Chapter 3
Atomic Structure

Chapter 3
Section 2
Atomic Structure

Objectives

Describe the experiments of Thomson, Millikan,
Rutherford 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.

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
Determined the charge of an electron.
Determined 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

Chapter 3
Section 3
Atomic Structure

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.

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.

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.

Isotopes
The most common type of hydrogen atom is called protium. It accounts for 99.985% of the hydrogen atoms found on earth.
The nucleus of protium consist of one proton and zero neutrons.

Isotopes
Another type of hydrogen atom is deuterium.
This accounts for 0.015% of hydrogen atoms.
The nucleus of deuterium consist of one proton and one neutron.

Isotopes
The third type of hydrogen atom is tritium. It exists in very small amounts in nature.
The nucleus of tritium consist of one proton and two neutrons.

Isotopes
Isotopes – are atoms of the same element that have different masses due to the varying number of neutrons.
Protium, deuterium and tritium are all isotopes of hydrogen.
The isotopes of an element all have the same number of protons and electrons but different number of neutrons.

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

Mass Numbers for Hydrogen

Homework
Worksheet: Atomic Number (B7)

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

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

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

Isotopes
Examples of hydrogen and helium:

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

Sample Problems
Classwork:
Problems 1-3, page 78

Homework
Page 85 - Section Review
Problems 1 (a-b), 2 (a-d), 3 (a-c),
Page 87 – Reviewing Concepts
Problems 6, 7, 8 and 9 (a-c)
Due: