Atomic Structure Online Test 9th Science Lesson 11 Questions in English

In 1911, Lord Rutherford, a scientist from New Zealand, performed his famous experiment of bombarding a thin gold foil with very small positively charged particles called alpha (α) particles. He selected a gold foil because, he wanted as thin layer as possible and gold is the most malleable metal.

Rutherford observed that:
1. Most of the alpha particles passed straight through the foil.
2. Some alpha particles were slightly deflected from their straight path.
3. Very few alpha particles completely bounced back.
Rutherford generalized these results of alpha particles scattering experiment and suggested a model of the atom that is known as Rutherford’s Atomic model.

According to Rutherford’s Atomic model:
i. The atom contains large empty space.
ii. There is a positively charged mass at the centre of the atom, known as nucleus.
iii. The size of the nucleus of an atom is very small compared to the size of an atom.
iv. The electrons revolve around the nucleus in close circular paths called orbits.
v. An atom as a whole is electrically neutral, i.e., the number of protons and electrons in an atom are equal.

Rutherford’s model of the atom was somewhat like that of the solar system. Rutherford’s model of atomic structure is similar to the structure of the solar system. Just as in the solar system, the Sun is at the centre and the planets revolve around it, similarly in an atom the nucleus present at the centre and the electrons revolve around it in orbits or shells.

According to Electromagnetic theory, a moving electron should accelerate and continuously lose energy. Due to the loss of energy, path of electron may reduce and finally the electron should fall into the nucleus. If it happens so, atom becomes unstable. But atoms are stable. Thus, Rutherford’s model failed to explain the stability of an atom.

In 1913, Neil’s Bohr, a Danish physicist, explained the causes of the stability of the atom in a different manner.

The main postulates of Bohr’s model of an atom are:
i. In atoms, the electrons revolve around the nucleus in stationary circular paths called orbits or shells or energy levels.
ii. While revolving around the nucleus in an orbit, an electron neither loses nor gains energy.
iii. An electron in a shell can move to a higher or lower energy shell by absorbing or releasing a fixed amount of energy.
iv. The orbits or shells are represented by the letters K, L, M, N, … or the numbers, n= 1, 2, 3, 4, ….

The orbit closest to the nucleus is the K shell. It has the least amount of energy and the electrons present in it are called K electrons, and so on with the successive shells and their electrons. These orbits are associated with fixed amount of energy, so Bohr called them as energy level or energy shells.

One main limitation was that Bohr’s model was applicable only to hydrogen and hydrogen like ions (example, He+, Li2+, Be3+, and so on). It could not be extended to multi electron nucleus.

In 1932 James Chadwick observed when Beryllium was exposed to alpha particles, particles with about the same mass as protons were emitted. In 1920, Rutherford predicted the presence of another particle in the nucleus as neutral. James Chadwick, the inventor of neutron was student of Rutherford.

Properties of Neutrons
1. This particle was not found to be deflected by any magnetic or electric field, proving that it is electrically neutral.
2. Its mass is equal to 1.676 × 10−24 g (1 amu).

The atom is built up of a number of subatomic particles. The three sub-atomic particles of great importance in understanding the structure of an atom are electrons, protons and neutrons.

There are two structural parts of an atom, the nucleus and the empty space in which there are imaginary paths called orbits. Orbit is defined as the path, by which electrons revolve around the nucleus. The protons and neutrons [collectively called nucleons] are found in the nucleus of an atom.

Explanation
There are two structural parts of an atom, the nucleus and the empty space in which there are imaginary paths called orbits. Orbit is defined as the path, by which electrons revolve around the nucleus. The protons and neutrons [collectively called nucleons] are found in the nucleus of an atom.

Besides the fundamental particles like protons, electrons and neutrons some more particles are discovered in the nucleus of an atom. They include mesons, neutrino, antineutrino, positrons etc.

Besides the fundamental particles like protons, electrons and neutrons some more particles are discovered in the nucleus of an atom. They include mesons, neutrino, antineutrino, positrons etc.

Atomic number (Z) = Number of protons = Number of electrons
This is a very important number is known as the atomic number (proton number, given the symbol Z) of an atom.

