Test: Basic Units Of Matter - Class 9

The most fundamental building block of matter was a particle called the atom. Now we know that the atom is made of many smaller pieces, known as subatomic particles. Every atom contains a central core called the nucleus, made of particles called protons and neutrons.

Atoms usually exist in two ways (1) in the form of molecules (2) in the form of ions 

The atom with the smallest mass is the hydrogen atom; its mass is about 10^-27 kg. 

A scanning tunneling microscope (STM) is an instrument for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer (at IBM Zrich), the Nobel Prize in Physics in 1986. For an STM, good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm (10 pm) depth resolution.With this resolution, individual atoms within materials are routinely imaged and manipulated. The STM can be used not only in ultra-high vacuum but also in air, water, and various other liquid or gas ambients, and at temperatures ranging from near zero kelvin to over 1000 DEGC.

Atomicity is the total number of atoms present in one molecule of an element or a compound. For example: one molecule of hydrogen (H₂) contains two atoms of hydrogen. Therefore, atomicity of hydrogen is 2. Simialrly, 1 molecule of O₃ contains 3 atoms. Therefore, its atomicity  is 3. molecule of Argon exist as Ar and hence its a monoatomic compound.

A geologist is a person who studies the earth. Geologists study the structure of the Earth, and they have found that it is made up of four main parts. The earth has a solid inner core, a liquid outer core, a mantle, and a crust.hence, option (D) is correct.

Chlorine is called Cl₂ because it is a diatomic molecule.
Diatomic means there are two atoms of the same elements in the molecules.
There are other diatomic molecules, such as hydrogen (H2), Nitrogen (N2), Oxygen (O2), Fluorine (F2), Chlorine (Cl2), Bromine (Br2), Iodine (I2).

Argon is a Noble gas. Noble gases are not very reactive since they have a very stable electron configuration. It is only about 1% of the air in the atmosphere but it is more abundant than any other element on the Periodic Table behind Nitrogen and Oxygen.

A combination of atoms is called a molecule. We know that atoms of most of the elements are very reactive and cannot exist in the free state (as single atoms). This is not so with molecules. Molecules can exist in free state because they are very stable.

An atom can be defined as the smallest particle of an element, which can take part in chemical change or reaction and still retain the identity of the element. Notice that the atom is not the smallest particle of an element, as there are electrons, protons and neutrons which are smaller, but, it is the smallest which can be involved in chemical change and still retain the chemical properties of the element.