Structure of Atom

Groups in chemistry refer to the arrangement of elements of the periodic table into vertical columns. These vertical columns, called groups, are numbered from 1 to 18 and run from top to bottom. The first column on the left is labelled Group 1, and the last column on the right is labelled Group 18. 

The elements in the group have the same number of electrons (valence electrons) in their outermost shell, also known as the valence shell, due to which they all have the same chemical or physical properties.

Periods in chemistry refer to the horizontal rows of elements in the periodic table, which represent all 118 elements across 7 rows. 

The periods run from left to right, with each consecutive element increasing in atomic number by 1.  

Periods 1, 2, and 3 have 2, 8, and 8 elements, whereas periods 4, 5, 6, and 7 have 18, 18, 32, and 32 elements.

The outermost electrons of an atom that are mostly involved in bonding reactions are called valence electrons. These electrons are farthest from the nucleus and have high energy.

For example, the total number of electrons in Lithium is three distributed in two energy levels, closest to farthest from the nucleus- 1s² 2s¹.

An atom has a central, dense, tightly packed nucleus with a diameter of 10^-15 m compared to an atom at 10^-10 m, a size equivalent to a marble in a football stadium. Therefore, the nucleus makes up less than 0.01 % of the atom's volume.

The nucleus contains positively charged protons and neutral neutrons as subatomic particles bound firmly by the nuclear force.

Molecular mass is the sum of the masses of all the atoms in the molecule, denoted as m.

The mass of the atoms is measured in the average atomic mass unit, a weighted average of all the element's naturally occurring isotopes.

The sum is represented in amu (μ) or Da unit.

A molecule is an electrically neutral chemical entity that consists of two or more bond atoms.

The joining of two or more atoms by a linkage known as a covalent bond creates molecules such that changing the connection, or the arrangement will change the molecule and its properties. 

The matter is composed of invisible, indivisible, innumerable particles called atoms. Atoms are responsible for giving each matter its unique properties and identity. The uniqueness comes from the nature of the atom and its composition. Atoms are composed of three subatomic particles- electron, neutron, and proton. For example, an Oxygen atom has 8 electrons, 8 protons, and 8 neutrons.

The Average Atomic mass of an element is the weighted average of the atomic masses of all naturally occurring isotopes of an element. It is also known as the atomic weight and is expressed in amu.

The weighted average gives a more accurate value than the normal average by considering all isotopic forms of an element.

The unit used to describe the mass of an atom is the unified atomic mass unit, symbolized as amu or μ.  

The standard used for measuring the mass of an atom is a ¹²C atom. Carbon-12 (¹²C) is an isotope of Carbon with 6 protons, 6 neutrons, and 6 electrons.

The unified atomic mass unit is defined as exactly 1/12 ^th the mass of one Carbon-12 atom.

The mass number is the total count of the number of protons and neutrons present in the nucleus of an atom, denoted as A.

For example, ¹²C has 6 protons and 6 neutrons; its mass number is 12.

A mass number is always a whole number. The mass number is also known as the atomic mass number or nucleon number.

The Atomic number (symbol Z) uniquely identifies an element and is equal to the number of protons present in the nucleus of an atom.

For example, atomic number 6 identifies the Carbon atom with 6 protons in its nucleus. An uncharged Carbon atom will also have 6 electrons in the outer nuclear region, equal to its atomic number.

Isotopes are a group of atoms belonging to an element with the same atomic number but a different mass number. Such a group of atoms have the same number of protons in their nuclei but differ only in the number of neutrons.

For example, the Oxygen atom has three isotopes – ¹⁶O, ¹⁷O, and ¹⁸O. All three isotopes have 8 protons; however, the number of neutrons is 8 in ¹⁶O, 9 in ¹⁷O, and 10 in ¹⁸O.

A Neutron is an uncharged, neutral subatomic particle found in an atom's nucleus, denoted as n or n⁰.

A proton is one of the subatomic particles found in every atom’s nucleus, denoted as p or p⁺.

An electron is a negatively charged elementary particle that constitutes an atom, denoted as e^- or β^-. The electrons are present in the outer-nuclear region of an atom as clouds.