The modern model of the atom has evolved over a long period of time through the work of many scientists.
Each atom has a nucleus, with an overall positive charge, surrounded by negatively charged electrons.
Subatomic particles contained in the nucleus include protons and neutrons.
The proton is positively charged, and the neutron has no charge. The electron is negatively charged.
Protons and electrons have equal but opposite charges. The number of protons equals the number of electrons in an atom.
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.
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: 14 C, 14 C, carbon-14, C-14. 6
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).
Each electron in an atom has its own distinct amount of energy.
When an electron in an atom gains a specific amount of energy, the electron is at a higher energy state (excited state).
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 element.
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.
Atoms of an element that contain the same number of protons but a different number of neutrons are called isotopes of that element.
The average atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes.
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 radiation.
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.
Matter is classified as a pure substance or as a mixture of substances.
A pure substance (element or compound) has a constant composition and constant properties throughout a given sample, and from sample to sample.
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.
The proportions of components in a mixture can be varied. Each component in a mixture retains its original properties.
Elements are substances that are composed of atoms that have the same atomic number. Elements cannot be broken down by chemical change.
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.
Elements can be differentiated by physical properties. Physical properties of substances, such as density, conductivity, malleability, solubility, and hardness, differ among elements.
Elements can also be differentiated by chemical properties. Chemical properties describe how an element behaves during a chemical reaction.
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.
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 similar chemical properties.
The succession of elements within the same group demonstrates characteristic trends: differences in atomic radius, ionic radius, electronegativity, first ionization energy, metallic/ nonmetallic properties.
The succession of elements across the same period demonstrates characteristic trends: differences in atomic radius, ionic radius, electronegativity, first ionization energy, metallic/ nonmetallic properties.
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.
Compounds can be differentiated by their physical and chemical properties.
Types of chemical formulas include empirical, molecular, and structural.
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.
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.
Organic acids, alcohols, esters, aldehydes, ketones, ethers, halides, amines, amides, and amino acids are categories of organic compounds that differ in their structures. Functional groups impart distinctive physical and chemical properties to organic compounds.
Isomers of organic compounds have the same molecular formula, but different structures and properties.
The structure and arrangement of particles and their interactions determine the physical state of a substance at a given temperature and pressure.
The three phases of matter (solids, liquids, and gases) have different properties.
Entropy is a measure of the randomness or disorder of a system. A system with greater disorder has greater entropy.
Systems in nature tend to undergo changes toward lower energy and higher entropy.
Differences in properties such as density, particle size, molecular polarity, boiling and freezing points, and solubility permit physical separation of the components of the mixture.
A solution is a homogeneous mixture of a solute dissolved in a solvent. The solubility 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.
The concentration of a solution may be expressed in molarity (M), percent by volume, percent by mass, or parts per million (ppm).
The addition of a nonvolatile solute to a solvent causes the boiling point of the solvent to increase and the freezing point of the solvent to decrease. The greater the concentration of solute particles, the greater the effect.
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.
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 indicators.
On the pH scale, each decrease of one unit of pH represents a tenfold increase in hydronium ion concentration.
A physical change results in the rearrangement of existing particles in a substance. A chemical change results in the formation of different substances with changed properties.
Types of chemical reactions include synthesis, decomposition, single replacement, and double replacement.
Types of organic reactions include addition, substitution, polymerization, esterification, fermentation, saponification, and combustion.
An oxidation-reduction (redox) reaction involves the transfer of electrons (e -).
Reduction is the gain of electrons.
A half-reaction can be written to represent reduction.
Oxidation is the loss of electrons.
A half-reaction can be written to represent oxidation.
Oxidation numbers (states) can be assigned to atoms and ions. Changes in oxidation numbers indicate that oxidation and reduction have occurred.
Heat is a transfer of energy (usually thermal energy) from a body of higher temperature to a body of lower temperature. Thermal energy is the energy associated with the random motion of atoms and molecules.
Temperature is a measurement of the average kinetic energy of the particles in a sample of material. Temperature is not a form of energy.
The concepts of kinetic and potential energy can be used to explain physical processes that include: fusion (melting), solidification (freezing), vaporization (boiling, evaporation), condensation, sublimation, and deposition.
transferred from one atom to another (ionic)
shared between atoms (covalent)
mobile within a metal (metallic)
Atoms attain a stable valence electron configuration by bonding with other atoms. Noble gases have stable valence configurations and tend not to bond.
When an atom gains one or more electrons, it becomes a negative ion and its radius increases. When an atom loses one or more electrons, it becomes a positive ion and its radius decreases.
Electron-dot diagrams (Lewis structures) can represent the valence electron arrangement in elements, compounds, and ions.
In a multiple covalent bond, more than one pair of electrons are shared between two atoms. Unsaturated organic compounds contain at least one double or triple bond.
Some elements exist in two or more forms in the same phase. These forms differ in their molecular or crystal structure, and hence in their properties.
Two major categories of compounds are ionic and molecular (covalent) compounds.
Metals tend to react with nonmetals to form ionic compounds. Nonmetals tend to react with other nonmetals to form molecular (covalent) compounds. Ionic compounds containing polyatomic ions have both ionic and covalent bonding.
When a bond is broken, energy is absorbed. When a bond is formed, energy is released.
Electronegativity indicates how strongly an atom of an element attracts electrons in a chemical bond. Electronegativity values are assigned according to arbitrary scales.
The electronegativity difference between two bonded atoms is used to assess the degree of polarity in the bond.
Molecular polarity can be determined by the shape of the molecule and distribution of charge. Symmetrical (nonpolar) molecules include CO2 , CH4 , and diatomic elements. Asymmetrical (polar) molecules include HCl, NH3 , and H2 O.
Intermolecular forces created by the unequal distribution of charge result in varying degrees of attraction between molecules. Hydrogen bonding is an example of a strong intermolecular force.
Physical properties of substances can be explained in terms of chemical bonds and intermolecular forces. These properties include conductivity, malleability, solubility, hardness, melting point, and boiling point.
A change in the nucleus of an atom that converts it from one element to another is called transmutation. This can occur naturally or can be induced by the bombardment of the nucleus with high-energy particles.
Energy released in a nuclear reaction (fission or fusion) comes from the fractional amount of mass that is converted into energy. Nuclear changes convert matter into energy.
Energy released during nuclear reactions is much greater than the energy released during chemical reactions.