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Atoms and their Composition – Atomic Theory and Examples

Grade 11
May 15, 2023

Atoms and their Composition


Anything that has mass and occupies space is called matter. It exists in three physical states “solid, liquid, and gas.”

Elements are the fundamental substances that make up all matter. Nearly 90 elements exist in the universe naturally. Hydrogen and helium are the least dense and smallest elements. But their abundance is nearly 98% of the mass of the entire universe!

Atoms and their Composition

Each element is made up of only a single kind of atom. For example, the element hydrogen contains only hydrogen atoms. The element helium contains only helium atoms.

Element: Helium gas, made up of the same type of atoms and represented by He.


Element: Hydrogen gas, made up of the same type of atoms and represented by H.

The Atomic Theory of Matter

  • All matter is made up of the smallest particles called atoms.
  • An atom cannot be created, destroyed, or divided into smaller particles.
  • The atoms of one element cannot be converted into the atoms of any other element.
  • All the atoms of one element have the same properties, such as mass and size. These properties are different from the properties of the atoms of any other element.

Atomic Theory of Matter

  • Atoms of different elements combine in specific proportions to form compounds.
  • Example: Hydrogen and oxygen combine in a 1:8 ratio by mass.

Hydrogen and oxygen


An element is made up of the smallest particle called an atom. In an element, all the atoms will retain the identity and properties of the element.

For example, the writing material in your pencil is a carbon atom. (Pencil, “lead,” is a substance called graphite. Graphite is a form of the element carbon.)



Atoms are made up of even smaller particles called subatomic particles like protons, neutrons, and electrons. Protons and neutrons together form the central part or nucleus of an atom.

nucleus of an atom

Expressing The Mass of Subatomic Particles

The size of the subatomic particles is very small. Suppose the count of protons or neutrons is equal to 602 000 000 000 000 000 000 000 (or 6.02 × 1023). They would have a mass of about 1 g. This means that one proton or neutron has a mass of:

= 1g/ 6.02 × 1023

= 0.000 000 000 000 000 000 000 001 66 g

= 1.66 × 10-24 g

So, it is difficult to measure the mass of subatomic particles using the units like grams. Instead, scientists use a unit called an atomic mass unit (symbol u). The mass of a proton is about 1 u, which is equal to 1.66 × 10-24 g.

The Nucleus of An Atom

In an element, all the atoms will have the same number of protons in their nucleus. For example, hydrogen atoms have one proton. Helium has two protons.   Eight protons in oxygen atoms. In an element, scientists use the term atomic number (symbol Z) to refer to the number of protons in the nucleus of an atom.

Nucleus of An Atom

The Nucleus of An Atom

The nucleus of an atom contains protons and neutrons in it. Both protons and neutrons make up the nucleus. Therefore, an element’s mass number is the total number of protons and neutrons in the nucleus of an atom. It is represented by the symbol ‘A’.

The mass of each proton or neutron is taken as 1 u. For example, a nitrogen atom, which has 7 protons and 7 neutrons in its nucleus, the mass number is 7+7 = 14. A uranium atom has a mass number of 238. It has 92 protons and 146 neutrons.

Nucleus of An Atom

Nucleus of An Atom

To represent the atomic symbol for an element (or element symbol), the letter X is used. Every element has a different atomic symbol.

For example, let us take the element fluorine:

The mass number A is 19 (which is the sum of the number of protons and neutrons).

The atomic number Z is 9.

atomic number


Number of neutrons

= Mass number − atomic number

= A − Z

Thus, for fluorine, the Number of neutrons

= A – Z

= 19 − 9

= 10

atomic number

Using the Atomic Number to Infer the Number of Electrons

The atomic number A and mass number Z directly do not give information regarding the number of neutrons in an element, nor do they give the number of electrons. The element as a whole is electrically neutral.

This means that the number of protons (positive charge) is equal to the number of electrons (negative charge). Therefore, in atom all the opposite charges balance each other.

For example, a neutral helium atom contains two protons, so to balance the charge, an atom contains two electrons. A neutral sodium atom contains eleven protons, so it must contain eleven electrons.

Number of Electrons

The number of protons = the number of electrons

Isotopes and Atomic Mass

An atom that is electrically neutral contains the same number of protons and, therefore, the same number of electrons. But the number of neutrons can vary.

Isotopes and Atomic Mass

Oxygen can exist in three forms that are,

The number of neutrons is 8, 9, and 10, respectively.

Their mass number is 16, 17, and 18, respectively.

These three forms of oxygen are called isotopes. The atoms of an element having the same number of protons and different mass numbers (or the number of neutrons) are called isotopes.

The isotopes of an element have similar chemical properties due to the same number of electrons and have different physical properties like mass due to the different number of neutrons.

There are some isotopes that are more unstable than others; due to this, their nuclei are more likely to decay, releasing energy and producing subatomic particles. This process is called radioactivity. This process takes place spontaneously.


For example: All uranium isotopes have unstable nuclei. So they are called radioactive isotopes or radioisotopes. All isotopes are not radioisotopes. Oxygen exists in three forms, and all are stable. Chemists have synthesized ten other isotopes which are unstable in nature.


Electrons in Atoms

Electrons are present around the nucleus. Therefore, they are the first subatomic particles that are likely to interact when atoms come near one another. The arrangements of electrons in an atom are responsible for their reactivity with other atoms.

The chemical properties of an element are described by the valence electrons (electrons in the outermost shell).

Electrons in Atoms

Revisiting the Atomic Theory

John Dalton did not know about subatomic particles when he developed his atomic theory. Even so, the modern atomic theory retains many of Dalton’s ideas, with only a few modifications.

Revisiting the Atomic Theory

Revisiting the Atomic Theory

Atoms and their composition


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