Atoms and Atomic Structure
Upon completion of this lesson, the students should be able to:
- Explain the atomic theory
- Identify the basic parts and sub-atomic particles of an atom
- Relate atomic numbers and mass numbers to the basic characteristics of an atoms
- Illustrate electron distribution in an atom
- Explain the four quantum number
- Relate atomic structure to the behavior of matter
Matter is composed of very small ultimate particles, or atoms of matter. The first philosophical statements relating to an idea similar to atoms was developed by Democritus in Greece in the fifth century. He was the one who coined the term atomos, which mean “uncuttable”. The idea was lost for century until scientific interest was rekindled during the Renaissance Period. Between 1803 and 1808, John Dalton performed several chemical experiments to determine the usefulness of atoms. He was the first to realize that the nature and properties of atoms could be used to explain the Law of Definite Composition of all substances developed earlier by Proust and the way and the proportions in which substances react with one another.
Whereas the word atom originally denoted a particle that cannot be cut into smaller particles, the atoms of modern parlance are composed of subatomic particles:
- electrons, which have a negative charge and are the least massive of the three;
- protons, which have a positive charge and are about 1836 times more massive than electrons; and
- neutrons, which have no charge and are about 1839 times more massive than electrons.
Dalton's Atomic Theory
Assumptions of Dalton’s Atomic Theory
- All matter is made up of minute, discreet, indivisible, and indestructible particles called atoms.
- Atoms of the same element are chemically alike: atoms of different elements are chemically different. in particular,the atoms of one element have a different mass than those of other elements.
- When atoms combine to form compounds or when such combination of atoms decomposes, each individual atom retain its identify.
- When atoms combine they do so in small numbers ratios.
Further studies were conducted to examine the assumptions of Dalton’s Atomic Theory. These are as follows:
- Atoms of elements today can be destroyed by artificial transmutation called bombardment of nuclear fission.
- With the existence of isotopes, the second assumption is only partially accepted. Isotopes are atoms of the same element having the same atomic number nut of different atomic mass.
- Atoms consist of smaller particles such as protons, neutrons, and electrons. These particles can be extracted to exist in individual form so that they are also called particles of matter.
- It is true that atoms combine to form molecules, but they combine not only in small ratio but also in large whole number ratio such as formation of organic compounds.
Other Atomic Theories
In 1908-1909, Ernest Rutherford and other scientists performed the experiment on bombarding gold foil with alpha particles. They observed that most (about 99.99%) of the particles passed through the film following a straight path, while some were deflected at large angles and few bounced back.
Based on the results, Rutherford proposed the following
- That the atom consists of a large empty space that explains why most of the particles went straight through the film.
- That the atom consists of a very small region where its positive electricity is concentrated, hence, heavy. The particles that bounced back were presumed to have hit this region.
- Those, which deflected, approached the positive nucleus; hence, there was repulsion since the alpha particles were also positive.
The above reasons describe the central part of the atom, called nucleus, to be with a very small volume yet a massive one.
Bohr’s Atomic Model
In 1913, Niels Bohr, a Danish scientist showed that the wavelength patterns of spectroscope could be related to electronic structures of atoms. According to him, the electrons travel in circular and elliptical orbits about the positive nucleus.
He explained that:
- An electron stays in an orbital, and when at such, it is characterized by a definite amount of energy, so that the energy of an electron is quantized.
- An electron jumps to a higher orbital to another, when it jumps from a lower to higher orbital, there is emission of energy.
- An electron revolves around the nucleus in a definite circular path.
Basic Parts of an Atom
Neil Bohr pictured the atoms as consisting of three basic kinds of particles: electrons, protons, and neutrons. The electron is a particle possessing a negative (-) electrical charge. The proton is a particle consisting of a positive charge (+) electrical charge equal in magnitude (but opposite in type) to the charge of the electron. The neutron is a particle with no electrical charge. The proton and neutron have essentially the same weight.The protons and the neutrons are packed together at the center of the atom to form what is known as the nucleus. Electrons travel around the nucleus of an atom, which are at relatively large distances from the nucleus.
Characteristics of an Atom
- All elements are observed to be electrically neutral, despite the presence of electrically charged particles in atoms. The number of positive protons in the nucleus of an atom is equal to the number of electrons surrounding the nucleus.
- Since elements differ from one another, their atoms must differ structurally. Each element has an atomic number, the atomic number is equal to the number of electrons revolving about the nucleus of the atom. Since atoms are electrically neutral, the atomic number also equals the number of protons present in the nucleus of an atom.
- Equal number of atoms of different elements weighed under the same conditions has a different weight. Atoms of different elements have different atomic weights, the atomic weight of an atom is equal to the sum of the number of protons and the number of neutrons in the nucleus of an atom. Thus, all of the weight of an atom comes from its nucleus. Atomic weights are relative, they do not give the number of grams that an atom weights, but they merely tell how much heavier or lighter an atom of one element is than another.
History of the Atomic Theory and the Structure of the Atom
Atoms of the same element can have the same atomic number but will differ in their atomic masses and these are called isotopes. The first isotopes discovered were those of neon by Thomson and Aston in 1912-1913. The mass spectrograph is a precise instrument used to determine the atomic masses to 1 part in 10,000.
