Atoms and Atomic Structure

By Prof. Liwayway Memije-CruzCAS

Introduction

Matter is composed of very small ultimate particles, or atoms of matter.

State of the Environment: Issues and Concerns

Democritus (c. 460 – c. 370 BC) an ancient Greek pre-Socratic philosopher

He developed the first philosophical statements relating to an idea similar to atoms

He was the one who coined the term atomos, which mean “uncuttable”

John Dalton: (6 September 1766 – 27 July 1844) an English chemist, meteorologist and physicist

determined the usefulness of atoms.

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.

The atoms are composed of subatomic particles:

Electrons have a negative charge and are the least massive of the three;

Protons have a positive charge and are about 1836 times more massive than electrons; and

Neutrons have no charge and are about 1839 times more massive than electrons.

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.

Illustration of Dalton’s Theory

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.

Law of Definite Composition

When atoms combine to form compounds or when such combination of atoms decomposes, each individual atom retain its identify.

Rutherford’s Experiment

He 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

1. That the atom consists of a large empty space that explains why most of the particles went straight through the film.

2. 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.

3. 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.

known for his foundational contributions to further understanding of the atomic structure and quantum theory.

Niel Bohr (1885 – 1962) a Danish physicist

Isotopes

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.

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.

Distribution of Electrons

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 Where: Number = maximum number of electrons possible in

the shell S = the number of the shell (K=1, I=2. m=3, etc)

Sublevels

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 space where the probability of finding the electrons 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.

Wave Mechanics/Quantum Mechanics or Orbital 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.

Quantum Numbers 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 counter clockwise spin.

Pauli’s Exclusion Principle states that no two electrons can have the same set of four quantum numbers.

Shapes of Orbitals

Exercise: C.HOPKINS CaFe Mighty good in a pinch of salt)

Element At. no

At. mass e _ p+ n0 Atomic

Structure

nG P Valence Nature

Self-Progress Test

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 configurations of the following: Carbon, Magnesium,

Potassium, Calcium and iron.

Thank you!