Revision: Term - 1 >> Matter Around Us Science SSLC (English Medium) Class 7 Tamil Nadu Board of Secondary Education

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

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")`

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.

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

Ex.: Copper, Iron, Silver, etc.

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

Ex.: Oxygen, Hydrogen, Gold & Helium.

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 : Number of protons present in the nucleus of an atom.

Mass number : Numbr of protons and neutrons present in the nucleus of an atom.

Atomic weight : It is the ratio that tells how many times an atom of an element is heavier than atom of Hydrogen

Atomic number— Atomic number refers to the number of protons present in an atom. It is denoted by Z.

Atom: An atom is the smallest indivisible unit of an element which exhibits all the properties of that element and may or may not have an independent existence. An atom is the smallest indivisible unit of an element which exhibits all the properties of that element and may or may not have an independent existence. 

Element is a substance which cannot be broken further into simpler substances and has a definite set of properties. Elements are made up of only one kind of atoms.

Formula: Formula is a short way of representing the molecule of an element or a compound

Element is a substance which cannot be split up into two or more simple substances by usual chemical methods of applying heat, light or electric energy; for example, hydrogen, oxygen and chlorine.

Molecule: A molecule can be defined as the smallest unit of an element or a compound which exhibits all the properties of that element or compound and has an independent existence. They are divisible into atoms.

Compounds are pure substances composed of two or more elements in definite proportion by mass and has properties, entirely different from those of its constituents elements.
Compound, are made up of different types of atoms combined chemically.

Elements: An element is defined as a pure substance made up of only one kind of atoms that cannot be converted into anything simpler than itself by any physical or chemical process. 

“Mixtures can be defined as. a kind of matter which is formed by mixing two or more pure substances (elements and compounds) in any proportion, such that they do not undergo any chemical change and retain their individual properties. Therefore they are impure substances.

The number of atoms in a molecule of an element is called its atomicity. 

Certain elements which show properties of both metals and non metals are called metalloids or semi-metals.

Non-metals, except hydrogen, are the elements which form negative ions by the gain of electrons.

Elements : An element is the basic form of matter that cannot be broken down into simpler substances by chemical reactions.

Compounds : A compound is a pure substance formed by the chemical combination of two or more elements in a fixed ratio by mass.

Alchemy was a form of chemistry studied in the middle age, which was concerned with trying to discover ways to change ordinary metals into gold.

Compounds: Compounds are pure substances composed of two or more elements in definite proportion by mass and has a definite set of properties. The compound is made up of only one kind of molecules

The number of atoms in a molecule of an element is called its atomicity. 

Carbonic acid

• Types of Molecules: Monoatomic (He), Diatomic (H₂), Triatomic (O₃), Tetratomic (P₄), Octatomic (S₈).
• Homoatomic & Heteroatomic: Same atoms = homoatomic (O₃), different atoms = heteroatomic (HCl).
• Gay-Lussac’s Law: Gases react in simple volume ratios at the same temperature and pressure.
• Avogadro’s Law: Equal gas volumes at the same conditions have an equal number of molecules.
• Molecular Formula from Volumes: Volume ratios help find formulas, e.g., NH₃ from 2 NH₃ → 1 N₂ + 3.