Chapter 2:Unit 4. Structure of Atom

Structure of Atom

Now that we have talked about how atoms are combined to make other substances, let’s talk about the particles that make up the atom. Particles that are smaller than the atom are called subatomic particles. The three main subatomic particles that form an atom are protons, neutrons, and electrons. The center of the atom is called the nucleus. First, let’s learn a bit about protons and neutrons, and then we will talk about electrons a little later.

An atom is composed of two parts: a dense nucleus containing positively charged protons and neutral neutrons. And an electron cloud containing negatively charged electrons. These are called subatomic particles. Most of the mass of an atom resides in the nucleus, while the electron cloud contains most of its volume. Electrons are massless, protons and neutrons have their own mass of 1 amu. Protons have one positive unit of charge and electrons have one negative unit of charge. Neutrons are neutral in charge.

The atomic number(Z) of a neutral atom tells the number of protons and the number of electrons. The mass number (A) is the sum of the number of protons (z) and number of neutrons. General symbol of an element:

If an atom could be expanded to the size of a football stadium, the nucleus would be the size of a single blueberry. (credit middle: modification of work by “babyknight”/Wikimedia Commons; credit right: modification of work by Paxson Woelber)

https://phet.colorado.edu/en/simulation/build-an-atom

There is a difference between mass number and atomic mass. Atomic mass is a weighted average of all the isotopic masses and it has a fractional value; whereas mass number is an integer number and only indicates total number of protons and neutrons in an isotope of an element. Please watch the following video.

Questions:

1. Determine the number of neutrons in each atom of the following element:
   a) Carbon-14
   b) Magnesium-25

2. Complete the table below:

Ans:

1. a) 8

b)13