1st Term Week 6 Lesson Note: Atomic Structure for SS 1

Class: Senior Secondary School 1 (SSS 1)
Subject: Chemistry
Topic: Atomic Structure
Sub-Topics: Structure of the Atom; Fundamental Subatomic Particles

• Duration: 40 Minutes

1. Definition of an Atom

An atom is the smallest, indivisible particle of an element that can take part in a chemical reaction. The word "atom" originates from the Greek word atomos, meaning indivisible.

While early scientists like John Dalton believed atoms could not be split, modern chemistry has proved that an atom has an internal structure and is made up of even smaller particles called subatomic particles.

2. Overall Structure of the Atom

An atom is spherically shaped and consists of two main regions:

1. The Nucleus: This is the extremely dense, positively charged central core of the atom. It contains almost all the mass of the atom but occupies an incredibly small volume. The nucleus contains protons and neutrons (collectively called nucleons).

2. The Electron Cloud (Shells/Orbits): This is the vast, relatively empty space surrounding the nucleus. In this region, tiny negatively charged particles called electrons revolve around the nucleus at very high speeds in fixed pathways known as energy levels, shells, or orbits.

3. The Fundamental Subatomic Particles

There are three primary subatomic particles that determine the mass, identity, and chemical behavior of an atom: Protons, Neutrons, and Electrons.

A. The Proton (p)

• Location: Found inside the nucleus.

• Charge: Carries a relative electrical charge of +1 (positive charge).

• Mass: Has a relative mass of 1 atomic mass unit (amu) (1.67 \times 10^{-27}\text{ kg}).

• Significance: The number of protons determines the identity of the element. For example, any atom with exactly 6 protons is always carbon.

B. The Neutron (n)

• Location: Found inside the nucleus alongside the protons.

• Charge: Carries no electrical charge (it is neutral, 0).

• Mass: Has a relative mass of 1 atomic mass unit (amu). It is slightly heavier than a proton, but for introductory purposes, their relative masses are treated as equal.

• Significance: Together with protons, neutrons make up the atomic mass. Variations in the number of neutrons in atoms of the same element lead to isotopes.

C. The Electron (e-)

• Location: Found outside the nucleus, revolving in energy shells.

• Charge: Carries a relative electrical charge of -1 (negative charge).

• Mass: It is extremely light, with a relative mass of \frac{1}{1840} amu (often approximated as 0). Because its mass is negligible, electrons do not significantly contribute to the overall weight of the atom.

• Significance: Electrons are responsible for chemical bonding and reactions.

4. Summary Table of Subatomic Particles

Particle	Symbol	Location	Relative Charge	Relative Mass (amu)	Actual Mass (kg)	Discoverer
Proton	$p$	Inside Nucleus	+1	1	1.673 \times 10^{-27}	Ernest Rutherford
Neutron	$n$	Inside Nucleus	0	1	1.675 \times 10^{-27}	James Chadwick
Electron	e-	Outside Nucleus	-1	\frac{1}{1840}	9.109 \times 10^{-31}	J.J. Thomson

5. Electrical Neutrality of an Atom

In a normal, uncombined state, an atom is electrically neutral. This means it carries no net electrical charge.

An atom is neutral because the number of protons (positive charges) in its nucleus is exactly equal to the number of electrons (negative charges) surrounding the nucleus. The positive and negative charges cancel each other out perfectly.

$$\text{Number of Protons} = \text{Number of Electrons} \implies \text{Net Charge} = 0$$

If an atom loses or gains electrons during a chemical reaction, this balance is disrupted, and the atom transforms into a charged particle called an ion.

Evaluation Exercises (For Classwork/Assignment)

1. Draw a simple, labeled diagram of an atom showing the positions of the three subatomic particles.

2. Complete the following sentences:

   • a) The subatomic particle with a negative charge is the __________.

   • b) __________ and __________ are found tightly packed inside the nucleus.

   • c) The mass of an atom is concentrated heavily in its __________.

3. Why is an atom considered to be electrically neutral despite containing charged particles?

4. Compare a proton and an electron in terms of mass and electrical charge.