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Model Of Matter - Atoms And Molecules

In this article, the following are the objectives that will be discussed.

  • Describe what an atom is made up of.
  • State the location, relative charges and relative masses of the subatomic particles.
  • Describe molecules as molecules of elements and molecules of compounds.

What is an Atom?

An atom is the smallest particle of an element that has the properties of the element. 
All matter (e.g. solid, liquid and gas) is made up of atoms.
All atoms of the same element are identical. However, atoms of one element are different from atoms of another element.

Size of an Atom

What is the size of an atom?
An atom has an average diameter of \(1 \times 10^{-11} \text{ m}\).

Let’s compare the size of an atom to other substances!

  1. A pencil

A pencil

Image Credit: unsplash - David Perkins

Diameter of a pencil lead: \(1 \times 10^{-2} \text{ m}\)
 

  1. Red blood cell

Red blood cell

Image Credit: unsplash - ANIRUDH

Diameter of a red blood cell: \(1 \times 10^{-5} \text{ m}\)
 

  1. Dust particle

Dust particle

Image Credit: Kunj Parekh - unsplash.com

Diameter of a dust particle: \(1 \times 10^{-7} \text{ m}\)

How are elements represented?

Atoms of the elements are represented using chemical symbols. This can be observed in the Periodic Table shown below.

Periodic Table

Image Credit: SEAB

Structure of an Atom

What is an atom made up of?

An atom is made up of subatomic particles:

  • protons;
  • neutrons; and
  • electrons.

Structure Of An Atom

 

Properties of Subatomic Particles

The table below shows the location, relative mass, and relative charge of the subatomic particles in an atom.

  Location Relative Mass Relative Charge
proton nucleus \(1\) \(+1\)
neutron nucleus \(1\) \(0\)
electron electron shell \(\frac{1}{1840}\) \(-1\)

Revision Questions

Here are some questions for us to look into in the first part of this article.

Question 1: 

Which of the following describes an atom?

  1. An atom can be divided into smaller parts without losing its properties.
  2. An atom has negligible mass.
  3. An atom is densely packed with protons and neutrons only.
  4. An atom is too small to be seen by the naked eye.

Answer :

(D) An atom is too small to be seen by the naked eye.

Explanation: 

An atom cannot be divided into smaller parts without losing its properties.

An atom has mass, which mainly comes from the protons and neutrons.

An atom is densely packed with protons, neutrons and electrons.

An atom can only be observed through an electron microscope.

Answer: (4) An atom is too small to be seen by the naked eye.

 

Question 2:

Which of the following is found moving around the nucleus in fixed orbits?

  1. electrons
  2. neutrons
  3. nucleus
  4. protons

Answer:

(A) electrons

Explanation:

Protons and neutrons are found in the nucleus of an atom. Electrons are found orbiting around the electron shells surrounding the nucleus.

 

Question 3:

Which of the following has the smallest relative mass in an atom?

  1. electron
  2. neutron
  3. nucleus
  4. proton

Answer:

(A) electron

Explanation:

An electron has a relative mass of \(\frac {1}{1840}\) while a proton and a neutron have a relative mass of 1 each. 

 

Question 4:

Which of the following is not a subatomic particle?

  1. electron
  2. molecule
  3. neutron
  4. proton

Answer: 

(B) molecule

Explanation: 

An atom is made up of subatomic particles, which consist of protons, neutrons and electrons. 

Molecule

What is a molecule?

A molecule is made up of two or more atoms chemically combined together.

Examples of molecules that are present around us are: 

  1. water (H2O)
  2. carbon dioxide (CO2)

Classification of Molecules

Molecules can exist in two kinds:

  1. molecules of elements; and
  2. molecules of compounds.

What is a molecule of an element?

A molecule of an element is made up of a fixed number of one type of atom chemically combined together.

Examples of molecules of elements:

  1. hydrogen (H2)
  2. oxygen (O2)
  3. ozone (O2)

What is a molecule of a compound?

A molecule of a compound is made up of a fixed number of two or more different atoms chemically combined together.

  1. water (H2O)
  2. ammonia (NH3)
  3. carbon dioxide (CO2)
  4. methane (CH4)

Revision Question

Here are some questions for us to look into in the second part of this article.

Question 1:

Which of the following correctly describes a molecule?

  1. A molecule is made up of one type of atom only.
  2. A molecule is made up of two or more different types of atoms only.
  3. A molecule is made up of the same type of atoms only.
  4. A molecule is made up of the same or different types of atoms.

Answer: 

(D) A molecule is made up of the same or different types of atoms.

 

Question 2:

Which of the following correctly describes a molecule of an element?

  1. A molecule of an element consists of a fixed number of more than one type of atom chemically combined.
  2. A molecule of an element consists of a fixed number of more than one type of atom not chemically combined.
  3. A molecule of an element consists of one type of atom chemically combined.
  4. A molecule of an element consists of one type of atom not chemically combined.

Answer:

(C) A molecule of an element consists of one type of atom chemically combined.

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