3.1a The modern model of the atom has evolved over a long period of time through the

work of many scientists.

3.1b Each atom has a nucleus, with an overall positive charge, surrounded by

negatively charged electrons.

3.1c Subatomic particles contained in the nucleus include protons and neutrons.

3.1d The proton is positively charged, and the neutron has no charge. The electron is

negatively charged.

3.1e Protons and electrons have equal but opposite charges. The number of protons

equals the number of electrons in an atom.

3.1f The mass of each proton and each neutron is approximately equal to one atomic

mass unit. An electron is much less massive than a proton or a neutron.

3.1g The number of protons in an atom (atomic number) identifies the element. The sum

of the protons and neutrons in an atom (mass number) identifies an isotope. Common

notations that represent isotopes include:




C, carbon-14, C-14. 6

3.1h In the wave-mechanical model (electron cloud model) the electrons are in orbitals,

which are defined as the regions of the most probable electron location (ground state).

3.1i Each electron in an atom has its own distinct amount of energy.

3.1j When an electron in an atom gains a specific amount of energy, the electron is at a

higher energy state (excited state).

3.1k When an electron returns from a higher energy state to a lower energy state, a

specific amount of energy is emitted. This emitted energy can be used to identify an


3.1l The outermost electrons in an atom are called the valence electrons. In general, the

number of valence electrons affects the chemical properties of an element.

3.1m Atoms of an element that contain the same number of protons but a different num-ber

of neutrons are called isotopes of that element.

3.1n The average atomic mass of an element is the weighted average of the masses of

its naturally occurring isotopes.

3.1o Stability of an isotope is based on the ratio of neutrons and protons in its nucleus.

Although most nuclei are stable, some are unstable and spontaneously decay, emitting


3.1p Spontaneous decay can involve the release of alpha particles, beta particles,

positrons, and/or gamma radiation from the nucleus of an unstable isotope. These

emissions differ in mass, charge, ionizing power, and penetrating power.

3.1q Matter is classified as a pure substance or as a mixture of substances.

3.1r A pure substance (element or compound) has a constant composition and constant

properties throughout a given sample, and from sample to sample.

3.1s Mixtures are composed of two or more different substances that can be separated

by physical means. When different substances are mixed together, a homogeneous or

heterogeneous mixture is formed.

3.1t The proportions of components in a mixture can be varied. Each component in a

mixture retains its original properties.

3.1u Elements are substances that are composed of atoms that have the same atomic

number. Elements cannot be broken down by chemical change.

3.1v Elements can be classified by their properties and located on the Periodic Table as

metals, nonmetals, metalloids (B, Si, Ge, As, Sb, Te), and noble gases.

3.1w Elements can be differentiated by physical properties. Physical properties of sub-stances,

such as density, conductivity, malleability, solubility, and hardness, differ

among elements.

3.1x Elements can also be differentiated by chemical properties. Chemical properties

describe how an element behaves during a chemical reaction.

3.1y The placement or location of an element on the Periodic Table gives an indication

of the physical and chemical properties of that element. The elements on the Periodic

Table are arranged in order of increasing atomic number.

3.1z For Groups 1, 2, and 13-18 on the Periodic Table, elements within the same group

have the same number of valence electrons (helium is an exception) and therefore simi-lar

chemical properties.

3.1aaThe succession of elements within the same group demonstrates characteristic

trends: differences in atomic radius, ionic radius, electronegativity, first ionization

energy, metallic/nonmetallic properties.

3.1bb The succession of elements across the same period demonstrates characteristic

trends: differences in atomic radius, ionic radius, electronegativity, first ionization

energy, metallic/nonmetallic properties.

3.1cc A compound is a substance composed of two or more different elements that are

chemically combined in a fixed proportion. A chemical compound can be broken down

by chemical means. A chemical compound can be represented by a specific chemical

formula and assigned a name based on the IUPAC system.

3.1dd Compounds can be differentiated by their physical and chemical properties.

3.1eeTypes of chemical formulas include empirical, molecular, and structural.

3.1ff Organic compounds contain carbon atoms, which bond to one another in chains,

rings, and networks to form a variety of structures. Organic compounds can be named

using the IUPAC system.

3.1gg Hydrocarbons are compounds that contain only carbon and hydrogen. Saturated

hydrocarbons contain only single carbon-carbon bonds. Unsaturated hydrocarbons

contain at least one multiple carbon-carbon bond.

3.1hh Organic acids, alcohols, esters, aldehydes, ketones, ethers, halides, amines,

amides, and amino acids are categories of organic compounds that differ in their struc-tures.

Functional groups impart distinctive physical and chemical properties to organic


3.1ii Isomers of organic compounds have the same molecular formula, but different

structures and properties.

3.1jj The structure and arrangement of particles and their interactions determine the

physical state of a substance at a given temperature and pressure.

3.1kkThe three phases of matter (solids, liquids, and gases) have different properties.

3.1ll Entropy is a measure of the randomness or disorder of a system. A system with

greater disorder has greater entropy.

3.1mm Systems in nature tend to undergo changes toward lower energy and higher


3.1nnDifferences in properties such as density, particle size, molecular polarity, boiling

and freezing points, and solubility permit physical separation of the components of the


3.1oo A solution is a homogeneous mixture of a solute dissolved in a solvent. The solu-bility of a solute in a given amount of solvent is dependent on the temperature, the

pressure, and the chemical natures of the solute and solvent.

3.1ppThe concentration of a solution may be expressed in molarity (M), percent by vol-ume,percent by mass, or parts per million (ppm).

3.1qqThe addition of a nonvolatile solute to a solvent causes the boiling point of the sol-vent

to increase and the freezing point of the solvent to decrease. The greater the concentration

of solute particles, the greater the effect.

3.1rr An electrolyte is a substance which, when dissolved in water, forms a solution

capable of conducting an electric current. The ability of a solution to conduct an electric

current depends on the concentration of ions.

3.1ss The acidity or alkalinity of an aqueous solution can be measured by its pH value.

The relative level of acidity or alkalinity of these solutions can be shown by using


3.1tt On the pH scale, each decrease of one unit of pH represents a tenfold increase in

hydronium ion concentration.

3.1uuBehavior of many acids and bases can be explained by the Arrhenius theory.

Arrhenius acids and bases are electrolytes.

3.1vvArrhenius acids yield H+(aq) (hydrogen ion) as the only positive ion in an aque-ous

solution. The hydrogen ion may also be written as H 3 O + (aq), hydronium ion.

3.1ww Arrhenius bases yield OH-(aq), hydroxide ion as the only negative ion in an

aqueous solution.

3.1xx In the process of neutralization, an Arrhenius acid and an Arrhenius base react to

form a salt and water.

3.1yy There are alternate acid-base theories. One theory states that an acid is an H +

donor and a base is an H + acceptor.

3.1zz Titration is a laboratory process in which a volume of a solution of known

concentration is used to determine the concentration of another solution.