Protons alone do not make up all of the mass of an atom. The neutrons in the nucleus also contribute to the total mass. The mass of the electron can be regarded as so small that it can be ignored. As a proton and a neutron have the same mass, the mass of a particular atom depends on the total number of protons and neutrons present. This number is called the mass number (or nucleon number, given the symbol A) of an atom. Mass number = Number of protons + Number of neutrons

For any element, the atomic numbers are shown as subscripts and mass number are shown as superscripts. For example, nitrogen is written as 14 N 7. Here 7 is its atomic number and 14 is its mass number.

Z stands for Zahl, which means NUMBER in German. Z can be called Atomzahl or atomic number A is the symbol recommened in the ACS style guide instead of M (massenzahl in German).

The forces between the protons and the neutrons in the nucleus are of special kind called Yukawa forces. This strong force is more powerful than gravity.

Atoms are so tiny their mass number cannot be expressed in grams but expressed in amu (atomic mass unit). New unit is U. Size of an atom can be measured in nano metre (1 nm = 10−9 m) Even though atom is an invisible tiny particle now-a-days atoms can be viewed through SEM that is Scanning Electron Microscope.

The outermost shell of an atom is called valence shell and the electrons present in the valence shell are known as valence electrons. The chemical properties of elements are decided by these valence electrons, since they are the ones that take part in chemical reactions.

The elements with same number of electrons in the valence shell show similar properties and those with different number of valence electrons show different chemical properties. Elements, which have 1 or 2 or 3 valence electrons (except Hydrogen) are metals. Elements with 4 to 7 electrons in their valence shell are non-metals.

Valency of an element is the combining capacity of the element with other elements and is equal to the number of electrons that take part in a chemical reaction. Valency of the elements having valence electrons 1, 2, 3, 4 is 1, 2, 3, 4 respectively. Valency of an element with 5, 6 and 7 valence electrons is 3, 2 and 1 (8–valence electrons) respectively. Because 8 is the number of electrons required by an element to attain stable electronic configuration. Elements having completely filled outermost shell show Zero valency. For example: The electronic configuration of Neon is 2,8 (completely filled). So, valency is 0.

Electronic configuration of magnesium is 2, 8, 2. So valency is 2.
Electronic configuration of sulphur is 2, 8, 6. So valency is 2 i.e. (8 – 6).

In nature, a number of atoms of elements have been identified, which have the same atomic number but different mass numbers. For example, take the case of hydrogen atom, it has three atomic species as shown below:

On the basis of these examples, isotopes are defined as the different atoms of the same element, having same atomic number but different mass numbers. There are two types of isotopes: stable and unstable. The isotopes which are unstable, as a result of the extra neutrons in their nuclei are radioactive and are called radioisotopes. For example, uranium-235, which is a source of nuclear reactors, and cobalt-60, which is used in radiotherapy treatment are both radioisotopes.

Let us consider two elements – calcium (atomic number 20), and argon (atomic number 18). They have different number of protons and electrons. But, the mass number of both these elements is 40. It follows that the total number of nucleons in both the atoms are the same. They are called isobars. Atoms of different elements with different atomic numbers, and same mass number are known as isobars.

No of neutrons in boron = 11 – 5 = 6
No of neutrons in carbon = 12 – 6 = 6
The above pair of elements Boron and Carbon has the same number of neutrons but different number of protons and hence different atomic numbers. Atoms of different elements with different atomic numbers and different mass numbers, but with same number of neutrons are called isotones.

Law of multiple proportions was proposed by John Dalton in 1804. It states that, “When two elements A and B combine together to form more than one compound, then different masses of A which separately combine with a fixed mass of B are in simple ratio”.

Carbon combines with oxygen to form two different oxides, carbon monoxide (CO) and carbon dioxide (CO2). The ratio of masses of oxygen in CO and CO2 for fixed mass of carbon is 1: 2.

The law of reciprocal proportions was proposed by Jeremias Ritcher in 1792. It states that, “If two different elements combine separately with the same weight of a third element, the ratio of the masses in which they do so are either same or a simple multiple of the mass ratio in which they combine among themselves.”

we have four ways of defining the properties of an electron, i.e. four quantum numbers. Thus, the numbers which designate and distinguish various atomic orbitals and electrons present in an atom are called quantum numbers.

According to Gay Lussac’s Law, whenever gases react together, the volumes of the reacting gases bear a simple ratio, and the ratio is extended to the product when the product is also in gaseous state, provided all the volumes are measured under similar conditions of temperature and pressure.