Revision: Atomic Structure Chemistry ICSE ICSE Class 7 CISCE

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.

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

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

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

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

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

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

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.

A tabular arrangement of elements in groups (vertical columns) and periods (horizontal rows), highlighting the regular trends in properties of elements, is called a Periodic Table.

OR

The classification of elements resulting from an arrangement of the elements in an increasing order of their atomic numbers is the modern periodic table.

The horizontal rows are called periods. The table consists of 7 periods, numbered from 1 to 7 from top to bottom.

The periodic table is a tabular arrangement of elements in horizontal rows, called periods, and vertical columns, called groups, to classify elements and facilitate their systematic study.

In the modern periodic table, groups (also known as families) are the 18 vertical columns that organize chemical elements by shared properties. Unlike periods, which are horizontal rows representing electron shells, groups are vertical columns of elements with similar chemical properties.

Atomic size is the distance between the center of an atom i.e., from the nucleus to the outermost shell (valence shell) of that atom.

Elements are pure substances made up of one type of atom.

Molecular formula: It is a symbolic representation of a molecule. It shows the number of atoms of each element present in it. These atoms combine in whole number to form the molecule.

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.

• Dobereiner grouped elements in threes (triads) with similar properties and a pattern in atomic masses.
• Newlands found that every 8th element shared similar properties (the Law of Octaves).
• Mendeleev arranged elements by atomic mass and predicted new elements, but couldn’t explain isotopes and rare earths.
• Moseley fixed the flaws by arranging elements by atomic number, forming the modern periodic table.
• Bohr proposed the long-form periodic table based on electron arrangement.