Samacheer Class 7 Science - Atomic Structure

A proton is a sub-atomic particle with a positive electric charge. Protons are located in the nucleus of an atom at its center. The positive charge of a proton is equal in magnitude but opposite in sign to the negative charge of an electron. This means that one proton has the same amount of charge as one electron, just with opposite polarity. Protons have a mass approximately equal to that of a neutron. The number of protons in an atom determines the atomic number and the identity of the element. For example, all atoms with one proton are hydrogen atoms, all atoms with six protons are carbon atoms, and so on.

Neutrons are called neutral particles because they carry no electric charge. Unlike protons, which are positively charged, and electrons, which are negatively charged, neutrons are electrically neutral. This means they do not participate in electrical interactions within the atom. Neutrons are located in the nucleus along with protons. Despite having no charge, neutrons have mass approximately equal to that of protons. The presence of neutrons in the nucleus contributes to the mass number of an atom. Different isotopes of the same element have the same number of protons but different numbers of neutrons, which is why neutrons are important in distinguishing between isotopes.

Isotopes and isobars are two different ways of classifying atoms based on their subatomic composition. Isotopes are atoms of the same element that have the same atomic number but different mass numbers. This means isotopes have the same number of protons but different numbers of neutrons. For example, hydrogen has three isotopes: protium with mass number 1, deuterium with mass number 2, and tritium with mass number 3. All three have atomic number 1 but differ in their number of neutrons. Isobars, on the other hand, are atoms of different elements that have the same mass number but different atomic numbers. This means isobars have different numbers of protons but the same total number of nucleons. For example, calcium and argon are isobars because both have a mass number of 40, but calcium has atomic number 20 while argon has atomic number 18. Isotopes are chemically similar because they have the same number of electrons, while isobars are chemically different because they have different numbers of electrons.

The atomic number and the mass number are two fundamental properties of an atom that help identify an element and describe its structure. The atomic number (represented by Z) is defined as the total number of protons in the nucleus of an atom, or equivalently, the number of electrons in a neutral atom. It uniquely identifies a chemical element. For example, Helium has an atomic number of 2 because it has 2 protons. The mass number (represented by A) is the total number of protons and neutrons in the nucleus of an atom. It represents the approximate mass of the atom in atomic mass units. For instance, Sodium (Na) has 11 protons and 12 neutrons, so its mass number is 11 + 12 = 23. While the atomic number is constant for an element, the mass number can vary due to the presence of isotopes, which are atoms of the same element with different numbers of neutrons.

The atomic number of an element is determined by the number of protons in its nucleus. Given that the atomic number is 9, this means the element has 9 protons. The mass number of an atom is the sum of the number of protons and neutrons in its nucleus. We are given that the atom has 10 neutrons. Therefore, the mass number can be calculated as: Mass Number (A) = Number of Protons (p) + Number of Neutrons (n). Substituting the given values, A = 9 + 10 = 19. An element with an atomic number of 9 is Fluorine (F). Thus, the element is Fluorine, and its mass number is 19.

The structure of an atom consists of a central nucleus surrounded by electron shells. The nucleus is located at the center of the atom and contains two types of sub-atomic particles: protons, which carry a positive charge, and neutrons, which are electrically neutral. The nucleus is extremely dense and contains most of the mass of the atom. Surrounding the nucleus are electron shells or energy levels, which are the outermost regions of the atom. These shells contain electrons, which are negatively charged particles. Electrons orbit around the nucleus in these shells, held in place by the attractive electrostatic force between the negatively charged electrons and the positively charged nucleus. The electrons are arranged in specific shells or orbitals, with the innermost shell being closest to the nucleus. The number of electrons in an atom equals the number of protons in a neutral atom, making the atom electrically neutral overall. The size of the atom is determined mainly by the electron shells, while the nucleus is extremely small in comparison.

The atomic number of an element is equal to the number of protons in its nucleus. Given that the atomic number is 26, the atom has 26 protons. In a neutral atom, the number of electrons is equal to the number of protons, so there are also 26 electrons. The mass number is the sum of protons and neutrons. Given the mass number is 56 and the number of protons is 26, we can calculate the number of neutrons: Number of Neutrons (n) = Mass Number (A) - Number of Protons (p). So, n = 56 - 26 = 30. The element with atomic number 26 is Iron (Fe). Therefore, an atom of Iron with a mass number of 56 has 26 protons, 26 electrons, and 30 neutrons.

