Samacheer Class 7 Science - Matter Around Us

A chemical formula is a symbolic representation that shows the types and numbers of atoms present in one molecule of an element or compound. It uses the symbols of elements along with numerical subscripts to indicate the exact proportion in which atoms are combined. The significance of a chemical formula is that it provides essential information about the composition of a substance. It tells us which elements are present in the compound and how many atoms of each element are bonded together in a single molecule. For example, the chemical formula H₂O indicates that water contains two hydrogen atoms and one oxygen atom. The chemical formula also helps in understanding the properties of the compound, calculating its molecular weight, and predicting how it will react with other substances. Furthermore, the chemical formula is a universal language that allows scientists worldwide to communicate the exact composition of compounds without ambiguity.

Elements are the simplest forms of matter that cannot be broken down into simpler substances by chemical reactions. They are the fundamental building blocks of all matter. Elements are made up entirely of one type of atom, meaning all atoms in an element are identical and have the same atomic structure. Each element is characterized by a specific number of protons in the nucleus of its atoms, which determines its atomic number and its position in the periodic table. Two examples of elements are hydrogen and oxygen. Hydrogen is the lightest and most abundant element in the universe, consisting of atoms with one proton and one electron. Oxygen is a non-metal element that is essential for respiration in most living organisms and exists as diatomic molecules (O₂) in its elemental form. Both hydrogen and oxygen are pure substances that cannot be decomposed further by ordinary chemical means.

A molecule is the smallest unit of a substance that retains all the chemical properties of that substance. It is formed when two or more atoms combine together through chemical bonding. A molecule can consist of atoms of the same element, such as oxygen molecules (O₂) where two oxygen atoms bond together, or it can consist of atoms of different elements, such as water molecules (H₂O) where hydrogen and oxygen atoms combine. The atoms within a molecule are held together by strong chemical bonds, and the specific arrangement and bonding of atoms determine the properties and behavior of the molecule. Molecules are the fundamental units that make up compounds and many elemental substances.

A compound is a pure substance formed when atoms of two or more different elements combine chemically in a fixed and definite proportion. The elements in a compound are held together by strong chemical bonds, and the properties of a compound are entirely different from the properties of the individual elements that compose it. For example, hydrogen and oxygen are both gases at room temperature, but when they combine in the ratio 2:1, they form water (H₂O), which is a liquid at room temperature. Similarly, sodium is a highly reactive metal and chlorine is a toxic gas, but when they combine chemically, they form sodium chloride or common salt (NaCl), which is a white crystalline solid used in cooking and food preservation. These examples demonstrate that compounds have unique properties that differ significantly from their constituent elements.

Metals
Non-Metals
Metals are lustrous. They have a shiny
Non metals are non lustrous. They
surface
have non- shiny surface
Metals are generally hard
Non-metals are generally soft
Most metals are bendable
Non-metals are non bendable
Most metals can be bent, beaten into sheets and they can drawn into wires
Non-metals are non ductile
Most metals are good conductors of electricity
Non-metals are bad conductors of electricity
Most metals are good conductors of heat
Non-metals are bad conductors of heat
Most metals are making ringing sound when struck. Hence, they are used to make objects like bells
Non-metals does not make any sound when they struck
Ex. : Copper, Lead, Tin, Nickel
Ex. : Carbon, Iodine, Sulphur

Compounds have several important characteristics that define their nature and behavior. A compound is formed only when the constituent elements combine in a fixed proportion by mass, which means the ratio of elements in a compound is always constant. The properties of a compound are completely different from those of its constituent elements. For example, sodium chloride (table salt) is formed from sodium and chlorine, but it has entirely different properties from both elements. A compound cannot be broken down into its constituent elements by physical methods such as filtration, evaporation, or magnetic separation because the elements are chemically combined at the atomic level. However, a compound can be separated into its constituent elements by chemical methods only, such as electrolysis or chemical reactions. This is because the bonds between atoms in a compound are strong and require chemical energy to break them. The fundamental particles of a compound are molecules, which consist of atoms of different elements bonded together in fixed ratios.

The following rules are followed while assigning symbol to an elements:
Chemical symbols usually consist of one or two letters.
The symbols of most elements correspond to the first letter (which is capitalized) of their English name. For example, the symbol for oxygen is “O” and that for hydrogen is “H”.
When there is more than one element that begins with the same letter, their symbols take two letters.
The first letter is capitalised while the second letter has a lower case.
For example, the names of both hydrogen and helium begin with H. So, hydrogen is represented by the symbol H and Helium by He.
Example:

Elements and compounds are fundamentally different forms of matter with distinct characteristics. An element is the simplest substance that cannot be broken down into simpler substances by any chemical method. It consists of only one type of atom. In contrast, a compound is a chemical substance formed by the combination of two or more elements in a fixed proportion. Elements combine chemically to form compounds, and compounds can be broken down into their constituent elements through chemical methods. The fundamental particle of an element is an atom, which is the smallest unit that retains the properties of the element. The fundamental particle of a compound is a molecule, which consists of atoms of different elements bonded together. Elements have their own characteristic properties, such as melting point, boiling point, and density, which remain constant. Compounds have different properties from their constituent elements. For example, hydrogen and oxygen are elements with their own properties, but when they combine chemically in a fixed ratio, they form water, which is a compound with completely different properties from both hydrogen and oxygen.

