CHAPTER 5 Atomic Models

Keywords

nucleus

nuclide

photo-electric

emission

photo-electrons

thermionic emission

By the end of this chapter, you will be able to:

1. understand the structure of an atom in terms of a positive nucleus and negative electrons.
2. understand the terms: Atomic number, mass number, and isotopes, and use them to represent different nuclides.
3. understand the methods by which electrons are ejected from matter/atoms and how these electrons are useful.

5.1: Introduction

In S.1, you learnt that matter is made up of very small particles that are in a state of continuous motion. The smallest particles of matter that can exist on their own are called atoms. But are the atoms the smallest particles of matter or they themselves can be subdivided into smaller particles? If so, what are these particles and how are they arranged in the atom? Is there any use we can make of these smaller particles in the atom? In this chapter, you will be able to learn that atoms are made up of other smaller particles and you will also be able to learn how these particles characterise the atoms of different substances.

5.2: The Atom

Just as bricks are the building blocks of a wall, an atom is the building block of any piece of matter. When atoms join together, they form molecules. Similarly, the molecules join to form big structures/objects around you. To understand the nature of atoms, let us consider that a piece of paper is continuously cut into smaller pieces. At some point, you may end up with a piece of paper, which is so small that it can no longer be cut into pieces again. Such small piece of paper is then said to be “indivisible”. This smallest indivisible piece of paper can be likened to an atom which is the smallest indivisible particle of matter. wikiHow

Figure 5.1: Cutting matter into small pieces

The same situation would happen if other forms of matter like wood were cut several times until the smallest indivisible particle is obtained. DID YOU KNOW? Did you know that the atom is the smallest indivisible particle of matter, which can take part in a chemical reaction?

5.3: The Structure of an Atom

Atomic structure refers to the arrangement of sub-atomic elements in the atom. The sub-atomic elements include protons, neutrons and electrons. Protons and Neutrons form the central nucleus of the atom while the electrons orbit (move in circular paths) around the nucleus.

Figure 5.2: Atomic structure

The mass of the electron is very small compared to the mass of a proton or a neutron. Therefore, the total mass of the atom is concentrated in the nucleus of the atom.

DID YOU KNOW?

Did you know that the total number of protons and neutrons in the nucleus of an atom is referred to as the atomic mass of the atom? The electrons carry a negative electric charge while the protons carry a positive electric charge. The neutrons have no electric charge. In a neutral atom, the number of electrons is equal to the number of protons in the nucleus of the atom.

DID YOU KNOW?

Did you know that the number of protons in the nucleus of an atom is referred to as the Atomic number of the atom? Atoms of different elements have different atomic structures because they have different atomic numbers. This is the reason for the unique characteristics of different elements.

4. From Figure 5.3, what do you think is the maximum number of electrons that can be accommodated in the innermost orbit? What about in the second innermost orbit?

5. With reference to a Periodic table, draw the atomic structures of the fifth to the twelveth element.

5.4: Representing Different Atoms

Each atom of a given element has got a unique symbol that represents it. If the symbol of the atom is X, then the atom would be represented as 2X, where A is the atomic mass and Z is the atomic number.

5.5: Charged Atomic

Recall that an electron carries a negative charge while a proton carries a positive charge. These two charges are equal but opposite. When an atom has the same number of electrons and protons, it has equal number of negative charges and positive charges. Therefore, the charges cancel and the atom is said to be a neutral atom. At times, an atom may gain or loose electrons. If an atom looses an electron, it now has more protons than electrons. Therefore, it acquires a positive charge. However, if the atom gains an electron, it will have more electrons than protons. Therefore, it acquires a negative charge.

ASSIGNMENT 5.1

Open any internet browser and search for a video describing atoms on YouTube. Watch the video and answer some questions relating to the video like: what is an atom? What is at the centre of the atom? What two sub-atomic particles make up the centre of the atom? What kind of charge do protons, neutrons and electrons have? Which of these sub-atomic particles are heavier?

5.6: Isotopes

Activity 5.1 Getting to know an isotope

Key question: What are isotopes?

What you need

Figure 5.4

Figure 5.4: Structure of some atoms

What to do

Study the structure of the atoms in Figure 5.4 and answer the questions that follow:

1. Which element does the figure above demonstrate?

2. Write down the atomic number and the mass number of each of the atoms.

3. Comment on their atomic number and mass numbers.

4. Basing on your reasoning, what causes the difference in their mass numbers?

5. Do you think the three elements have the same characteristics in terms of the chemical properties and their weight? Explain your response.

6. What are isotopes?

DID YOU KNOW?

With regard to an atom, did you know that atomic mass and mass number mean the same thing?

Each element has a unique atomic number, which determines its chemical properties. An element may have different atomic masses due to differences in the number of neutrons in the nucleus. The atoms of the same element, having the same number of protons but different number of neutrons, are called Isotopes. Isotopes have similar chemical properties but different physical properties.

Examples of Isotopes

1. Hydrogen – It exists in three different isotopic forms:

5.6.2: A Nuclide

A nuclide is the nucleus of an atom of a specific isotope. A nuclide is characterised by the number of positively charged protons (Z), neutrons (N) and the energy state of the nucleus. Examples of nuclides include:

(i) Chlorine-37, an isotope of chlorine: The nucleus consists of 17 protons and 20 neutrons.

(ii) Sodium-23, an isotope of sodium: The nucleus consists of 11 protons and 12 neutrons.

(iii) Chlorine-35, an isotope of chlorine: The nucleus consists of 17 protons and 18 neutrons.

5.7: Electron Emission

Quiz:

Consider a metal plate with a large surface area, placed in the open such that it is exposed to direct heat of the Sun. What do you think will happen to the surface particles of the metal plate?

Activity 5.2 Investigating the processes of electron emission

Key question: Describe photo-electric and thermionic emission. What you need Internet resource Library resource What to do

a) Research about:

(i) The cathode

(ii) The anode

(iii) How thermionic emissions work.

(iv) How photo-electric emission occurs.

b) Make a report about your findings and make a presentation to your class.

Activity 5.3 Investigating how cathode rays are produced

Key question:

Describe how cathode rays are produced.

What you need

Relevant textbooks

Internet resource

What to do (in groups)

a) Research about:

(i) Characteristics of cathode rays.

(ii) How cathode rays are produced.

(iii) The charge on cathode rays.

(iv) What happens when a maltase cross is put in the cathode ray tube.

b) Write a report about your findings and make a presentation to your class.

EXERCISE 5.4

Explain:

1. The difference between thermionic emission and photo-electric emission.

2. How cathode rays and X-rays are different.

3. Prepare a presentation of your findings or you can put your findings on a poster using a manilla in case you cannot access a computer.

5.7.1: X-rays

When the cathode rays strike a hard surface, an interaction of the particles of the metal occurs which result in the conversion of kinetic energy into heat (about 99%) and X-ray photons (1%). The X-ray photons are released in a beam with a range of energies.

Activity 5.4 Investigating how x-rays are produced

Key question:

Describe how x-rays are produced.

What you need

Relevant textbooks

Internet resource

What to do (in groups)

a) Research about:

(i) Properties of x-rays. (ii) How x-rays are produced.

(iii) Types of x-rays and their characteristics.

(iv) Why a tungsten target is used.

(v) Why the x-ray tube is evacuated.

b) Write a report and make a presentation to your class.

Watch the video on atomic models