UNIT # 3 Introduction Objectives 3.1 Structure of the Atom 3.2 3.3 Arrangement of Electrons 3.4 3.5 Atomic Mass Problems 1 | 2 | 3

# ATOMIC STRUCTURE 3.1 - Structure of the Atom

In Unit 2 (section 2.1) we learned that matter refers to the substances that make up our universe. In this unit, we will learn that all matter on earth and in the universe is composed of atoms. Atoms are the basic blocks of matter. If you are interested in knowing the early chemical discoveries and the atomic theory, take a look at the suggested reading on Dalton's Atomic Theory.

Let's look at what is inside an atom.

There are three subatomic particles that make up all atoms:

The main things we need to know about these three subatomic particles are their charge and mass.

Mass (g)
Mass (amu) Relative Charge Located
proton
1.673 x 10-24
1
+1
at the center of the atom in the nucleus
neutron
1.675 x 10 -24
1
0
at the center of the atom in the nucleus
electron
9.110 x 10 -28
1/1835
-1
outside the nucleus

Points to note:

1. Protons and neutrons are found at the center of atom in the nucleus. Electrons surround the nucleus of the atom.
2. Notice that the mass of the electron is much less, about 1/1835 that of the proton. In other words, it takes about 1835 electrons to equal the mass of one proton. So, most of the mass of the atom is concentrated at the center of the atom in the nucleus.
3. Protons and electrons carry electric charges (positive and negative charges respectively).

 species with like charges REPEL species with unlike charges ATTRACT

Protons repel each other but electrons and protons are attracted to each other.

4. Neutrons have no charge. There is no electrostatic force of attraction associated with neutrons. The mass of the neutron is very close to that of the proton.

Physical size of the atom:

• The diameter of atoms is of the order of 1 Å.
• The diameter of the nuclei is of the order of 10-4 Å.

The atom is made up of mostly empty space in which electrons move. Electrostatic attraction exists between the electrons and protons and keeps the negatively electrons attracted to the positively charged nucleus.

An atom is electrically neutral. Since the relative charge of the proton is +1 and the relative charge of the electron is -1, there MUST BE equal number of electrons and protons in an atom.

Our next task is to learn how to:

1. count the number of protons in an atom (Section 3.1.1).
2. count the number of electrons in an atom (Section 3.1.2).
3. count the number of neutrons in an atom (Section 3.1.3).

Section 4.4
Electrons, Protons, and Neutrons. Atomic Number ..p83

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