Atom

ATOM

is tiny basic building block of matter. They are the smallest particles of a chemical element that still exhibit all the chemical properties unique to that element. the name atom comes from the Greek átomos (from α-, "un-" – temno, "to cut"), which means uncuttable, or indivisible, something that cannot be divided further.

INDIVIDUAL ATOMS OF ELEMENT SILICON SHOWN USING SCANNING TRANSMISSION EECTRON MICROSCOPE. THE ATOMS IN EACH PAIR ARE LESS THAN A TEN-MILLIONTH OF A MILLIMETER (LESS THAN A HUNDRED-MILLIONTH OF AN INCH) APART

EARLY CONCEPTS ABOUT ATOM

ARISTOTLE – believed that matter was continuous and could be divided endlessly into smaller portions

LEUCIPPUS – concluded that there must be ultimate particles that could not be further subdivided

DEMUCRITUS - said that all matter was composed of tiny, indivisible particles called atoms and gave the particles name “atomos”

•JOSEPH PRIESTLEY -a British chemist, who isolated and described several gases, including oxygen though he did not recognize it as a new element.

ANTOINE LAVOISIER - he demonstrated that burning is a process that involves the combination of a substance with oxygen. He formulated the theory of combustion: substances combine with oxygen of the air as they burn. He also demonstrated the role of oxygen in animal and plant respiration.

HENRY CAVENDISH – discovered the properties of the element hydrogen and determined its specific gravity. His most celebrated work was the discovery of the composition of water; he stated that “water consists of dephlogisticated air (oxygen) united with phlogiston (hydrogen).”

JOSEPH LOUIS PROUST– established the law of definite proportions, sometimes called Proust's law, which states that the elements in a compound are all present in a fixed proportion by weight, regardless of how the compound is prepared.

JOHN DALTON –formulated the law of multiple proportions stating that if two elements form more than one compound, the differing masses of one element that combine with a fixed mass of the second element are simple, whole-number ratio.

1. All elements are made up of tiny, invisible particles called atoms. Atoms can be neither created nor destroyed during chemical reactions.

2. All atoms of a given element are identical, but the atoms of one element differ from the atoms of every other element.

DALTON’S ATOMIC THEORY:

3. Atoms of different elements form compounds by combining in fixed, small whole-number ratios such as 1 atom of A to 1 atom of B, 2 atoms of A to 1 atom of B, and 3 atoms of A to 2 atoms of B.

4. If the same elements form more than one compound, there is a different, but definite, small whole-number mass ratio and atom ratio for each compound.

5. A chemical reaction involves a change not in the atoms themselves, but in the way atoms are combined to form compounds.

J. J. THOMSON discovered electron. His work suggested that the atom was not an "indivisible" particle but a jigsaw puzzle made of smaller pieces. He imagined that atoms looked like pieces of raisin bread, a structure in which clumps of small, negatively charged electrons, the ("raisins") were scattered inside a smear of positive charges later called proton.

He showed that electrons have charge/mass equal to 1.76 x 10 11 C/kg

R.A. Millikan Measured the electronic charge (1.60 x 10 -19

C) therefore the mass of the electron is 9.11 x 10 -31

Kg)

EUGEN GOLDSTEIN discovered that atoms had positive charges. He suggested this claim upon observing the stream of light in a cathode ray tube.

JAMES CHADWICK discovered the neutron

ERNEST RUTHERFORD suggested that the atom consisted of a small, dense core of positively charged particles in the center (or nucleus) of the atom, surrounded by a swirling ring of electrons.

Rutherford's atom resembled a tiny solar system with the positively charged nucleus always at the center and the electrons revolving around the nucleus.

ATOMIC STRUCTURE

• ERNEST RUTHERFORD theorized that positive charge and mass were concentrated in the center of the atom. He called this concentrated region of electric charge the nucleus of the atom.

ATOMIC STRUCTURE NIELS BOHR proposed nuclear model of the atom, in which the atom is seen as a compact nucleus surrounded by a swarm of much lighter electrons. His model posits that an atom emits electromagnetic radiation only when an electron in the atom jumps from one quantum level to another.

ATOMIC STRUCTURE ERWIN

SCRHÖDINGER formulated the theory of wave

mechanics, which describes the

behavior of the tiny particles that make

up matter in terms of waves. He

formulated the wave equation to describe

the behavior of electrons

ATOMIC STRUCTURE

ATOMIC STRUCTURE

FUNDAMENTAL PARTICLES OF ATOM ELECTRONS are negatively charged particle with a mass of 0 amu (atomic mass unit) that occupy the space around the nucleus of an atom. They carry a negative electric charge of –1.602 x 10-19 coulomb and has a mass of 9.109 x 10-31

ATOMIC STRUCTURE

FUNDAMENTAL PARTICLES OF ATOM

PROTONS are positively charged particles with a mass of 1 amu. Along with neutron they build the nucleus of the atom. They determine the element’s identity. They have positive electrical charge of 1.602 x 10-19 and mass of 1.67 x 10-27 kg.

