Chapter 4: Atoms and Elements

4.1: Elements and Symbols

• Elements
  • These are pure substances from which all other things are built.
  • These cannot be broken down into simpler substances.
• Chemical Symbols
  • These are one- or two-letter abbreviations for the names of the elements.
  • Only the first letter of an element’s symbol is capitalized.
  • If the symbol has a second letter, it is lowercase so that we know when a different element is indicated.
  • If two letters are capitalized, they represent the symbols of two different elements.

4.2: The Periodic Table

• Periodic Table: The arrangement of 118 elements.

• Period: Each horizontal row in the periodic table.

• Group: Each vertical column on the periodic table.

• Representative Elements: These have group numbers 1A to 8A.

• Transition Elements: These are block elements in the center of a periodic table that have group numbers followed by the letter ‘B’.

• The two rows of 14 elements called the lanthanides and actinides, which are part of Periods 6 and 7, are placed at the bottom of the periodic table to allow them to fit on a page.

• Group 1A: These are alkali metals except for Hydrogen.

• Group 2A: These are alkaline earth metals; these are shiny metals but not as reactive.

• Group 7A: These are halogens; highly reactive and form compounds with most of the elements.

• Group 8A: These are noble gases; they are quite unreactive and are seldom found in combination with other elements.

• Metals: These are shiny solids, such as copper, gold, and silver. It can be shaped into wires or hammered into a flat sheet.

  • They are good conductors of heat and electricity. They usually heat at a higher temperature than nonmetals.

• Nonmetals: They are often poor conductors of heat and electricity.

  • They have low melting points and low densities.

• Metalloids: These are elements that exhibit some properties that are typical of metals and other properties that are characteristic of nonmetals.

  • These are semiconductors because they can be modified to function as conductors or insulators.

  

4.3: The Atom

• Atom: The smallest unit particle of an element that retains the characteristics of that element.
• Subatomic Particles: These are even smaller bits of matter.
• Cathode Rays: These are streams of small particles produced when electricity is applied to a glass tube.
• Protons: These are the atom’s positively charged particles.
• Electrons: These are the atom’s negatively charged particles.
• Neutrons: These are the atom’s neutral particles.
• Nucleus: The center of the atom.
• Atomic Mass Unit: A very small unit of mass.
  • It is defined as one-twelfth of the mass of a carbon atom which has a nucleus containing 6 protons and 6 neutrons.
• Dalton: The atomic mass unit.

Dalton’s Atomic Theory

1. All matter is made up of tiny particles called atoms.
2. All atoms of a given element are similar to one another and different from atoms of other elements.
3. Atoms of two or more different elements combine to form compounds. A particular compound is always made up of the same kinds of atoms and always has the same number of each kind of atom.
4. A chemical reaction involves the rearrangement, separation, or combination of atoms. Atoms are never created or destroyed during a chemical reaction.

4.4: Atomic Number and Mass Number

• Atomic Number: This is equal to the number of protons in every atom of that element.
  • It is the whole number that appears above the symbol of each element on the periodic table.
  • Atomic number = number of protons in an atom
• Mass Number: The total number of protons and neutrons in its nucleus.
  • It does not appear on the periodic table because it applies to single atoms only.
  • Mass number = number of protons + number of neutrons
  • If we are given the mass number of an atom and its atomic number, we can calculate the number of neutrons in its nucleus.
  • Number of neutrons in a nucleus = mass number - number of protons

4.5: Isotopes and Atomic Mass

• Isotopes: These are atoms of the same element that have the same atomic number but different numbers of neutrons.
• Atomic Symbol: It indicates the mass number in the upper left corner and the atomic number in the lower left corner.
• Because each isotope has a different mass, chemists have calculated an atomic mass for an “average atom,” which is a weighted average of the masses of all the naturally occurring isotopes of that element.

4.6: Electron and Energy Levels

• Electromagnetic spectrum: It shows the arrangement of different types of electromagnetic radiation in order of increasing energy.
• When the light from the Sun passes through a prism, the light separates into a continuous color spectrum, which consists of the colors we see in a rainbow.
• When light from a heated element passes through a prism, it separates into distinct lines of color separated by dark areas called an atomic spectrum.
• In an atom, each electron has a specific energy known as its energy level, which is assigned values called principal quantum numbers.
• An electron can change from one energy level to a higher level only if it absorbs the energy equal to the difference in energy levels.
  • When an electron changes to a lower energy level, it emits energy equal to the difference between the two levels.
  • If the energy emitted is in the visible range, we see one of the colors of visible light.
• The electron arrangement of an atom gives the number of electrons in each energy level.

4.7: Trends in Periodic Properties

• Period Properties: These consists of the valence electrons in atoms, the trends in atomic size, ionization energy, and metallic character.
• Valence Electrons: These are the electrons in the outermost energy level.
• Electron-Dot Symbol: Also known as a Lewis Structure, represents the valence electrons as dots that are placed on the sides, top, or bottom of the symbol for the element.
• Ionization Energy: A quantity of energy that is required to remove one of the outermost electrons.
• Cation: A positive particle.
• Anion: A negative particle.
• The ionization energy decreases going down a group.
• Going across a period from left to right, the ionization energy increases.
• Metallic Character: An element that loses valence electrons easily.
  • It is more prevalent in the elements on the left side of the periodic table and decreases going from left to right across a period.
  • The elements on the right side of the periodic table do not easily lose electrons, which means they are the least metallic.
  • Atoms at the bottom of any group have more electron levels, which makes it easier to lose electrons