Chapter 2 Section 1: Models of the Atom Section 2: The Nucleus Inside the Atom Chapter 2 Section 1: Models of the Atom Section 2: The Nucleus

Models of the Atom Greek philosophers devised a theory of atoms, or tiny particles.

John Dalton Combined the idea of elements with the Greek theory of the atom. Matter is made up of atoms. Atoms cannot be divided into smaller pieces. All atoms of an element are exactly alike. Different elements are made of different atoms. Dalton’s theory was tested by William Crookes and his cathode-ray tube experiment.

J.J. Thompson Discovered negatively charged particles, electrons, which are a part of every atom. Thomson revised Dalton’s model to include a sphere with a positive charge and negatively charged electrons spread evenly within the positive charge. The negatively charged electrons and the positive charge in the sphere neutralized each other.

Earnest Rutherford Tested Thompson’s model, which was found to be an inaccurate model of the atom

An atomic model (Bohr’s model) With a nucleus was developed. The positively charged proton is located in a very small space at the center of an atom. Most of an atom is empty space occupied by nearly massless electrons. Electrically neutral particles, neutrons, are also located in the nucleus. The number of electrons equals the number of protons in an atom.

The Nucleus Atomic number – number of protons in the nucleus of an atom Isotopes of an atom have the same number of protons but different numbers of electrons Mass number is the number of neutrons plus the number of protons Average atomic mass – the average mass of the mixture of an elements isotopes The strong nuclear force holds tightly packed protons together in a nucleus

Electron Cloud Model Explains the unpredictable wave behavior of electrons, which could be anywhere in the area surrounding the nucleus.

Electrons move rapidly around the nucleus in areas called shells. Electron shells Electrons move rapidly around the nucleus in areas called shells.

Each shell can hold up to 8 electrons except the first shell which can only hold 2 electrons The number of the period (row) an element is in, is the same as the number of shells it has

The first shell needs to be full before the second shell gets any electrons. The last, or outer, shell is called the VALENCE SHELL. The electrons in the valence shell are called valence electrons

Electron Shell Diagrams (Also called Bohr diagrams after the man who started them.)

The atomic number tells you the number of protons an element has. In its natural state, each element has the same number of electrons as protons. So: We use the atomic number to know how many electrons we need to draw. Then we start filling up shells from the inside out.

Reminders We write the element symbol in the center. The first shell is full at 2. Every other shell is full at 8. We always draw 4 single electrons before we start to pair them up.