Atoms: Building Blocks of Matter Physical Science Chapter 4

An Atomic Model of Matter Scientists have tried to determine the composition of matter for 2400 years using various methods Indirect evidence Evidence gathered without actually seeing or touching the object Model A model is a representation of evidence & ideas that explains facts or a concept

The Greek Model Democritus devised a model with the following ideas: Matter could not forever be divided into smaller pieces. The smallest piece possible was an atom Atoms were all made of the same material but different sizes and shapes Atoms are infinite in number, constantly moving and joining together

Dalton’s Model John Dalton, an English chemist, conducted many experiments and came up with the following theory: All elements are composed of atoms. Atoms are indivisible & indestructible Atoms of the same element are alike Atoms of different elements are different Cmpnds are formed by the joining of elemental atoms

Thomson’s Model J.J. Thomson conducted experiments (passing electric current thru gas, causing negatively charged particles to escape). …..led him to believe atoms were made of smaller particles. Thomson’s Model: The atom is made up of pudding like positively charged material in which negatively charged electrons are scattered like plums in a pudding (or chips in a cookie).

Rutherford’s Model Ernest Rutherford experimented with shooting positively charged rays through gold foil. Some of the positive charges bounced off the gold foil rather than passing through it. He developed a new theory on the structure of the atom. Rutherford’s theory stated: The atom has a small, dense positively charged center. Negative electrons scattered around nucleus on edges of atom.

The Bohr Model Niels Bohr, a Danish scientist, proposed & tested that negatively charged e- were held by attraction to the + nucleus. Bohr’s Model stated: Electrons moved in orbits around the nucleus. These orbits were at certain distances from the nucleus depending upon the energy of the electrons.

The Wave Model Today’s atomic model is based on wave mechanics. The wave model states: Electrons do not travel around the nucleus in a specific path, but can be found in an area called the electron cloud. An atom has a small positively charged nucleus surrounded by a large region in which enough electrons are located to make the atom neutral.

Structure of an Atom An atom is made up of 3 subatomic particles: protons, neutrons, & electrons which have consistent structure in all atoms. Protons: positively charged particles in the nucleus Neutrons: neutrally charged particles in the nucleus Electrons: negatively charged particles surrounding the nucleus Neutral atoms have equal #s of protons & electrons, so that positive and negative charges balance.

Subatomic particle masses The mass of both protons & neutrons is one amu (atomic mass unit) Electrons have almost no mass.

Atomic Number The difference between elements is the number of protons they have in their nucleus. The number of protons in the nucleus is called the atomic number. The number of protons in an element always stays the same, however the number of neutrons can vary.

Mass Number and Atomic Mass All atoms have a mass number. The mass number is equal to the number of protons + the number of neutrons. Any sample of an element that occurs in nature contains a mixtures of isotopes. Therefore, the atomic mass is an average of the atomic masses of all the existing isotopes of a particular element. An element with a different number of neutrons (but same proton #) is called an isotope.

Electron Cloud The electron cloud is the area around the nucleus in which the electrons can be found. Electrons whirl around the nucleus billions of time in a second, however their movement is not random. Electrons are locked into an orbit a specific distance from the nucleus depending on how much energy they have.

Energy Levels Levels farther from the nucleus have more energy Distance an electron is from the nucleus is called it’s energy level. Each energy level can hold a specific number of electrons. The arrangement of the electrons in the energy levels accounts for how an atom bonds with other atoms. Levels farther from the nucleus have more energy

Arranging the Elements Russian chemist Dmitri Mendeleev was the first scientist to arrange the elements in a logical manner. He classified the elements according to chemical and physical properties & then put them in order of increasing atomic mass. This led to a table of elements that increased by atomic mass and were arranged in columns of similar characteristics.

Predictions Mendeleev found that there were holes in his periodic table of elements. Mendeleev predicted that elements would be discovered that would fit into the holes in his table of elements. Within his lifetime, three of the elements that Mendeleev predicted were discovered.

The Modern Periodic Table Mendeleev’s table had some problems. Some elements did not fit the characteristics of the columns where they were located. Mendeleev’s table was refined by Henry Moseley after the discovery of atomic particles. Moseley arranged the elements according to atomic number (# of protons) instead of atomic mass (protons + neutrons).

Periodic Law Moseley’s table was based on periodic law, which states, “the physical and chemical properties of the elements are a function of their atomic number.”

Periodic Table Design The periodic table is a classification system that organizes the elements in a logical, usable and meaningful way. The periodic table is designed so that it is easy to predict an element’s physical and chemical properties.

Columns of the Periodic Table The columns in the periodic table represent groups or families of elements. Families of elements have similar chemical and physical properties. BECAUSE each atom of these elements have the same number of electrons in the outer shell (valence).

Rows in the Periodic Table Each horizontal row in the periodic table represents a period. Elements in a period do not have similar properties. Elements in a period always increase by one proton or one electron from left to right. There are seven periods of elements. The rare earth elements are pulled out of their rows, however and are listed below the table.

Element Key Important information about the elements is given in each square of the periodic table including: Atomic number Chemical Symbol Name Atomic Mass Practice using the Periodic Table

Forces within the Atom not in ch 4, but impt The atom is held together by four forces. The four forces account for the behavior of the subatomic particles. The four forces are: Electromagnetic Force Strong Force Weak Force Gravity

Electromagnetic Force The electromagnetic force can either attract or repel particles. Particles with the same charge are repelled. Particles with different charges are attracted. Electrons are kept orbiting the nucleus cause they are attracted to positively charged protons. Protons in nucleus, however, are repelled by each other.

Strong Force The strong force glues protons (and neutrons) together to form the nucleus. The strong force only works when protons are close together. The strong force is the strongest of the four forces.

Weak Force The weak force is responsible for the process of radioactive decay. The weak force allows a neutron in the nucleus to change into 1 proton and 1 electron. Radioactive decay is what powers the sun.

Gravity Gravity is the force of attraction between all objects that have mass. The effect of gravity on the nucleus is not fully understood and is the weakest of the four forces.