Compounds possess five important characteristics that distinguish them from elements and mixtures. First, a compound is formed only when the constituent elements combine in a fixed proportion by mass, meaning the ratio of elements is always constant and definite. Second, the properties of a compound are different from those of its constituent elements. For example, sodium chloride has different properties from both sodium and chlorine. Third, a compound cannot be broken down by physical methods such as filtration, evaporation, or magnetic separation because the elements are chemically bonded together. Fourth, a compound is made up of different elements that are chemically combined at the atomic level, forming strong chemical bonds. Fifth, a compound can be separated into its constituent elements by chemical methods only, such as electrolysis, heating, or chemical reactions, because these methods provide enough energy to break the chemical bonds between atoms. These characteristics make compounds distinct from both elements and mixtures.

Metals and non-metals have distinctly different properties that allow us to classify elements into these two categories. Metals are generally lustrous, meaning they have a shiny and reflective surface. They are typically hard and strong, though some metals like sodium and potassium are exceptions. Most metals are malleable, which means they can be bent and beaten into thin sheets without breaking. Metals are also ductile, meaning they can be drawn into thin wires. Additionally, metals are good conductors of heat and electricity. Examples of metals include iron, copper, and aluminum. Non-metals, on the other hand, are generally non-lustrous, having a dull surface without shine. They are usually soft and brittle, meaning they break easily when bent or struck. Non-metals are non-malleable and non-ductile, so they cannot be shaped into sheets or wires. Most non-metals are poor conductors of heat and electricity, with the exception of carbon in the form of graphite. Examples of non-metals include oxygen, nitrogen, and sulfur.

Metalloids are elements that possess properties intermediate between metals and non-metals, making them unique in the periodic table. Physically, metalloids usually look like metals with a shiny appearance, but they behave largely like non-metals in their chemical reactions. They are brittle solids, meaning they break easily when bent or struck, unlike the malleability of metals. Metalloids have intermediate to relatively good electrical conductivity, which is better than non-metals but less than metals, making them useful as semiconductors in electronic devices. They can form alloys with metals, combining with metallic elements to create new materials with useful properties. Most of their physical and chemical properties are intermediate in nature, falling between the extremes of metals and non-metals. All metalloids are solids at room temperature. Common examples of metalloids include silicon, which is widely used in computer chips and solar cells, germanium, boron, and antimony. These elements play an important role in modern technology, particularly in the semiconductor industry.

Metals and their uses in our houses & schools :
Magnesium is used in the laboratory as magnesium ribbon.
Copper is used cooking utensils.
Zinc is mainly used as a protective coat for iron is our school and house gates.
Aluminum is used as cooking utensils and electrical cables which are used in our schools and houses.
Iron and steel are widely used is construction of house and school.
Sodium is used as a table salt and for flavoring, preserving food.
Coins are made up of nickel.
Lead is used in car batteries. Lead based alloys find extensive use in printing
Non-metals and their uses in houses & school:
Oxygen is essential for the breathing. It is used as oxidising agent in laboratory. Chlorine is used for purifying water.
Graphite is used in pencil leads.
Carbon is used as a fuel.
Bromine is used in dyes.
Iodine is used in laboratory to test for starch.
Metalloids and their uses in houses & schools :
Silicon is used in glass items.
Antimony is used in optical discs.
Tellurium is used in solar cells.
Boron is used in washing powders.
Germanium is used to make transistors which is used in laboratory in electrical devices.
Comparsion of properties of metals, non-metals & metalloids
Metals
Non-metals
Metalloids
They have lustre
They are not lustrous
Intermediate
Conduct heat and electricity
Poor conductors of electricity except graphite
Intermediate (semi­conductors)
They are malleable and ductile
They are neither malleable nor ductile.
Intermediate

When a substance is heated, significant changes occur in the movement and arrangement of its particles. The internal energy of the substance increases, which causes the particles to move faster and more vigorously. The increased kinetic energy of the particles leads to greater movement and vibration. As the particles gain more energy, the bonds between them weaken, and the distance between particles increases. During phase changes such as melting or evaporation, the bonds between particles break, allowing them to move more freely. This causes the volume of the matter to increase, which is why substances expand when heated. The closeness and arrangement of particles change as they move farther apart, and the motion of particles becomes more rapid and energetic. However, despite these changes in movement and arrangement, the size of the individual particles remains the same. The mass of the matter also does not change during heating because no particles are added or removed from the substance. Only the spacing between particles and their kinetic energy change, not the particles themselves.