Revision: Inside the Atom General Science SSC (English Medium) 8th Standard Maharashtra State Board

Molecule : Molecule is the smallest unit of a compound (or an element) which always has an independent existance.

Covalent bond— When atoms of different non-metals neither donate nor accept electrons and hence no ions are formed, such a bond is called covalent bond.

Metal:  A chemical element that is an effective conductor of electricity and heat can be defined as a metal.

Ex.: Copper, Iron, Silver, etc.

Metalloid: Metalloid is a chemical element that exhibits some properties of metals and some of non-metals. Metalloids are generally semi-conductors.

Ex.: Silicon. Arsenic, Antimony and Boron.

An atom is the smallest particle of a chemical element that retains its chemical properties.

Chemical bond— A chemical bond is the binding force between two or more atoms of a molecule.

Element: It is a substance that cannot be broken down into simpler substance by chemical means

Ex.: Oxygen, Hydrogen, Gold & Helium.

An atom is the smallest particle of an element which retains its chemical identity in all physical and chemical changes.

Radicals : A radical is an atom of an element or a group of atoms of different elements that behaves as a single unit with a positive or negative charge on it.

An Atom: Smallest particle of an element that can exist and have properties of an element.

Relative atomic mass— Relative atomic mass is the mass of an atom of an element as a multiple of the standard atomic mass unit.

The relative atomic mass of an element is the ratio between the average mass of its isotopes to 1/12th part of the mass of a carbon – 12 atoms. It is denoted as A_r.

Relative atomic mass = `" Average mass of the isotopes of the element"/(1"/"12^{"th"}" of the mass of one Carbon- 12 atom")`

Compound: A compound is a pure substance that is formed when the atoms of two or more elements combine chemically in definite proportions.

Ex: H20, NaCl.

Non-Metal: Non-metal is an element that doesn’t have the characteristics of metal including, (i.e.) ability to conduct heat or electricity luster or flexibility.

Ex. Carbon Iodine, Sulphur.

Mass number— Mass number is the sum of the number of protons and neutrons present in the nucleus of an atom. It is denoted by A.

An atom which becomes charged by losing or gaining electrons is called an ion.

Atom: An atom is the smallest indivisible unit of an
OR
Atom is the smallest unit of matter.

The number of protons in the nucleus is known as the atomic number of the element and is denoted by Z.

The total number of neutrons and protons in the nucleus is called the mass number of the element and is denoted by A.

The atomic number of an atom is equal to the number of protons in its nucleus (which is same as the number of electrons in a neutral atom).

The mass number of an atom is equal to the total number of nucleons (i.e., the sum of the number of protons and the number of neutrons) in its nucleus.

It is the sum of total number of protons and neutrons present in a nucleus.

1. atomic mass number = number of protons + number of neutrons.
2. Atomic number = number of protons = number of electrons. It is denoted by Z.
3. Mass number = number of protons + number of neutrons. It is denoted by A.
4. For example: Carbon atom its,
   1. Number of proton = 6
   2. Number of neutrons = 6
   3. Number of electron = 6
5. Atomic number (Z) = number of protons = number of electrons = 6.
6. Mass number (A) = number of proton + number of neutrons = 6 + 6 =12.

A mass unit equal to exactly one-twelfth `(1/12^(th))` the mass of one atom of carbon-12 is called one atomic mass unit. It is written as ‘u’.

When the properties of elements in a period or a group of the modern periodic table are compared, certain regularity is observed in their variations. It is called the periodic trends in the modern periodic table.

The valency of an element is determined by the number of electrons present in the outermost shell of its atoms, that is, the valence electrons.

The atoms of the same element, having same atomic number Z, but different mass number A, are called isotopes.

OR

Atoms having the same atomic number (Z) but different mass numbers (A).

Moderator of a nuclear reactor is a substance that slows down the speed of neutrons. In traditional nuclear reactors, the moderator is the same thing as that of coolant like water.

Dalton's atomic theory laid the foundation of modern chemistry with four core postulates:

1. All matter is made up of extremely small particles called atoms.
2. Atoms of the same element are identical to each other in mass and properties; atoms of different elements differ.
3. Atoms can neither be created nor destroyed — they are indestructible.
4. Atoms combine in fixed, simple whole-number ratios to form compound atoms (molecules).

Note: Modern discoveries have refined some postulates (e.g., isotopes show atoms of the same element can differ in mass), but the core framework remains foundational.

• The structure of an atom and its nucleus was developed from the discovery of electrons by J.J. Thomson and alpha particle scattering experiments by Rutherford.
• An atom consists of electrons, protons, and neutrons, with protons and neutrons in the nucleus and electrons revolving in stationary orbits.
• The maximum number of electrons in a shell is given by 2n², and the shells are named K, L, M, N, O, P, and Q.

Isotopes are atoms of the same element that have the same atomic number but different mass numbers (different number of neutrons).

Same in isotopes:

• Atomic number (Z)
• Number of protons and electrons
• Electronic configuration
• Position in periodic table
• Chemical properties (nearly identical)

Different in isotopes:

• Mass number (A)
• Number of neutrons
• Physical properties

Examples: \[_1H^1and_1H^2\]