Atoms

All atoms are made of three basic particles: the proton, electron, and neutron. Let’s see how these were identified or discovered. We will also answer a set of related questions on atoms, protons, neutrons and electrons.


How were electrons identified?

The first strong evidence that something existed smaller than an atom came in 1897. English physicist J. J. Thomson discovered that electricity passing through a gas caused the gas to give off particles that were too small to be atoms.

Thomson’s new particles also had a negative electric charge while atoms have zero electric charge.

Thomson called his particles corpuscles, which were eventually named electrons, and proposed that they came from the inside of atoms.

How were the proton and the nucleus discovered?

In 1911, Ernest Rutherford, Hans Geiger, and Ernest Marsden did a clever experiment to test Thomson’s model of the atom.

  • They launched positively charged helium ions (a charged atom is called an ion) at extremely thin gold foil. They expected the helium ions to be deflected a small amount as they passed through the foil.
  • However, a few bounced back in the direction they came! The unexpected result prompted Rutherford to remark “it was as if you fired a five-inch (artillery) shell at a piece of tissue paper and it came back and hit you!”
  • The best way to explain the pass-through result was as if the gold atoms were mostly empty space, allowing most of the helium ions to go through virtually undeflected.
  • And, the best way to explain the bounceback result was as if nearly all the mass of a gold atom were concentrated in a tiny, hardcore at the center. Further experiments confirmed Rutherford’s ideas and we know that every atom has a tiny nucleus, which contains more than 99% of the atom’s mass.

How was the neutron identified?

Even after the discovery of the positively charged proton in the nucleus, still there was a serious problem with the atomic model. Protons could only account for about half the observed mass. This problem was solved in 1932 by James Chadwick.

Chadwick’s experiments revealed another particle in the nucleus which has no electric charge and a similar mass as the proton.

Chadwick’s neutral particle was named the neutron.

Protons, neutrons, and electrons

Today we know that atoms are made of three tiny subatomic particles: protons, neutrons, and electrons.

Protons have a positive charge. Electrons have a negative charge. Neutrons add mass but have zero charge.

The charge on a proton (+e) and an electron (-e) are exactly equal and opposite. Atoms that have the same number of protons and electrons have a total charge of precisely zero.

The nucleus

The protons and neutrons are grouped together in the nucleus, which is at the center of the atom.

The mass of the nucleus determines the mass of an atom because protons and neutrons are much larger and more massive than electrons. In fact, a proton is 1,836 times heavier than an electron.

All atoms have both protons and neutrons in their nuclei except for the simplest type of hydrogen, which only has one proton and no neutrons.

Electrons define the volume of an atom

Electrons take up the region outside the nucleus in a region called the electron cloud. The diameter of an atom is really the diameter of the electron cloud. Compared to the tiny nucleus, the electron cloud is enormous, more than 10,000 times larger than the nucleus.

The atomic number is the number of protons | How is an atom of one element different from an atom of another element?

The atoms of different elements contain different numbers of protons in the nucleus. For example, all atoms of carbon have six protons in the nucleus and all atoms of hydrogen have one proton in the nucleus.

Because the number of protons is so important, it is called the atomic number. The atomic number of an element is the number of protons in the nucleus of every atom of that element.

Atoms of the same element always have the same number of protons in the nucleus.

Complete atoms are electrically neutral

The number of protons and electrons in a complete atom is always equal. Thus, the negative charge of the electron/s cancels the positive charge of the proton/s. Hence, complete atoms are electrically neutral.

For example, hydrogen has one proton in its nucleus and one electron outside the nucleus. The total electric charge of a hydrogen atom is zero because the negative charge of the electron cancels the positive charge of the proton.

Each carbon atom has six electrons, one for each of carbon’s six protons. Like hydrogen, a complete carbon atom is electrically neutral.

What is Atomic number?

Chemists refer to the number of protons in the nucleus of an atom as the atomic number. So, hydrogen has atomic number 1, helium has atomic number 2, and so on.

atomic number = number of protons

Aluminium has 13 protons so its atomic number is 13. In the Periodic Table, the elements are arranged in order of atomic number, so aluminium is the thirteenth element in the Periodic Table.

What is Mass number?

Protons do not account for all the mass of an atom. Neutrons in the nucleus also contribute to the mass. Therefore, the mass of an atom depends on the number of protons plus the number of neutrons. This number is called the mass number of the atom.

mass number = number of protons + number of neutrons

Hydrogen atoms (with one proton and no neutrons) have a mass number of 1.

Helium atoms (two protons and two neutrons) have a mass number of 4 and lithium atoms (three protons and four neutrons) have a mass number of 7.

Atoms
Scroll to top
error: physicsTeacher.in