ATOMIC STRUCTURE

FUNDAMENTAL PARTICLES OF ATOM NEUTRONS are electrically neutral particles with a mass of 1 amu. The neutron is about 10-13 cm in diameter and weighs 1.6749 x 10-27 kg. Neutrons help stabilize the protons in the atom's nucleus by reducing the repulsion between protons.

ATOMIC STRUCTURE

Atomic mass unit (amu) – arbitrary units assigned to the relative masses or atomic weights of the elements. It is the basic unit of mass of atoms and molecules exactly 1/12 the mass of one 12C atom. I amu = 1.6606 x 10 -24 gAtomic weights – relative masses of average atoms of the elements expressed in amuCoulomb (C) - is the unit of electric charge

OTHER CONCEPTS ABOUT ATOM

• at ground state, every atom is neutral in electrical charge because the number of electrons equals the number of protons

• the mass number (A) is equal to the number of protons plus the number of neutrons.


• all atoms of a particular kind of element have the same number of protons. This number of protons in the nucleus of an atom is defined as the atomic number (Z). The atomic number for an element is the same as the number of protons in the nucleus of each atom of that element.

• an atom’s number of proton is equal to the number of electron

• an atom may have definite atomic number and mass number and is called nuclide. • atomic mass is the average atomic mass for the naturally occurring element given in atomic mass unit (amu)


Example:A particular kind of atom has 61 neutrons and a mass number of 108.(a) How many protons does this atom have?(b) How many electrons does this atom have?(c) What is the atomic number of this element?

(accounting for particles of atom)

OTHER CONCEPTS ABOUT ATOMSolution:(a)The atom has 47 protons because:

number of protons = mass number =number of neutrons

number of protons = 108-61=47

(b) The atom has 47 electrons, the same as the number of protons.(c) The atomic number is 47. The atomic number is the number of protons.


Exercise: Answer the same three questions for an atom that has 18 neutrons and a mass number of 35.


ELEMENT Atomic number

Mass number

No. of protons

No. of neutrons

No. of electrons

Carbon -12Carbon – 13AluminumUraniumChlorine

66

1213

238

613 14

1892

6 6 67 6

2713 1392 14692

17 35 1717


H. MOSELEY - calculated that elements have fixed number of protons specific to that element. Atoms of other elements have different number of protons. He established that chemical properties of atoms are determined by their nuclear charge.

He established that atomic numbers are more fundamental than atomic masses.

OTHER CONCEPTS ABOUT ATOMOTHER PARTICLES OF ATOM

nucleusmade up of

hadronsclassified into

Baryons(3

quarks)

Mesons(quark-antiquark pair)

Antibaryons(3

antiquarks)Ex. I up & I down quark

Neutron1 up & 2 down

quarksProton

2 up & 1 down quarks

• Hadrons are elementary particles made up of gluons or quarks or both and participate in the strong reaction. •Baryons are subatomic particles belonging to a group that undergoes strong reactions, have a mass greater than or equal to that of the proton, and consist of three quarks

• Mesons are medium-sized elementary particle such as pion or kaon that has a rest mass between that of an electron and a proton and participates in the strong interactions. It consists of a quark and an antiquark. •Antibaryons a subatomic particle that is antiparticle of a baryon consist of 3 antiquarks

•Quark is an elementary particle with an electrical charge equal to one-third or two-thirds that of the electron. It makes up the proton and neutron as fundamental particles. Protons and neutrons, the particles that make up the nuclei of atoms, consist of quarks. According to theorists, there are 6 types of quarks, up, down, beauty, truth, charmed, strangeness. Ups have a kind of charge equal to 2/3 and downs have equal to 1/3

OUTSIDE THE NUCLEUS

LEPTONS

ELECTRONS

MUONS

NEUTRINOS

• electron neutrinos

• tau neutrinos•muon neutrinos

consists of

TAU PARTI-CLES

•Leptons a family of extra-nuclear particles consist of electrons, muons, tau particles, and three corresponding types of neutrino (electron neutrinos, muon neutrinos, and tau neutrinos). Electrons are tiny, negatively charged particles that surround the nuclei of atoms. Muons are negatively charged particles found in cosmic rays, subatomic particles that travel to Earth from outer space. Tau particles are negatively charged particles and are extremely rare, but scientists have detected them in laboratory experiments. Electrons, muons, and tau particles are called the charged leptons. Neutrinos are tiny particles with little or no mass and no electric charge.