Distribution of Electrons and Electronic Configuration
Electrons revolve around the nucleus of an atom in a definite pattern. Groups of electrons maintain definite average distances from the nucleus forming shell or energy levels of electrons surrounding the nucleus. Each shell is capable of containing a definite number of electrons, the number increasing in distance from the nucleus increases. Letters – k, l, m, n, o, and p starting with the shell nearest the nucleus, designates the shells. The maximum number of electrons in any shell can be calculated from the relationship:
Number = 2s
Number = maximum number of electrons possible in the shell
S = the number of the shell (K=1, I=2. m=3, etc)
The energy levels are further subdivided into sublevels designated by the letters s, p, d, f, g … (in alphabetical order), the number of which corresponds to the number of the energy level.
Each sublevel has a set of orbital, which are of equal energy.
K (n=1) one sublevel: 1s
L (n=2) two sublevels: 2s and 2p
M (n=3) three sublevels: 3s, 3p, and 3d
S = 2 electrons
p = 6 electrons
d = 10 electrons
f = 14 electrons
Orbital is the home of the electrons or the region of spsce where the probability of finding the electrons is greatest
s sublevel has one orbital with 2 electrons
p sublevel has 3orbitals with 8 electrons
d sublevel has 5 orbitals with 18 electrons
Electrons in the atoms are found in the orbitals. The particular arrangement of electrons in the orbitals of an atom is called its electronic configuration. An orbital is the home of the electrons and the region of space where the probability of finding the electron is greatest.
Rules in building up the electronic configuration:
- The number of electrons entering the atom must be equal to its atomic number, z, and the number of protons. Thus, the atom is neutral.
- No more than 2 electrons with opposite spins can enter any single orbital (Pauli’s Exclusion Principle.)
- When there are orbitals of the same kind of energy, the electrons occupy the equivalent orbitals singly to the maximum and with their spins parallel (Hund’s Rule).
- The opposite spin may be represented by arrows pointing upwards and downwards.
Write the electronic configuration and the distribution of electrons in the main energy levels of the following:
- Na 11e 1s2 2s2 2p6 3s1
- K 19e 1s2 2s2 2p6 3s2 3p6 4s1
- O 8e 1s2 2s2 2p4
Quantum Mechanics or Orbital Theory
The Wave Mechanics or Quantum Mechanics or Orbital Theory of the atom replaced Bohr’s Theory.
Light exhibits dual wave-particle properties. Interference and diffraction patterns formed when light passes through slits can only be explained by the addition of waves. Discontinuous emission of light from hot bodies can only be explained by particle-like photons of emitted light. Louis de Broglie reasoned that if light can exhibit wave and particle properties, then tiny moving particles of matter might also exhibit wave properties. De Broglie’s theory of matter-waves was experimentally proven by the diffraction pattern produced by electrons directed towards crystals just as light waves are diffracted through slits.
Werner Heisenberg’s Principle of Uncertainty
We can measure a visible particle’s position, its direction and speed of motion. However, according to Heisenberg, if an invisible tiny particle like an electron exhibits properties of a wave, it is impossible for us to know both its instantaneous location in space and its exact energy. To describe the allowed energy states of an electron, mathematical expressions based on quantum mechanics were applied considering wave properties of electrons moving around the nucleus, the attraction of electrons for the nucleus and the kinetic energy of motion of electrons. Heisenberg’s Uncertainly Principle holds that if we know the energy of an electron with high accuracy, its location is uncertain. In the quantum mechanical description of the atom we can just state the probability of finding an electron in a certain region of space surrounding its nucleus at a given instant.
Regions of high electron density are regions where an electron is most likely to be found. These regions are called orbitals. The term orbital is used for an allowed energy state as well as the distribution of electron density in space. An orbital has both a characteristic energy and a characteristic shape in space around the nucleus is three- dimensional, three of the quantum numbers described the orbital occupied by an electron. The fourth quantum number describes the spin of an electron in space.
The Principal Quantum Number (n) is associated with the distance of the electron from the nucleus and it determines the gross energy of the electron.
The Second Quantum Number (Azimuthal Quantum Number) (I) gives the shape of the orbital. It has integral values from 0 to n-1.
The Third Quantum Number (Magnetic Quantum Number) (m ) describes the orientation of the orbital in space. The integral values may be l1, l-1, l-2 down to -l. Positive values of m describe orientation in the direction of applied magnetic field while negative values refer to orientation in the opposite direction.
The Fourth Quantum Number is the electron spin quantum number (m ). It describes the spinning of the electron on its axis. It can have a clockwise spin or counterclockwise spin.
Pauli’s Exclusion Principle states that no two electrons can have the same set of four quantum numbers.
- What is the modern atomic theory?
- What are the main parts of an atom? Be able to describe each of the particles.
- What is a shell or energy level?
- How are shells or energy levels designated?
- Define the following: a.orbital b.Quantum Mechanics c. Isotopes d. Atomic Number e.Atomic Weight
- describe Neil Bohr's Atomic Model
- Explain the Quantum Number Theory.
- Draw diagrams to show the distribution of electrons protons, and neutrons of atoms of helium, neon, argon, krypton and radon.
- What is the Heisenberg’s Uncertainly Principle? Why is it applicable to electrons but not a moving vehicle?
- Write the electronic configuration of the following: Carbon, Magnesium, Potassium, Calcium and iron.