Atomic Structure 7th Science Lesson 4 Questions in English

An atom is one and thousand times smaller than the thickest human hair. It has an average diameter of 0.0000000001m or 1×10-9 m.

John Dalton proposed the atomic theory in the year 1808. He proposed that matter consists of very small particles which he named atoms.

Nanometre is the smallest unit used to measure small lengths. One metre is equal to 1×109 nm or one nanometre is equal to 1×10-9 m.

John Dalton proposed an atom is smallest indivisible particle, it is spherical in shape. His theory does not propose anything about the positive and negative charges of an atom.

In 1897 J.J Thomson proposed a different theory. He compared an atom to a watermelon.

J. J. Thomson theory proposed that the atom has positively charged part like the red part of the watermelon and in it are embedded, like the seeds, negatively charged particles which he called electrons.

There were short coming in Thomson’s theory, Earnest Rutherford gave a better understanding. Earnest Rutherford conducted an experiment. He bombarded a very thin layer of gold with positively charged alpha rays.

Based on this experiment, Rutherford proposed his famous theory. In his opinion, – 1. The fact that most alpha particles pass through the gold sheet means that the atom consists mainly of empty space.

Rutherford’s theory proposes that 1. The nucleus at the centre of the atom has the positive charge. Most of the mass of the atom is concentrated in the nucleus. 2. The negatively charged electrons revolve around the nucleus in specific orbits. 3. In comparison with the size of the atom, the nucleus is very, very small.

The discoveries made during the twentieth century proved that atoms of all elements are made up of smaller components – electron, proton and neutron. An electron from hydrogen atom is no different from electron of a carbon atom. In the same manner, protons and neutrons of all elements also have same characteristics. These, particles that make up the atom are called Subatomic Particles.

The proton is the positively charged particle and its located in the nucleus. Its positive charge is of the same magnitude as that of the electron’s negative charge. Neutron is inside the nucleus.

Electron is a negatively charged particle. Electrons revolve around the nucleus of the atom in specific orbits.

Neutron is inside the nucleus. The neutron does not have any charge. Excepting hydrogen (protium), the nuclei of all atoms contain neutrons.

Protons and Neutrons are the two types of particles in the nucleus of an atom. They are called nucleons.

The total negative charge of all an electron outside the nucleus is equal to the total positive charge in the nucleus. That makes the atom electrically neutral.

Proton(p) – Ernest Rutherford
Electron(e) – Sir John Joseph Thomson
Neutron(n) – James Chadwick

The mass od proton is 1.6726 ×10-27. Its charge is +1.

The mass of Electron is 9.1093 × 10-31. The charge of electron is –1.

The number of electrons or protons in an atom is called the atomic number of that atom.

The mass of Neutron is 1.6749 × 10-27 kg. The charge of neutron is 0 i.e., no charge.

The atomic number is represented by the letter Z. if we know the atomic number of an atom, we know the number of electrons or protons in it.

In the Oxygen atom there are eight protons and eight electrons in orbit around the nucleus, so the atomic number of helium is z=8.

The atomic number is represented by the letter Z. if we know the atomic number of an atom, we know the number of electrons or protons in it.

In the Oxygen atom there are eight protons and eight electrons in orbit around the nucleus, so the atomic number of helium is z=8.

Mass number is equal to the sum of the number of protons(p) and neutrons (n) in the nucleus.
Atomic mass or mass number = Number of Protons + Number of Neutrons

The mass of an atom is concentrated in its nucleus.

The Mass number is represented by letter A.

A lithium atom contains 3 Protons and 4 neutrons. Its atomic mass number A = 3+4 = 7.

Atoms that have the same mass number but different atomic numbers is known as Isobars. for example, Calcium – 40 and Argon – 40.

In a sodium atom, there are 11 Protons and 12 neutrons. Hence, its atomic mass number A = 11 + 12 = 23.

In a sodium atom, there are 11 Protons and 12 neutrons. Hence, its atomic mass number A = 11 + 12 = 23.

Atoms of the same element can have different number of neutrons. Such atoms will have same atomic number but different mass numbers. These atoms are called isotopes. For example Hydrogen has three isotopes — Hydrogen (1H1), Deuterium (1H2), Tritium (1H3).

7 is the atomic number and 14 is the mass number.

7 is the atomic number and 14 is the mass number.

In an atom no of proton and no of electrons are same i.e., sodium has 11 proton and 11 electrons. So, its atomic number is 11.

In an atom no of proton and no of electrons are same i.e., sodium has 11 proton and 11 electrons. So, its atomic number is 11.

Some atoms can hold one electron, some can hold two, some can hold three, some can hold four and some cannot hold any electron. This property is called valency.

The combining property of an atom is called as Valency. It is a measure of how many hydrogen atoms it can combine with.

Oxygen can combine with two hydrogen atoms and create water molecule; the valency of oxygen atom is two.

Oxygen can combine with two hydrogen atoms and create water molecule; the valency of oxygen atom is two.

The combining property of an atom is called as Valency. It is a measure of how many hydrogen atoms it can combine with.

The combining property of an atom is called as Valency. It is a measure of how many hydrogen atoms it can combine with.

In case of chlorine, it can combine with only one hydrogen to create HCl (hydrochloric acid) here the valency of chlorine is one.

Methane has one carbon atom combining with four hydrogen atoms to form carbon molecule is methane (CH4).