Chapter 3 Atoms and the Periodic Table
For centuries, many have tried to learn about the smallest particle. As far as 2500 years ago, Democritus and Leucippus created an idea for the atom and called it ‘atmos’ meaning indivisible. Various philosophers and scientists have proposed models for what and how the atom should be. The initial theory proposed by Dalton in 1803 states that atoms are indivisible and that all atoms of a particular material are identical. He also stated that atoms of different materials join to form compounds. J.J. Thompson proposed the initial model of the atom. It is called the plum pudding model. He theorized that the atom consists of negatively charged particles placed in a positive charge as plums in a pudding. E. Rutherford brought in the nuclear model where he discovered that the atom has a positive nucleus where the electrons that are negatively charged revolve around the nucleus. He tried the gold foil experiment to prove this theory.
The Bhor’s model states that the electrons do not randomly revolve around the nucleus, but it is similar to the planets revolving around the sun in an orderly manner. The Quantum mechanical model by E. Schrödinger states that the electrons move around in a random manner and the area where the possibility of finding an electron is highest is called the orbital (Fig. 1).
An atom consists of electrons (negative charge) and a nucleus which has the neutrons (neutral charge) and the protons (positive charge). In a neutral atom, the number of electrons and protons are equal. The atomic number (A) of an atom denotes the number of protons, and the mass number (Z) denotes the number of nucleons (protons + neutrons). An element is usually represented byAtomic no.Mass no.Symbol.
Radioactivity is the process by which elements with an unstable nucleus decompose to form a stable element. The element emits radiation during decomposition. Radiation consists of Alpha, Beta and Gamma particles.
Atoms of a few elements have different mass numbers since the number of neutrons is different. The ones with different Z value are called isotopes.
Bohr proposed a model for the electromagnetic spectra of an atom. Based on their energy levels, electrons are placed in orbits around the nucleus. Adjacent orbits have an energy difference of one quantum energy. Electrons can shift orbits by either gaining or losing energy. Valence electrons are ones that are present in the last orbit or ‘shell’. Each orbit consists of orbitals where the probability of finding the electron is high. The electrons are arranged such that they remain in the lower energy to maintain stability.
Mendeleev proposed a widely accepted periodic table in 1869. He arranged elements based on similarities in their properties. The table helped him predict the properties of germanium before discovery. His table could not cover all the elements. A modern periodic table was created where all the elements were arranged based on their atomic and mass number. It consists of 103 elements and 18 groups(columns) and 7 periods(rows). Vertically, the elements are also classified into metals, non-metals, metalloids and transition metals.
Chapter 5 – Chemical Bonding And States Of Matter
Compounds consist of atoms of different materials bound together. This bonding is done based on the valence electrons of an atom. Two atoms hold because of the chemical bonding between them which can be either ionic or covalent. The properties of elements in the periodic table having the same number of valence electrons are similar. Nobel gases are the only elements which do not combine with other elements since their outer shell is filled and hence stable. Metals try to become stable by donating electrons while nonmetals become stable by receiving electrons and hence they mostly combine with each other. Electronegativity is the tendency of an atom to attract electrons and electropositivity is the tendency of an atom to donate electrons. In the periodic table, the electronegativity of elements increases across the row and decreases down the group. If the electronegativity is high among two atoms, the bond is ionic. If it is less, the bond is covalent.
Ionic bonding is noticed among metal and non metal bonding. The metal donates the electron and hence it becomes positively charged (cation) and the non metal gains an electron and becomes negatively charged (anion). The elements in the first two groups donate electrons while the elements in groups 15, 16 and 17 attract electrons. Based on the number of electrons an element will donate and the number of electrons another element will accept, we can predict how they combine with each other. For example, the element in group two will donate two electrons while the element in group 17 will only want one. Hence the atom of an element in group two will combine with two atoms of the element in group 17.
Naming a compound has certain rules. Before the anion, the symbol of the cation is written. The ratio in which they have to be taken is calculated and written in the subscript of the element. When two or more atoms combine, they form polyatomic ions, and they are usually negatively charged.
When 2 atoms share their valence electrons, a covalent bond is created. This bonding mainly occurs between atoms with a lesser electronegative difference between them. The first element is named fully and the second element ends with –ide while being named. These compounds are called molecular compounds.
The valences of the elements are calculated, and a pair of electrons connect the atoms. The remaining electrons are filled around the more electronegative atom such that it satisfies the octet rule. If not, double and triple bonds occur to make sure the octet rule is satisfied.
The above method gives a 2D view of the atom bonding. To have a 3D view, VSEPR theory is used. Based on the combination of the elements in a covalent bond, linear, trigonal planar, tetrahedral and trigonal bipyramidal shapes are formed.
There are three states of matter- solids, liquids and gases. In solids, the atoms are closely spaced and in an orderly arrangement. It is difficult to compress solids. They occupy finite volume. In liquids, the atoms are more loosely packed than the solids.
They do not have a finite volume and take the volume of the container. In gases, the atoms move freely. Hence they do not have any finite volume or thickness (Fig. 2).
Normally, gases on cooling become liquids (condensation) which on further cooling become solid (crystallization) e.g., steam, water, ice. The process where a gas converts to a solid directly is called sublimation e.g., CO2, dry ice (Fig. 3).
Works Cited:
[1] Buescher, Lee. Atomic Structure Timeline. Web. 20-Apr-2014.
[2] Bento, Yinon. Chemical Elements. Web. 20-Apr-2014.
