Revision: Structure of the Atom Science English Medium Class 9 CBSE

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.

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.

Atomic number refers to the number of protons present in an atom. It is denoted by Z. Example: An atom of oxygen contains 8 proton Therefore its atomic number is 8.

The protons and neutrons collectively are known as nucleons.

Mass number refers to the sum of the number of protons and neutrons present in the nucleus of an atom and denoted by A Mass number = Number of protons + Number of neutrons.

The outermost shell of an atom is known as its valence shell.

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.

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.

Mass number of an element is equal to the sum of protons and neutrons in the nucleus of the atom.

The number of protons in the nucleus of an atom, which is characteristic of a chemical element and determines its place in the periodic table. Atomic number is also equal to numbers of electrons in an atom.

For example:

• Atomic number of carbon = 6
• Atomic number of nitrogen = 7
• Atomic number of oxygen = 8
• Atomic number of magnesium = 12

Atomic number is the number of protons in the atom of an element.

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’.

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.

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).