Samacheer Class 7 Science - Atomic Structure

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.

Neutrons are the particles in an atom that have a neutral change (no charge). They are not positive like protons. They are not negative like electrons. So they are called as neutral particles.

Isotopes
Isobar
Isotopes are atoms of the same element thus having same atomic number but different mass number.
Isobars are such atoms which have same mass number but different atomic numbers. Thus isobars are different elements
Example : Hydrogen has three isotopes protium, with deuterium, tritium atomic number 1 but mass number 1,2,3.
Example : Calcium and Argon are isobars as they have atomic number 20 and 18 but their mass number is 40.

Atomic number
Mass number
The number of electrons or protons in an atom is called atomic number of that atom.
The number of protons and neutrons present is its nucleus is called mass number of that atom.
It is represented by the letter Z
It is represented by the letter ‘A’.
Example : Helium (He) has 2 protons and two electrons hence its atomic number will be 2.
Example : Sodium (Na) has 11 protons and 12 Neutrons hence its mass number A= 11 + 12 = 23

The element is Fluorine
Its mass number A
= n + p
= 10 + 9 = 19
∴Mass number of Fluorine = 19

Structure of an atom.Position of the subatomic – particles.
Atom consist of three sub-atomic.
Particles : 1. Protons, 2. Electrons, 3. Neutrons
The nucleus of the atom contains the protons (positively charged) and the neutrons (no change)
The outermost regions of the atom are called electron shells and contain the electrons (negatively charged)
The protons have a positive charge while neutrons have a neutral charge.
Electrons circle around the nucleus of an atom, they are negatively charged.

Atomic number A = 26
Mass number A = 56
No. of Protons P = ?
No. of Electrons = ?
No. of Neutrons N = ?
Name of the element is Iron (Fe)
No. of proton (P) = 26
Mass number A = n+p
56 = n = 26
n = 56-26
∴ No. of neutrons n = 30
In an element No. of protons and No. of electrons is equal.
∴No of electrons = no. of protons
= 26
Structure of Iron:

Nucleons are sub-atomic particles (Protons and Neutrons) which are located in the nucleus of atoms.
Neutron and proton exist within the nucleus of the atom, together, they are called as nucleons.
Properties of Nucleons :
(a) Nucleons (Proton and Neutrons) have same mass as if they were identical particles that differ only in their electric charge.
(b) The proton carries a charge +1 and the Neutron is neutral.
(c) The size nucleon is about 1 : 6 fm (Femto meters)
(d) The force that holds the nucleus together is very short range.

Valency is defined as the combining capacity of an element.
Valency of the element with atomic number : 8 Name of the element:
Oxygen : 0
Atomic number: 8
Valency : 2
Water (H 2 O) is the Oxide of Hydrogen and most familiar oxygen compound

Atoms with no electron will have mass, because mass depends on number of Protons and Neutrons though it has no electron.
Atoms can exist without electrons, Matter is build out of neutrons, protons (+) and electrons (-). Matter becomes stable only if it is electrically neutral.
So atoms without electrons do exist and must have their own states (charged or uncharged transfered back and forth in their environment)
Example : He 2+ . It has 2 protons and 2 neutrons but no electrons.

Common salt is mostly sodium chloride. The ionic compound with the formula NaCl, representing equal proportions of sodium and chlorine.
Elements present is common salt: Sodium & Chlorine.
Formula of common salt: NaCl Atomic number and mass number of NaCl:
Atomic number of Na = 1 Atomic number of Cl = 17 Mass number of Na = 23
Mass number of Cl = 35
Ions present in the compound :
Sodium ion Na+ and Chloride ion Cl
Try yourself:

The atomic number is always a whole number because there must always be a whole number of protons in a nucleus. In other words there can never exist fractions of a proton.
The atomic mass number is always a whole number for the same reason. Protons and neutrons always come in single units not in fractions.

Atomic number of Sulphur = No.of proton sulpher
= 16
Atomic mass number of Sulphur = No. of Protons + No. of Neutrons
= 16 + 16
= 32

Oxygen, Symbol – O.
Helium, Mass no. 4 Potassium, Atomic number: 19.
Phosphorous, No. of Protons : 15.
Carbon, Mass number: 12.