Ch 5: ATOMIC STRUCTURE

Atomic Structure

Atomic structure is the study of the particles present inside an atom and their arrangement. The main subatomic particles are electrons, protons and neutrons.

Dalton’s Atomic Theory

Dalton considered atoms to be the smallest and indivisible particles of matter. Later discoveries proved that atoms contain smaller subatomic particles.

Subatomic Particles

Subatomic particles are smaller particles present inside an atom. Electrons are negatively charged, protons are positively charged and neutrons have no charge.

Cathode Ray Tube

A cathode ray tube is a partially evacuated glass tube containing a cathode and an anode connected to a high voltage source. It was used to study cathode rays.

Cathode Rays

Cathode rays are streams of negatively charged particles emitted from the cathode and travelling towards the anode. They travel in straight lines and possess mass and kinetic energy.

Properties of Cathode Rays

Cathode rays carry a negative charge and are attracted towards a positive plate. Their nature does not depend on the gas in the tube or the material of the electrodes.

Discovery of the Electron

In 1897, J. J. Thomson proved that cathode rays consist of negatively charged particles called electrons. He concluded that electrons are present in all atoms.

Importance of the Discovery of the Electron

The discovery of the electron proved that atoms are divisible. Therefore, Dalton’s statement that atoms are indivisible was only partially correct.

Anode Rays

Anode rays are positively charged rays that travel in the direction opposite to cathode rays. They are also called positive rays or canal rays.

Canal Rays

Anode rays are called canal rays because they pass through the holes or canals in a perforated cathode. They were discovered during Eugen Goldstein’s experiments.

Properties of Anode Rays

Anode rays travel in straight lines and their particles possess mass and kinetic energy. Their particles are heavier than electrons and carry positive charges.

Formation of Anode Rays

Electrons collide with gas atoms and remove one or more electrons from them. The atoms become positively charged and move towards the cathode as anode rays.

Discovery of the Proton

When hydrogen gas was used, the positive particles obtained were the lightest and had the highest charge to mass ratio. Rutherford identified them as hydrogen ions and they were called protons.

Radioactivity

Radioactivity is the spontaneous emission of rays from atoms of certain elements. It provided further evidence that atoms are divisible.

Thomson’s Model of the Atom

Thomson proposed that an atom is a positively charged sphere with negatively charged electrons scattered throughout it. The positive and negative charges balance each other, making the atom neutral.

Plum Pudding Model

Thomson’s model was called the plum pudding model because the electrons were compared with plums scattered throughout a positively charged pudding.

Alpha Ray Scattering Experiment

Geiger and Marsden directed alpha particles towards a very thin gold foil. The experiment produced observations that could not be explained by Thomson’s model.

Observations of the Alpha Ray Scattering Experiment

Most alpha particles passed straight through the gold foil, some were deflected through small angles and a few were deflected through large angles. About one out of every 12,000 alpha particles rebounded.

Conclusion of the Alpha Ray Scattering Experiment

The observations showed that the positive charge could not be spread uniformly throughout the atom. Therefore, Thomson’s model was rejected.

Rutherford’s Model of the Atom

Rutherford proposed that an atom has a small, dense and positively charged nucleus at its centre. Electrons move around the nucleus and most of the atom is empty space.

Nucleus

The nucleus is the small, dense and positively charged central region of an atom. It contains almost all the positive charge and most of the mass of the atom.

Explanation of Alpha Particle Scattering

Most alpha particles passed through because most of the atom is empty space. Deflections occurred because alpha particles were repelled by the positively charged nucleus.

Size of the Nucleus

Rutherford estimated that the radius of the nucleus is at least 10,000 times smaller than the radius of the atom. This shows that the nucleus occupies a very small part of the atom.

Drawbacks of Rutherford’s Model

Rutherford’s model could not explain the stability of the atom or the arrangement of electrons around the nucleus. It also could not explain the relationship between atomic mass and atomic number.

Stability Problem in Rutherford’s Model

According to electromagnetic theory, moving electrons should continuously lose energy and eventually fall into the nucleus. This would make the atom unstable, but atoms are stable.

Bohr’s Model of the Atom

In 1913, Niels Bohr proposed that electrons move around the nucleus in definite energy levels. Electrons in permitted energy levels do not lose energy.

Stationary Orbits

Stationary orbits are definite paths with fixed energies in which electrons move without radiating energy. They explain why electrons do not fall into the nucleus.

Energy Shells

Energy shells are three dimensional regions around the nucleus where electrons are found. They are represented by K, L, M and N or by the numbers 1, 2, 3 and 4.

Energy of Shells

The energy of a shell increases as its shell number increases. The K shell has the lowest energy.

Maximum Number of Electrons

The maximum number of electrons that can be accommodated in a shell is calculated using 2n². Here, n represents the shell number.

Electron Capacity of Shells

The K shell can hold 2 electrons, the L shell can hold 8, the M shell can hold 18 and the N shell can hold 32.

Subshells

Each energy shell is divided into smaller regions called subshells.

Electron Absorption of Energy

When an electron moves from a lower energy level to a higher energy level, it absorbs energy. The absorbed energy is equal to the difference between the two energy levels.

Electron Emission of Energy

When an electron moves from a higher energy level to a lower energy level, it releases energy in the form of a photon.

Importance of Bohr’s Model

Bohr’s model explained the stability of atoms and the distribution of electrons around the nucleus. It overcame important limitations of Rutherford’s model.

Discovery of the Neutron

In 1932, James Chadwick discovered the neutron, a neutral particle present in the nucleus. The discovery helped explain the relationship between atomic mass and atomic number.

Neutron

A neutron is an electrically neutral particle found in the nucleus of an atom. Its mass is slightly greater than the mass of a proton.

Absence of Neutrons in Hydrogen

Neutrons are present in the nuclei of all atoms except ordinary hydrogen. An ordinary hydrogen atom contains one proton and no neutron.

Electron

The electron is a negatively charged subatomic particle found outside the nucleus. It has a very small mass compared with protons and neutrons.

Proton

The proton is a positively charged subatomic particle present in the nucleus. Its relative charge is plus 1 and it is approximately 1840 times heavier than an electron.

Neutron Properties

A neutron has no electric charge and is present in the nucleus. It has slightly more mass than a proton.

Comparison of Subatomic Particles

Electrons have a negative charge and are found outside the nucleus. Protons have a positive charge and neutrons have no charge, and both are found inside the nucleus.

Alpha Particle

An alpha particle contains two protons and two neutrons. It is positively charged and was used in Rutherford’s gold foil experiment.

Atomic Number

The atomic number is the number of protons present in the nucleus of an atom. It is represented by the symbol Z.

Atomic Number and Electrons

In a neutral atom, the number of protons is equal to the number of electrons. Therefore, atomic number equals the number of protons and the number of electrons.

Importance of Atomic Number

All atoms of the same element have the same atomic number. Different elements have different atomic numbers because they contain different numbers of protons.

Mass Number

The mass number is the total number of protons and neutrons present in the nucleus. It is represented by the symbol A.

Nucleons

Protons and neutrons present in the nucleus are collectively called nucleons. The mass number is the total number of nucleons.

Calculating the Number of Neutrons

The number of neutrons is found by subtracting the atomic number from the mass number. Number of neutrons equals mass number minus atomic number.

Representation of an Atom

An atom is represented using its element symbol along with its mass number and atomic number. The mass number is written at the upper left and the atomic number is written at the lower left.

Carbon Atom Example

Carbon has an atomic number of 6 and a mass number of 12. Therefore, it contains 6 protons, 6 electrons and 6 neutrons.