ELECTRONIC STRUCTURE OF ATOM

ELECTRONS • are elementary particles that are stable negatively charged elementary particle with a small mass that is fundamental constituent of matter and orbits the nucleus of an atom.


• they form electric currents by flowing in a stream and carrying their negative charge with them.•because they form the outer layers of atoms, they are also responsible for many of the physical and chemical properties of the chemical elements. Electrons help determine how atoms of an element behave with respect to each other and how they react with atoms of other elements.

ELECTRONIC STRUCTURE OF ATOM• they are among the smallest of all elementary particles and have no detectable shape or structure. At the same time, they do have a property that scientists can measure called spin, or intrinsic angular momentum. An electron’s spin makes it act as a tiny magnet. Electrons can spin clockwise or counterclockwise

ELECTRONIC STRUCTURE OF ATOM•Electrons occupy definite energy levels called principal energy levels, outside the nucleus. The principal energy levels are divided into sublevels and that the number of sublevels is equal to the value of the principal quantum number. Their values begin with 1 for the level having the lowest energy, followed by 2, 3 ,4, and 5. Principal energy levels are divided into sublevels and that the number of sublevels is equal to the value of the principal quantum number so that, energy level 1 has one sublevel, 2 has two, 3 has three and so on. The lowest energy level is labeled s the next p, the third d and the fourth f.


These letters stand for sharp, principal, diffuse, and fine respectively. Other sublevels existing at higher principal energy levels are represented alphabetically starting with g followed by h, I and so on. Each sublevel corresponds to orbital type. Principal energy level 2 has two types of orbitals: one s orbital and three p orbitals. An atomic orbital takes in no more than two electrons.


Thus, the p sublevel can hold six electrons. The electron arrangement in the available atomic orbitals in successive sublevels of the atom is called electronic configuration. Electronic configuration is the distribution of electrons in atomic orbitals



1 2 3 4 5

Z Ele-ment

s s p s p d s p d f s p d f

1 H 1

7 N 2 2 3

11 Na 2 2 6 1

16 S 2 2 6 2 4

29 Co 2 2 6 2 6 10 1

46 Pd 2 2 6 2 6 10 2 6 10

The Aufbau Principle

• used to predict the configurations. It says that the buildup of electrons in atoms results from continually increasing the quantum numbers.

RULES FOR THE BUILD-UP OF EC

1. Each added electron enters an orbital of the lowest energy level and sublevel available.2. No more than two electrons can be placed in any orbital.

3. Before a second two electron can be placed in any orbital , all the orbitals of that sublevel must contain at least one and then each could get a second electron (The Hund’s Rule)


Exercise:Work on the electron configuration of the first ten elements:

(in ground state)

ELECTRON SPIN

• electrons exhibit a property known as spin, a motion much like that of the earth rotating on its axis, however can be clockwise or counter clockwise. If one object spins in a clockwise direction, and another object spins in a counter clockwise direction, the objects are said to have opposite spins.

ELECTRON SPIN

• no two electrons in an atom may have the same set of four quantum numbers• an orbital may hold no more than two electrons• according to, in order for two electrons to occupy the same orbital, they must have opposite spins.

Pauli exclusion principle

ELECTRON SPIN

half-filled orbital is represented by a box (as orbital) containing a single arrow pointing either upward or downward

hydrogen 1 s

filled-orbital is represented by a box containing two arrows, one pointing upward and the other pointing downward

helium1s

ELECTRON SPIN

The arrow pointing upward represents an electron with one kind of spin. The arrow pointing downward represents an electron with opposite spin. Two oppositely spinning electrons occupying the same orbital are called an orbital pair.

ELECTRON SPIN

Example:

Boron (5 electrons)

Oxygen (8 electrons)

The Energies of Orbitals

• an atom with its electrons in the lowest-energy orbitals is said to be ground state the state of lowest energy • if electrons occupy any other orbitals, the atom is said to be in excited state

RELATIONSHIP BETWEEN SHELLS, SUBSHELLS, ORBITALS AND

ELECTRONS

Shell no. n

No. of sub-shell in shell

Subshelldesignation

No. of orbitals in subshell

Original designation

Maximum no. of e in subshell

Maximum no. of e in shell

1 1 1s 1 1s 2 2

2 2 2s 1 2s 2

2p 3 2p 6 8

3 3 3s 1 3s 2

3p 3 3p 6

3d 5 3d 10 18

4 4 4s 1 4s 2

4p 3 4p 6

4d 5 4d 10

4f 7 4f 14 32

THE QUANTUM NUMBERSA. Principal Quantum Number, n

• correlates with the distance of the electron from the nucleus or the average distance rather than a fixed distance. The greater the value of the principal quantum number, the farther from the nucleus will the electron most probably be found and the higher will be the energy of the electron.

