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Structure of an Atom

Structure of an Atom

This lesson aligns with Next Generation Science Standards (NGSS) PS1.C

Everything in the universe, including living and non-living organisms, is composed of matter. Matter can be defined as a substance that is made up of various types of particles that occupy space and has a mass. The fundamental building block of matter is a particle called an atom. Everything around us is a definite representation of that fundamental substance. Your computer monitor is composed of atoms. The chair you sit on is made up of atoms and the air you breathe in. In fact, if the human body breaks down to the basic level, it is made up of living cells that also contain atoms bonded together. In this article, we will discuss the atom and its structure. 

The term atom originates from the Greek word “Atomos” which means “indivisible”.  An atom is the smallest particle that comprises three subatomic particles such as protons, neutrons, and electrons.  Protons and neutrons are present inside the very center of the atom which is called the nucleus. Outside of the nucleus, electrons are present that orbit around it. All atoms have a unique identity in the periodic table which is specified by their atomic numbers and symbols. Examples include carbon atom [math]6C[/math], iron atom [math]26Fe[/math], etc.

Structure of an Atom 
One way to understand the structure of the atom is to think about a football stadium. Imagine a grain of pea is placed in the center of the field. This represents the nucleus. The outer row of seats in the large stadium shows the limit of the electron’s influence. The rest of the atom contains free space.

The nucleus is a central core of an atom where the majority of the mass is concentrated. It takes more than 99.9% of the mass of an atom and is ten thousand times smaller than an atom. The nucleus has a very tight arrangement of protons and neutrons within it, and electrons continuously revolve around in the fixed orbits of the nucleus. Protons and neutrons are firmly bound together by strong nuclear forces. 

  • Protons are positively charged. 
  • The charge on one proton is [math]+1 or 1.6 × 10^-19 C[/math].
  • The mass of a proton is approximately 1 Dalton which is [math]1.66 × 10^-27kg[/math].

  • Electrons are negatively charged. The charge on one electron is [math]-1 or -1.6 × 10^-19 C[/math].
  • The mass of an electron is negligible as it is 1800 times less than that of a hydrogen atom.

  • Neutrons are also a fundamental part of the nucleus. They are closely bound inside the nucleus.
  • Neutrons are neutrals. They have no charge at all.
  • The mass of a neutron is nearly equal to that of a proton, i.e. [math]1.66 × 10^-27kg[/math].

Atomic Number  
An atomic number is a simple positive whole number. It represents how many protons are present in an atom. Due to this reason, sometimes it is called a photon number. It is denoted by using the capital letter Z. Atomic number is a key factor in analyzing the properties of an element. This is because the modern periodic table is designed according to the ascending order of atomic number.

Atomic number = number of protons

For example, in a lithium atom 3Li, there are 3 electrons and 3 protons. Thus; the atomic number of Li = number of electrons = number of protons = 3

A neutral atom must have same number of protons and electrons. If a neutral atom has 1 proton, it must have one electron. Similarly, if a neutral atom has 5 protons, it must have 5 electrons. For example, a helium atom is written as [math]2He[/math] it is composed of two protons and two electrons as well. Different elements have different atomic numbers.

Mass Number
The mass number of an atom is the total number of protons and neutrons that are held in its nucleus. It is represented by using the capital letter “A”. For example, in a carbon atom [math]612C[/math], the mass number of carbon atom is 12.

Atomic number (Z) = number of protons = number of electrons = 6
Mass number (A) = number of protons + number of neutrons = 6 + 6 =12

Nuclear Symbol 
To write a nuclear symbol, the mass number is placed at the upper left corner of the chemical symbol and the atomic number is written at the lower left corner of the chemical symbol. The complete symbol for helium is written below: 

  • Atoms are building blocks of matter. They are made up of subatomic particles like protons, neutrons, and electrons.
  • Protons and neutrons are confined in a nucleus. However, electrons orbit around the nucleus in fixed orbits.
  • An atomic number is the total number of protons in the nucleus of an atom.
  • A mass number is a whole number that is the total sum of both protons and neutrons.  

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