Atomic Models, Structure, Isotopes & Electron Configuration

Unit 4: Atoms and Their Models

UNIT 4

Atoms are the basis of all the structures and organisms in the universe.

2. Models of the Atom

A model is a representation of a system in the real world. Models help us to understand systems and their properties.

2.1 Dalton's Model of the Atom

John Dalton proposed that all matter is composed of very small things which he called atoms.

2.2 Thomson's Model of the Atom

In this model, the atom is made up of negative electrons that float in a soup of positive charge, much like plums in a pudding or raisins in a fruit cake.

2.3 Rutherford's Model of the Atom

His new model described the atom as a tiny, dense, positively charged core called a nucleus surrounded by lighter, negatively charged electrons.

2.4 Bohr's Model of the Atom

Atoms only emit light at certain wavelengths or frequencies.

James Chadwick

In 1932 James Chadwick discovered the neutron and measured its mass.

3. Atomic Mass and Diameter

It is difficult to imagine the size of an atom, or its mass, because we cannot see an atom and also because we do not use very small measurements.

3.1 How Heavy Is an Atom?

Scientists use a different unit of mass when they are describing the mass of an atom. This unit is called the atomic mass unit, u.

4. Structure of the Atom

• The electron carries one unit of negative electric charge.
• The nucleus can be broken up into smaller building blocks called protons (+) and neutrons.
• The proton has positive electric charge.
• The neutron is electrically neutral.

5. Atomic Number and Atomic Mass Number

Atomic number (Z): the number of protons in an atom.

Atomic mass number (A): the number of protons and neutrons in the nucleus of an atom.

The atomic number (Z) and the mass number (A) are indicated using a standard notation, e.g.: carbon is written as ¹²₆C.

6. Isotopes

The chemical properties of an element depend on the number of protons and electrons inside the atom. So if a neutron or two is added or removed from the nucleus, the chemical properties will not change. This means that such an atom would remain in the same place in the periodic table.

Isotope: isotopes of an element have the same number of protons (same Z), but a different number of neutrons (different N).

7. Relative Atomic Mass

The relative atomic mass of an element is the average mass of all the naturally occurring isotopes of that element. The units for relative atomic mass are atomic mass units (u).

8. Electronic Configuration

The distribution of electrons into different orbits of an atom was suggested by Bohr and Bury. The following rules are followed for writing the number of electrons in different energy levels or shells.