Atomic Structure

RELATED TASK

Describe atomic structure, quantities and units of electro-magnetic radiations, x-ray and gamma rays, primary and secondary radiation.

ATOMIC STRUCTURE.

• Atom is the smallest unit of matter that define the chemical element.
• Atom made up of two main parts, that are,
• Nucleus: contains positive protons (p) and neutral neutrons (n)

Shells: Contain electrons (e) that circles the nucleus in their energy level.

NUCLEUS.

• Nucleus is the central part of an atom where almost the mass of an atom is present.
• Nucleus is composed of two main particles.
• Proton
• Neutron

Protons are positively charged particle present in nucleus.

Proton have a mass of 1.6726×10−27 kg. That is, they weigh 1,836 times the mass of electrons.

Nucleus

• Neutrons are charge less particle present in a nucleus of an atom.
• Neutron have a mass of 1.6929×10−27 kg. that is, they weigh 1,839 times mass of electrons

Both protons and neutrons in a nucleus of an atom are called Nucleons.

Nucleus

The number of protons present in nucleus of an atom is called Atomic number. And is denoted by a letter ‘Z’.

The total number of protons and neutron in a nucleus of an atom is called Mass number. And is denoted by a letter ‘A’.

Shell

Shell is an orbit that consist of an electron(s) which evolves around the nuclear of an atom.

• Shell are also known as Energy level.
• Electron is negatively charged particle that revolves on its orbit around the nucleus.
• Electron has a mass of 9.11×10−31 kg.

Each shell (orbit) has its maximum carrying capacity of electron.

Shell

• The capacity of shell to carry the electrons is determined by the formula.

2n2 where n is the number (position) of shell from nuclear.

The number of shells are numbered from the inner most shell to outer ward from the nuclear.

The inner most shell are numbered as one (n=1) and others are continuously numbered in a sequence.

Shells are also named as K, L, M, N

Shell

Types of Electrons

Electrons are either bound or free.

Bound electrons: These are the electrons that are held in orbit around the nucleus in the electron shells by the attractive force of the positive nucleus.

The binding energy is the positive energy required to overcome the pull of the nucleus and release the electron from the shell.

Types of Electrons

Free electrons: These are the electrons that are not bound in an electron shell around a nucleus.

• They have a kinetic energy of:
• Kinetic energy = ½mv2

where: m = mass, v = velocity

The actual binding energy of electrons is expressed in electron volts (eV) or keV (1keV = 1000 eV) 1 eV = 1.6022 x 10 -19 joules

Free electron

Key points

Increase in the atomic number = increase in the binding energy of the electrons (there are more protons and, therefore, more energy is needed to release the electrons from the greater positive pull) Increase in the distance between the nucleus and the electron = decrease in the binding energy of the electron (decrease in the positive pull of the protons in the nucleus)

Atom

The number of the proton and electron in an atom is equal, thus, this make the atom to be electrically neutral.

If either number of proton or electron exceed in an atom, it become electrically charged and is called an Ion.

• If proton exceed electron it become positively charged. And the atom is named Cation.

And if electron exceed it become negatively charged. And the atom is named Anion.

Atom

Atom

The number of proton in an atom determine it’s chemical element properties since each element has it’s own number of proton.

For example hydrogen atom has only one proton in its nucleus, while Sodium atom has eleven protons in its nucleus.

If the number of proton changed in the nucleus of an atom that atom changes its chemical element properties.

Atom

Atom of an element become charged by either losing or gaining electron on its outer most shells.

Atom if gain extra electron it become negatively charged. And if loses the electron it become positively charged.

The process of either gaining or losing of electron in an atom is called Ionization.

Ionization

Atom

The atom of the same chemical element is said to have the same number of neutrons in its nucleus.

But there are occasion where the number of neutrons in atom of the same chemical element differs.

If the number of neutrons in atoms of the same chemical element differs these atoms are called Isotopes

Isotopes

Isotope are atoms of the same chemical element but different number of neutrons in its nucleus.

Isotope

Atomic Structure

Isotones are atoms (nuclides) having the same number of neutrons but differ proton number. Example boron-12 and carbon 13 both contain 7 neutrons.

Isobars are atoms (nuclides) of different chemical elements having the same atomic mass (number of nucleons). Example 40Ar, 40K and 40Ca.

Isobars

Isotones

Periodic table

The periodic table is a tabular arrangement of the chemical elements, organized on the basis of their atomic number, electron configurations, and recurring chemical properties.

Elements are presented in order of increasing atomic number, which is typically listed with the chemical symbol in each box.

Periodic table

The standard form of the table consists of a grid of elements laid out in 18 columns and 7 rows, with a double row of elements below that.

The table can also be deconstructed into four rectangular blocks: the s-block to the left, the p-block to the right, the d-block in the middle, and the f-block below that.

• Periods: Are rows of the table

Groups: Are the columns of the table

Periodic table

Periodic table

In the standard periodic table, the elements are listed in order of increasing atomic number.

A new row (period) is started when a new electron shell has its first electron.

Columns (groups) are determined by the electron configuration of the atom; elements with the same number of electrons in a particular subshell fall into the same columns.

For example oxygen and selenium are in the same column because they both have four electrons in the outermost p-subshell.

