How does filtration separate different parts of a mixture?

Separates a mixture based on particle size by passing a liquid-solid mixture through a porous barrier (like filter paper), trapping insoluble solids while allowing liquid to pass through.

How does crystallization separate a solid that is dissolved in a liquid?

Separates a dissolved solid from a liquid by evaporating the liquid solvent until the solute precipitates out as pure solid crystals.

How does distillation separate a mixture made of two liquids?

Separates a mixture of two or more liquids based on differences in their boiling points by heating the mixture and condensing the vapor of the liquid with the lower boiling point.

What is a physical change? Give one simple example of a phase change.

A physical change alters the form or appearance of a material without changing its chemical composition. Phase change example: Ice melting into liquid water.

What is a chemical property, and when can you actually see it?

A chemical property is the ability or inability of a substance to combine with or change into one or more other substances. It can only be observed when the substance undergoes a chemical change/reaction.

What are flammability and reactivity?

Flammability is the ease with which a material ignites and burns in the presence of oxygen. Reactivity is the speed and ease with which a substance chemically combines with other substances.

What are four signs that a chemical change has taken place?

1) Unexplained color change, 2) formation of a gas (bubbling/fizzing), 3) formation of a precipitate (solid settling out of solution), and 4) energy/heat/light release or absorption.

What does the Law of Conservation of Mass state?

Mass is neither created nor destroyed during a chemical reaction; total mass of reactants equals total mass of products.

What simple equation shows the mass relationship in a chemical reaction?

Mass of Reactants = Mass of Products

Why do we need the Law of Conservation of Mass to balance chemical equations?

Equations must be balanced because atoms cannot appear or disappear out of nowhere during a reaction (satisfying the Law of Conservation of Mass so that every atom present before the reaction is accounted for after).

What criteria are used to classify materials as solids, liquids, or gases?

Materials are classified based on whether their shape is definite or variable, whether their volume is definite or variable, and the kinetic energy/arrangement of their particles.

List the four most familiar states of matter, along with the fifth state that exists at extremely cold temperatures.

Four main states: Solid, Liquid, Gas, Plasma. Fifth state at near absolute zero: Bose-Einstein Condensate (BEC).

Compare solids, liquids, and gases in terms of whether their shape and volume are definite or variable.

Solid: Definite shape, definite volume. | Liquid: Variable shape, definite volume. | Gas: Variable shape, variable volume.

Rank the main states of matter in order of increasing particle energy levels.

Bose-Einstein Condensate < Solid < Liquid < Gas < Plasma.

What is plasma, what types of particles make it up, and where is it most commonly found in the universe?

Plasma is a high-energy, ionized gas composed of positively charged ions and free electrons. It is the most common state of matter in the universe (found in stars, lightning, and neon signs).

At what temperature does a Bose-Einstein Condensate (BEC) form, and how do atoms behave at this threshold?

Forms at temperatures extremely close to Absolute Zero (0 K or -273.15°C). At this threshold, individual atoms lose their separate identity and clump together into a single "super-atom" acting as a single wave.

What is the Greek origin of the word "kinetic," and how is kinetic energy defined?

Comes from the Greek word kinesis (meaning "motion"). Kinetic energy is the energy an object possesses due to its motion.

State the fundamental premise of the Kinetic Theory of Matter.

All matter is composed of tiny particles (atoms and molecules) that are in constant, random motion.

When was the Kinetic Theory of Matter developed, and what was it originally created to explain?

Developed mainly during the mid-to-late 19th century (1800s) to explain the physical behavior, pressures, and temperatures of gases.

How does the speed of particle motion affect an object's kinetic energy?

As particle speed increases, kinetic energy increases (higher temperature means faster-moving particles).

Describe the specific motion of atoms inside a solid object.

Particles in a solid do not move freely; they vibrate in fixed, tightly packed positions.

How do kinetic energy levels and intermolecular attractions interact to keep solids in a fixed structure?

Strong intermolecular attractive forces hold particles tightly together, overcoming their low kinetic energy and keeping them in a rigid, fixed lattice.

Why do liquids have no definite shape while still maintaining a definite volume?

Particles have enough kinetic energy to partially overcome intermolecular forces and slide past each other (no definite shape), but not enough energy to completely separate (maintaining a definite volume).

Why do gases lack both a definite shape and a definite volume?

Gas particles have high kinetic energy that completely overcomes attractive forces, allowing them to spread out freely and fill any container shape or volume.

How does the distance between gas particles compare to the spacing of particles in solids or liquids?

The distance between gas particles is vast compared to their size—much farther apart than in tightly packed solids or closely contacting liquids.

Define a physical property of matter.

A characteristic that can be observed or measured without altering the chemical composition of the substance.

List five specific examples of physical properties.

Color, density, melting point, mass, and viscosity.

Explain the difference between an extensive physical property and an intensive physical property.

Extensive properties depend on the amount of matter present. Intensive properties depend only on the type/identity of matter present, not the amount.

Is mass classified as an extensive or intensive property? Explain your choice.

Mass is an extensive property because it directly increases or decreases when you change the amount of matter in the sample.

Why is density considered an intensive physical property rather than an extensive one?

Density is an intensive property because it is a ratio (mass / volume). Doubling the sample size doubles both mass and volume equally, keeping the density value identical regardless of sample size.