Matter
Universe is made up of material which scientists have named “matter”. The air we breathe, the food we eat, house, where we live, stones, clouds, stars, plants and animals, even a small drop of water or a particle of sand — Every thing in our surrounding is matter.
Anything that occupies space and has mass is called matter . It's composed of atoms, which are the basic building blocks of all matter.
Atom
An atom is the smallest unit of a chemical element. It is made up of a nucleus containing protons and neutrons, surrounded by a cloud of electrons. It is the basic building block of matter and retains the chemical properties of the element.
Modern atomic theory states that:
1. All matter is composed of indivisible particles called atoms.
2. Atoms of the same element are identical in size, mass, and chemical properties.
3. Atoms of different elements have different properties.
4. Atoms combine in simple, whole-number ratios to form compounds.
5. In chemical reactions, atoms are rearranged, but they are neither created nor destroyed.
This theory was proposed by John Dalton in the early 19th century. While some aspects of Dalton's theory have been modified over time, it remains a fundamental concept in chemistry.
Size of atoms range between 30 and 300 pm (pico meter), or between 0.3 and 3 angstroms. Helium (He) is the smallest element, and francium (Fr) is the largest.
Sub Atomic Particles
According to Dalton theory, atoms are the ultimate particles of matter and that the atoms cannot be divided further ie indivisible. The experiments conducted during the latter half of the nineteenth century revealed that the atom is not the ultimate particle.
A particle which is smaller than an atom in size is termed as sub atomic particles . Atom can be broken down into three subatomic particles, They are : protons, electrons, and neutrons.
An electron carries a negative charge of 1.602176634 × 10^−19 coulomb, which is considered the basic unit of electric charge and rest mass of the electron is 9.1093837015 × 10^−28 g, which is approximately 1/1837 of the mass of a proton.
Protons are positively charged sub atomic particle and are found in every atomic nucleus of element along with neutrons. Mass of proton is 1.6726 x 10^-24 g. and 1836 times heavier than an electron.
Neutrons are neutrally charged subatomic particles and mass is approximal equal to 1.674*10^-24 g.
Atom is extremely small and it is mostly empty. It has positively charged nucleus (containing Protons and Neutrons)surrounded by a cloud of negatively charged electrons. The nucleus is less than one ten-thousandth the size of the atom and contains more that 99.9% of the mass of the atom. Nucleus held positively charged protons and electrically neutral neutrons together by a nuclear force. This force is much stronger than the electrostatic force that binds electrons to the nucleus. Atom also have a number of shells or orbits (discrete energy level)in which electrons rotate.
The number of protons present in the nucleus is called the atomic number (Z), the atomic number defines the element properties. The number of neutrons in the nucleus is denoted by N. The mass number (A) of the nucleus is equal to A=Z + N .
Atoms of the same element can have different number of neutrons ,they are called isotopes of that element. As an example hydrogen has three (3) isotopes: hydrogen-1 (hydrogen), hydrogen-2 (deuterium) and hydrogen-3 (tritium), all have same no of proton (1) but different number of neutrons 0,1 and 2 respectively. They have same chemical property but has the different physical property.
Atoms of the different elements can have same number of neutrons ,they are called isobars of that element. As an example , Iron and Nickel. Both are Isobar, have the same mass number which is 58 whereas the atomic number of iron is 26, and the atomic number of nickel is 28.They have different chemical property but has the same physical property.
Molecule
Molecules consist of two or more atoms which are held together by chemical bonds.
Some examples of molecules:
1. Water (H2O)
2. Carbon dioxide (CO2)
3. Oxygen (O2)
4. Glucose (C6H12O6)
5. Sodium chloride (NaCl)
They form the building blocks of all materials and are essential for understanding the properties and behaviors of substances in chemistry.
Atomicity of Molecules
The atomicity of a molecule means the number of atoms that constitute the molecule. It can be classified based on their atomicity:
Monoatomic Molecules
Molecules of some elements have only one atom. Their atomicity is equal to one and they are called monoatomic. For example, all noble gases such as helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn) are monoatomic.
Diatomic Molecules
When two atoms of the same or different elements bonded together and form a molecule are called diatomic molecules .Examples are oxygen gas (O2), nitrogen gas (N2), and hydrogen chloride (HCl)
Triatomic Molecules
These molecules consist of three atoms bonded together. Examples include carbon dioxide (CO2) and ozone (O3).
Tetratomic Molecules
These molecules consist of four atoms bonded together. An example is methane (CH4).
Polyatomic Molecules
These molecules consist of more than four atoms bonded together. Examples include water (H2O), ammonia (NH3), and glucose (C6H12O6).
Frequently Asked Questions
Q: What does photoelectric current depend on?
Ans : The photoelectric current depends upon the intensity of the incident radiation.Higher the intensity of incident radiation more number of photon strike the metal surface and release more photoelectrons .Q: What is the term used for the minimum energy required to remove an photoelectron ?
Ans : Work function .Q: What is the time lag between the incidence of photons and the ejection of photoelectrons?
Ans : No time leg (approximetly less 1 ns) .Q: How does the intensity affect the photoelectric current?
Ans : As intensity increases, the photoelectric effect increases.Q: How does the intensity affect the photoelectric current?
Ans : As intensity increases, the photoelectric effect increases.Q: Light of wavelength 3500 Å is incident on two metals A and B. Which metal will yield more photoelectrons if their work functions are 5.5 eV and 2.5 eV respectively?
Ans : Metal B.Explanation :
Given, λ = 3500 Å = 3.5 X 10^-7m
Energy of incident photons = hv = hc/λ
= 3.536 eV
Work function of metal B is less than 3.536 eV.