Periodic table

A group or family is a vertical column in the periodic table.

Elements within the same group generally have the same electron configurations in their valence shell.

Consequently, elements in the same group tend to have a shared chemistry and exhibit a clear trend in properties with increasing atomic number.

Blocks of periodic table

Electronic configuration

• Electronic configuration is the arrangement of electrons within the shells of an atom.
• Shells are divided into subshells within it.
• The subshells are named s, p, d, f.
• The outer most shell (subshell) are called Valence shell.

The shell (subshell) are also known as Energy levels.

Valence electron

Electronic configuration

• The shell K or 1 has only one subshell s and the shell L or 2 has two subshell s and p.

As the number of shell increases also the number of subshell increases.

The number of shell in the atom of the chemical element are represented by the number of period in the periodic table.

Electronic configuration

Electron occupies first the electronic shell of the lowest energy level available in the atom.

• The electronic shell near to the nucleus are of lower energy level.

The energy associated to an electron is that of its orbital.

The configuration that corresponds to the lowest electronic energy is called the ground state.

Electronic configuration

The electron occupies the lowest available energy level is said to be at a ground state.

When electron absorb energy, it jumps to higher energy level. And this state is known as Excited state.

• Also electron can absorb energy that allows it to move out of electron shells of an atom.

If this happen the atom is said to be Ionized .

Electronic configuration

• And the ejected electron is known as ionized electron.

The energy used to remove electron from an atom is known as Ionization energy.

Ionization energy is the minimum energy required to eject electron completely out of electron shell of an atom.

Ionization energy is also known as Binding energy.

Bohr’s atomic theory

In an atom, electrons (negatively charged) revolve around the positively charged nucleus in a definite circular path called orbits or shells.

Each orbit or shell has a fixed energy and these circular orbits are known as orbital shells.

Atomic Structure

The energy levels are represented by an integer (n=1, 2, 3…) known as the quantum number. This range of quantum number starts from nucleus side with n=1 having the lowest energy level. The orbits n=1, 2, 3, 4… are assigned as K, L, M, N…. shells and when an electron attains the lowest energy level, it is said to be in the ground state.

The electrons in an atom move from a lower energy level to a higher energy level by gaining the required energy and an electron moves from a higher energy level to lower energy level by losing energy.

Bohr’s atomic theory

• ∆E = E2 – E1 = hf
• where h is the Planck's constant=6.626 x 10-34

j-sec.

Atomic Structure

Introduction

Introduction

• Atom is the smallest unit of matter that define the chemical element.
• Atom made up of two main parts, that are,
• Nucleus: contains positive protons (p) and neutral neutrons (n)

Shells: Contain electrons (e) that circles the nucleus in their energy level

Types of Electrons

Electrons are either bound or free.

Bound electrons: These are the electrons that are held in orbit around the nucleus in the electron shells by the attractive force of the positive nucleus.

The binding energy is the positive energy required to overcome the pull of the nucleus and release the electron from the shell.

This is of the same magnitude as the actual (negative) energy of the electron that is released if the electron is freed.

Atom

Isotopes are atoms of the same chemical element that have different number of neutrons in its nucleus.

For example cobalt-60 atoms and cobalt atoms.

Both radioactive cobalt-60 atoms and cobalt atoms have 27 protons but cobalt-60 atoms have 33 neutrons, instead of 32 neutrons which are present in a stable cobalt atoms.

Carbon has three naturally occurring isotopes: all of its atoms have six protons and most have six neutrons as well, but about one per cent have seven neutrons, and a very small fraction have eight neutrons.

Isotopes

Atom

Isotones are atoms (nuclides) having the same number of neutrons but differ proton number. Example boron-12 and carbon 13 both contain 7 neutrons.

Isobars are atoms (nuclides) of different chemical elements having the same atomic mass (number of nucleons). Example 40S, 40Cl, 40Ar, 40K and 40Ca.

Periodic table

The standard form of the table consists of a grid of elements laid out in 18 columns and 7 rows, with a double row of elements below that.

The table can also be deconstructed into four rectangular blocks: the s-block to the left, the p-block to the right, the d-block in the middle, and the f-block below that.

• The rows of the table are called periods.

The columns of the table are called groups.

Periodic table

n the standard periodic table, the elements are listed in order of increasing atomic number.

A new row (period) is started when a new electron shell has its first electron.

Columns (groups) are determined by the electron configuration of the atom; elements with the same number of electrons in a particular subshell fall into the same columns.

For example oxygen and selenium are in the same column because they both have four electrons in the outermost p-subshell.

Electronic configuration

Bohr’s atomic theory

In an atom, electrons (negatively charged) revolve around the positively charged nucleus in a definite circular path called orbits or shells.

Each orbit or shell has a fixed energy and these circular orbits are known as orbital shells.

The energy levels are represented by an integer (n=1, 2, 3…) known as the quantum number. This range of quantum number starts from nucleus side with n=1 having the lowest energy level. The orbits n=1, 2, 3, 4… are assigned as K, L, M, N…. shells and when an electron attains the lowest energy level, it is said to be in the ground state.

The electrons in an atom move from a lower energy level to a higher energy level by gaining the required energy and an electron moves from a higher energy level to lower energy level by losing energy.