Chapter 4 Atomic Structure

Review Chapter 4

Objectives Describe the experiments of Thomson, Millikan, Rutherford, Chadwick and Bohr and explain how they contributed to our present understanding of atomic structure. Describe the mass and charge differences among electrons, protons and neutrons. Define atomic number, mass number and isotopes in terms of the number of electrons, protons and neutrons. Determine the atomic mass of an element from the percent abundance of its isotopes, or the abundance from its atomic mass.

The Modern Atom Today we define the atom as the smallest particle of an element that retains the chemical properties of that element. All atoms consist of two regions: The nucleus is a very small region located near the center of an atom and contains positively charged particles called protons.

Surrounding the nucleus is a region occupied by negatively charged particles called electrons. This region is very large compared to the size of the nucleus. Protons, electrons and neutrons are called subatomic particles.

The Electron

Robert Millikan Robert Millikan was an American physicist. In 1909 conducted experiments to prove the electron existed. Oil drop experiment. Determined the charge of a single electron. (1.592 × 10−19 coulomb)

Robert Millikan The electron does carry a negative charge. The mass of an electron is very small. (9.1 x 10-31 Kg) Because atoms are electrically neutral, they must contain a positive charge to balance the negative charge. Because of the low mass of electrons, atoms must contain other particles that account for most of the mass.

Robert Millikan

The Nucleus

Neutron In 1932 the English physicist James Chadwick discovered the neutron. The neuron is found in the nucleus of the atom along with the proton. A neutron has no charge, yet has the same relative mass as a proton.

Model of the Atom

Bohr Model While the Bohr model works well for hydrogen, it does have its limitations: It did not work well with atoms with more than one electron. It does not account for electron-electron repulsions. Additional electron-nucleus interactions present problems.

Atomic Nucleus

Atomic Nucleus Except for the hydrogen atom, all atomic nuclei are made up of two kinds of particles, protons and neutrons. A proton has a positive charge equal in magnitude to the negative charge of an electron. Atoms are electrically neutral. A neutron which has no charge.

Atomic Number Atomic Number (Z) – represents the number of protons in the nucleus of each atom of that element. Atoms of the same element have the same number of protons. Atoms of different elements have different number of protons.

Atomic Number On the periodic table, the elements are placed in order of increasing atomic number. At the top left of the table is hydrogen, H, which has an atomic number of 1. All atoms of the element hydrogen have one proton in the nucleus.

Next is helium, which has two protons, and therefore an atomic number of two. This is followed by lithium with an atomic number of three – three proton in the nucleus. The atomic number for each element is listed on the periodic table above the elements symbol.

# of protons = # of electrons The atomic number identifies an element. Because all atoms are neutral (no charge), we know the atomic number is equal to the number of electrons. # of protons = # of electrons Example: Silver (Ag) has an atomic number of 47. Therefore, silver has 47 protons and 47 electrons in each silver atom.

Classwork Example problem 4.1 Page 116 Complete the chart

Isotopes Isotopes – are atoms of the same element that have a different number of neutrons. The number of protons is the same among isotopes.

Isotopes The simplest atoms are those of hydrogen. All hydrogen atoms contain one proton. However, hydrogen atoms can contain different number of neutrons. Three types of hydrogen atoms are known with a neutron count of 0, 1 or 2.

Mass Number Mass Number – represents the total number of protons and neutrons in the nucleus of an isotope. Identifying an isotope requires knowing both the atomic number and mass number of the element.

Mass Number Calculating the mass number: Tritium: Mass number = atomic number + number of neutrons Tritium: Atomic # = 1 (1 proton) # of neutrons = 2 Mass number = 3

Isotopes Example: Uranium-235 contains 92 protons and 143 neutrons. Mass number = atomic number + number of neutrons 235 (protons + neutrons) = 92 protons + 143 neutrons

Homework Worksheet: Atomic Number (B7) or Element list with atomic number

hydrogen-3 represents tritium Isotopes Isotopes of an element are usually identified by specifying their mass number. There are two common methods: The mass number is written with a hyphen after the name of the element. hydrogen-3 represents tritium

hydrogen-3 represents tritium is written as: Isotopes 2) The second method shows the elements symbol written with the atomic and mass numbers. hydrogen-3 represents tritium is written as: H 3 1

Isotopes Examples of hydrogen and helium:

Sample Problem How many protons, electrons and neutrons are there in an atom of chlorine-37?

Sample Problems Sample Problem 4-2, page 118 Classwork:

Classwork Page 129 Problems 66, 68, 72, 74 and 75

Homework Worksheet 1 Due: