The Atom and its Structure

1.Structure

Objectives:

- Know the prominent models of an atom.
- Understand the particles that are found in the nucleus and their respective characteristics
- Recognize the particles found outside the nucleus
- Know the characteristics of an atom ( isotopes, representation, mass, charge)

A.Discontinuity of matter

Matter is discontinuous, that is made up of cluster of small particles called atoms. The atom has long been considered as the smallest indivisible unit of matter.

B.Structure of the atom

There are essentially 2 important and prominent models.These are :
1. Thomson and the discontinuity of the electrons

Many experiments performed on metals had provided evidence for the existence of the electrons. Metals can emit electrons efficiently. When subjected to a large potential, a metal emits a beam of particles. The path of these particles is straight in the absence of electric or magnetic interaction and deflected under their effect. The beam is deflected toward the positive plate under the effect of an electric field. We can deduce that these particles are negatively charged. They carry an electrically negative charge.

2. Rutherford and the discontinuity of the nucleus.

A radioactive source emits a beam of positive charged particles. These particles travel towards a very thin gold foil in front of a fluorescent screen. Most of the particles pass straight through the gold foil without deviation. The rest of the particles are deflected as they pass through the gold foil. They have been repelled. In this foil, matter is concentrated in positive charged portion called nuclei.
The atom has a particular structure: It consists of smaller particles. The fundamental particles that constitute the atom are the protons, neutrons and the electrons.

Main Results:

i. Most alpha particles go straight through the thin gold foil
ii. Some alpha particles are scattered through a small angle.
iii. A very small number are deflected through very large angles i.e. they bounce back.

Conclusions :

i. An atom is mostly empty space
ii. All of the positive charge in an atom is contained within a small central part of the atom called the NUCLEUS.
iii. Most of the mass of the atom is contained within the nucleus.

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1.1 The Nucleus

A- The Nucleon

Nucleus is not a simple particle. The nucleus itself contains particles called nucleons. Nucleons that carry an electrical charge are called protons. Nucleons that don t carry an electrical charge are called protons.
Protons carry the elementary positive charge: + e = 1.6x10^-19 C whose mass is m_p = 1.6724x10^-24 Kg.
Neutron's mass is very close to the proton's mass : m_n = 1.6747x10^-27 Kg.
The mass of a nucleus is equal to 1.67x10^-27 Kg.

B. Characteristics

The nucleus is characterized by :

- The number of protons in the nucleus Symbol Z.
- The number of nucleons in the nucleus Symbol A.
- A = Z + N where N is the number of neutrons in the nucleus.

The nucleus of an atom of symbol X is represented by ^A_ZX.
   Example: Chemical symbol for the element Li.

Each nucleus is characterized by the pair ( Z , A ) and it's called a nuclide.
   Example: The nucleus of the hydrogen atom is represented by ¹₁H.

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1.2 The electron

A. Characteristics

Electrons are negatively charged. They carry the elementary electrical charge -e where -e = -1.6x10^-19 C.
Electrons and protons carry charges equal in absolute value but are of opposite signs.

Mass of electron = m_p / 1836 = 9.1x10^-31 Kg. This mass is smaller than the mass of the proton.
The atom is electrically neutral. It contains as many protons as electrons. Thus the electrical charge of the electrons in an atom is -Ze.

B.Energy of an atom

The electron possess a potential energy Ep , a kinetic energy E_k thus E_total = Ep + E_k .

1.3 Characteristics of the Atom

Mass & Charge: mass atom = mass nucleus + mass electrons The charge of the nucleus of an atom is +Ze, the charge of its electrons is -Ze. Thus, the charge of an atom is 0.You can see in the image that the number of protons (inside) is equal to the number of electrons (revolving outside).				Mass and Atomic Number: Mass proton = Mass neutron. Mass of nucleus = Number of nucleons it contains. The atomic number of an atom is the charge number of its nucleus. Symbol : Z.
Symbolic representation and dimension of Nucleus and atoms: The number of protons in the nucleus of an atom is the essential characteristic by which we define categories of atoms, called elements. The atom is considered as a sphere, withthe nucleus as its center. It has an empty-space structure.				Isotopes: Atoms of the same element have the same number of protons, but may not have the same number of neutrons, consequently, the yhave the same atomic number Z but different mass number A. These atoms are called isotopes. Isotopes of the same element are the nuclides that have the same same number of protons but a different number of neutrons. The number of isotopes may vary from one element to another.

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