Chapter 18 Structure of an Atom

Section 1 Structure of an Atom Scientists have developed their own shorthand for dealing with long, complicated names = chemical symbols which is a short or abbreviated way to write the name of an element. The symbols have a capital letter by itself or with one or two lower case letters Argentum is Latin for “silver” = Ag Some are named for scientists, for places, or for their properties. See/memorize chart on pg. 544 Because scientists worldwide use this system, everyone understands what the symbols mean.

Atomic Components An element is matter that is composed of one type of atom, which is the smallest piece of matter that still retains the property of the element. Atoms are composed of particles called protons, neutrons, and electrons. Proton=positive charge Neutron=neutral charge Electron=negative charge

Quarks Protons and neutrons are composed of smaller components called quarks. So far, we know of six quarks. To study quarks, scientists accelerate charged particles to tremendous speeds and then force them to collide with protons. Then they view the tracks they leave behind for evidence of what the inner structure of atoms looks like. Sixth quark is called “top” quark.

Atomic Models Scientists and engineers use models to represent things that are to small or difficult to visualize. Atoms are so small that it would take 24,400 atoms stacked on top of each other to equal the thickness of a sheet of aluminum foil. The atomic model has changed over time because technology continues to get better. 400 BC Democritus proposed the idea that atoms make up all substances. Aristotle, a famous Greek philosopher, disputed Democritus’s theory and proposed that matter was uniform throughout and did not have smaller particles. He was wrong but his theory was accepted for 2000 years!

In the1800’s, John Dalton, an English scientist, came up with proof that atoms exist. Dalton’s model = solid sphere Over time more and more data was found and the model changed. Since 1926 we have accepted the electron cloud model. This model has a center nucleus made of protons and neutrons and a cloud with electrons outside the nucleus. The cloud is 100,000 times larger than the diameter of the nucleus. Each electron is much smaller than each proton. The electrons are orbiting the nucleus very fast in random patterns not set paths. Each electron travels at an average distance from the nucleus, depending on its energy level.

Section 2 Masses of Atoms and Atomic Numbers The nucleus contains most of the mass of the atom. Protons and Neutrons mass = 1.6726 X 10 to the negative 24th grams That is 1,836 times greater than that of the electrons. Electron mass = 9,1093 X 10 to the negative 28th grams. To make it easier think of the mass of a proton or a neutron as 1amu. The majority of the mass of an atom is in the nucleus. Protons identify the element. The number of protons tells you what type of atom you have and vice versa. All atoms with six protons are carbon atoms. All atoms with eight protons are oxygen atoms. We call the number of protons the ATOMIC NUMBER!

Mass Number The mass number of an atom is the sum of the number of protons and the number of neutrons in the nucleus of an atom. # of Neutrons =Mass # - Atomic # Carbon 12 is the most common form of carbon but carbon 14 which is radioactive is also present in small quantities.

Isotopes When all the atoms of an element do not have the same number of neutrons we call it an isotope. Example: Boron can have 5 or 6 neutrons depending on its mass number. The avg. atomic mass of an element is the weighted-avg. mass of the mixture of isotopes. For Boron the avg. atomic mass is 10.8amu 4/5(11amu) + 1/5(10amu)=10.8amu Uranium 238 has 92 protons. How many neutrons does it have? Answer: 146 neutrons

Section 3 The Periodic Table Periodic means “repeated in a pattern”. The calendar is a periodic table of days and months. Dmitri Mendeleev, a Russian chemist, searched for ways in the 1800’s to organize the elements. Today’s periodic table of elements are arranged by increasing atomic number and by changes in physical and chemical properties. Medeleev left blanks in his chart for elements not yet discovered. His predictions were later proven correct!

Mendeleev’s table was arranged by atomic mass that gradually increased from left to right. Today’s table is arranged by atomic number thanks to the work of Henry Moseley, an English scientist.

The Atom and the Periodic Table The vertical columns in the periodic table are called groups or families, and are numbered 1-18. Elements in each group have similar properties. Exp. Group #11 includes copper, silver, and gold which are all shiny metals and are good conductors of electricity and heat.

Electron Cloud Structure Energy levels in atoms can be represented by a flight of stairs. Each stair step away from the nucleus represents an increase in the amount of energy within the electrons. The higher energy levels contain more electrons. Elements that are in the same group have the same number of electrons in their outer energy level. That outer level is what determines the chemical properties of the element. See Fig. 10 pg. 558

Energy Levels Energy level #1 can contain a max of two electrons. Energy level #2 can contain a max of eight electrons. The outer level is complete when it reaches eight electrons. The elements in group #1 all have one electron in the outer energy level. The elements in group #17 have seven electrons in the outer level. Thus the elements in group #1 and group #17 react easily with one another.

Group #17 are the halogens. Sodium in group #1 reacts well with chlorine in group #17 and the result is the compound sodium chloride, or NaCl-ordinary table salt. Not all elements will combine readily with other elements. Group #18 elements have complete outer energy levels so they are unreactive.

Regions on the Periodic Table The horizontal rows of the elements on the periodic table are called periods. The elements increases by one proton and one electron as you go from left to right in a period. The blue squares are the metals. Most are solid at room temperature. Exp. iron, zinc, and copper The right side of the table are the yellow squares which are the non-metals. Exp. oxygen, bromine, and carbon The green are metalloids or semimetals.