Nucleons are the sub-atomic particles that are located in the nucleus of an atom. Specifically, nucleons refer to protons and neutrons together. They are called nucleons because they are the particles that make up the nucleus. The term nucleon comes from the word nucleus, emphasizing that these particles are the fundamental components of the atomic nucleus. Nucleons have several important properties. First, protons and neutrons have approximately the same mass, making them nearly identical in terms of mass, differing mainly in their electrical charge. The proton carries a positive charge of +1, while the neutron is electrically neutral with zero charge. Second, both nucleons are extremely small, with a size of approximately 1 to 6 femtometers (fm), where one femtometer is 10 to the power of negative 15 meters. Third, nucleons are held together in the nucleus by a very strong nuclear force that acts over an extremely short range. This nuclear force is much stronger than the electrostatic repulsion between protons, which is why the nucleus remains stable despite the positive charges of protons repelling each other.

Valency is defined as the combining capacity of an element, which represents the number of electrons an atom can lose, gain, or share when forming chemical bonds with other atoms. The valency determines how many atoms of another element a particular element can combine with. For the element with atomic number 8, which is oxygen, the valency is 2. Oxygen has 8 protons and typically 8 electrons arranged as 2, 6 in its electron shells. To achieve a stable electron configuration, oxygen needs to gain 2 electrons or share 2 electrons, giving it a valency of 2. When oxygen combines with hydrogen, which has a valency of 1, the compound formed is water with the chemical formula H₂O. In water, two hydrogen atoms combine with one oxygen atom because oxygen needs 2 electrons to complete its outer shell, and each hydrogen atom can provide 1 electron through sharing. Water is the most common and familiar compound of oxygen with hydrogen.

An atom with no electrons will still have mass because mass depends on the number of protons and neutrons in the nucleus, not on the presence of electrons. Electrons have negligible mass compared to protons and neutrons, so removing electrons does not significantly affect the mass of an atom. Therefore, an atom without electrons will retain most of its original mass. Atoms can indeed exist without electrons. Such atoms are called ions or ionized atoms. When an atom loses electrons, it becomes positively charged because it has more protons than electrons. Matter is built from protons, neutrons, and electrons, and while atoms are normally electrically neutral with equal numbers of protons and electrons, atoms can exist in ionized states where this balance is disrupted. An example of an atom without electrons is He²⁺, which is a helium nucleus with 2 protons and 2 neutrons but no electrons. This is also called an alpha particle. Other examples include H⁺ (a hydrogen nucleus with 1 proton and no electrons) and O²⁻ (an oxygen ion with 8 protons but 10 electrons, though this has extra electrons rather than none).

Common salt, scientifically known as sodium chloride, is an ionic compound essential for life and widely used in cooking and industry. It is formed by the chemical combination of two elements: Sodium (Na) and Chlorine (Cl). The chemical formula for common salt is NaCl, indicating that one atom of sodium combines with one atom of chlorine. The atomic number of Sodium (Na) is 11, and its mass number is approximately 23. The atomic number of Chlorine (Cl) is 17, and its mass number is approximately 35. In the ionic compound NaCl, sodium loses one electron to form a positively charged ion, Na⁺ (sodium ion), and chlorine gains one electron to form a negatively charged ion, Cl⁻ (chloride ion). These oppositely charged ions are held together by strong electrostatic forces.

Atomic numbers and mass numbers are always whole numbers because they are based on counting discrete sub-atomic particles. The atomic number represents the number of protons in the nucleus of an atom. Since protons are indivisible particles that cannot be split into fractions, the number of protons must always be a whole number. There cannot exist a fraction of a proton in an atom; an atom either has a proton or it does not. Similarly, the mass number represents the total count of nucleons, which includes both protons and neutrons. Since both protons and neutrons are discrete particles that come in single units and cannot be divided, the total number of nucleons must also be a whole number. For example, an atom cannot have 2.5 protons or 3.7 neutrons. Each proton and neutron is a complete particle, so when we count them, we always get whole numbers. This is why atomic numbers and mass numbers are always expressed as whole numbers and never as decimals or fractions.

Atomic number of Sulphur = No.of proton sulpher
= 16
Atomic mass number of Sulphur = No. of Protons + No. of Neutrons
= 16 + 16
= 32

The element used for breathing, essential for life, is Oxygen, and its chemical symbol is O. The gas used in filling balloons, known for its lightness, is Helium, and its most common isotope has a mass number of 4. The element present in bananas that is important for nerve function and muscle contraction is Potassium, and its atomic number is 19. The element found in crackers that contributes to their crispness and is also found in bones and teeth is Phosphorous, and it has 15 protons in its atoms. The most valuable element, known for its role in organic chemistry and life itself, is Carbon, and its most common isotope has a mass number of 12.