• all electrons having the same value of n are said to be the same shell or principal energy level• with respect to size, the larger the value of n the larger the orbital

B. AZIMUTHAL QUANTUM NUMBER, l

-describes the orbital’s shape labelled as s, p, d, f

s – 0 sphericalp – 1 two-lobedd – 2 four-lobedf – 3 six or eight-lobed

orbital number shape

• for n=1, there is only s orbital• for n=2, there is both s and p orbitals• for n=3, there are s, p, and d orbitals• for n=4, there are s, p, d and f orbitals

the shorthand designation for the energy level and type of orbital consisting of the principal quantum number followed by the letter corresponding to the azimuthal quantum number. And Thus:n=1 and in the orbital for which l=0, the designation is 1 s, in the orbital with n=3 and l=2, the designation is 3d. Then, the set of orbitals having both the same prinicpal quantum number, n, and the same azimuthal quantum number, l, is called a subshell

SHAPES AND ORIENTATIONS OF SUBLEVELS

C. MAGNETIC QUANTUM NUMBER ml

-specifies the orientation of an orbital in space and can take any integral value from -1 to +1 including 0. The possible values for ml are 0, ±1, ±2 …… ±l• the spherical s orbital (l=0) has only one orbital in a sublevel because ml can have only the value 0.

C. MAGNETIC QUANTUM NUMBER ml

• the two-lobed p orbitals (l=1) occur in sets of three because ml can have values of -1, 0, and +1

• the d orbitals (l=2) occur in sets of five, having values of -2, -1, 0, +1 and +2• the f orbitals occur in sets of seven ml =-3, -2, -1, 0, +1, +2 and +3.

D. SPIN QUANTUM NUMBER ms

• The spin quantum number can have a value of + ½ or -½

• describes as the spin of the electron about its own axis which can occur in either clockwise or a counter clockwise direction

Quantum number

Values Number of Values

significance

Principal, n

1, 2, 3, . . . .

- Distance from the nucleus

Azimuthal , l

0, 1, 2,. . . N - 1

n Shape of orbitals

Magnetic, ml

0, ±1, ±2, . . ±1

2l + 1 Orientation of orbital

Spin, ms -½, + ½ 2 Direction of electron spin

Sub Energy LevelsThe number of sub energy levels in any main energy level is equal to the value of n.

n sublevel l ml ms

n=1

s 0 0

n=2

s,p 0,1 -1,0,+1

n=3

s,p,d 0,1,2 -2,-1,0,+1,+2

n=4

s,p,d,f 0,1,2,3

-3,-2,-1,0,+1,+2,+3

ELEMENTAL CLASSIFICATION ON THE BASIS OF ELECTRON

CONFIGURATION

Elemental Classification on the basis of electronic configurations

Noble gases configuration – an electronic configuration in which the last 8 electrons occupy and fill the s and p subshells of the highest occupied shellRepresentative elements where the distinguishing electron is found in an s and p subshell

Elemental Classification on the basis of electronic configurations

Transition elements – the distinguishing electron is found in a d subshellInner transition elements – distinguishing electron is found in an f subshell

Electron configurations of transition metals

•The difference in energy between ns and (n-1)d orbitals is not very large. This causes some transition metal atoms to have irregular electron configuration patterns.

1 2 3 4 5

Z Element s s p s p d s p d f s p d f

21 Sc 2 2 6 2 6 1 222 Ti 2 2 6 2 6 2 223 V 2 2 6 2 6 3 224 Cr 5 1

25 Mn 2 2 6 2 6 5 226 Fe 6 227 Co 7 228 Ni 8 229 Cu 10 1

• there is gradual filling of the 3d orbital that can be caused by acquisition of an electron from the 4s orbital •At chromium, both the 3d and 4s orbitals are occupied, but neither is completely filled .This suggests that the energies of the 3d and 4s orbitals are relatively close for atoms in this row.

ISOTOPES

Isotopes are atoms of the same element having different mass numbers due to different numbers of neutrons in their nuclei. All atoms have corresponding isotopes.

When writing symbols for isotopes, the mass number is written as a superscript and the atomic number as a subscript, both to the left of the symbol of the element.

Example: Hydrogen has 3 isotopes which are given corresponding names – Protium (I proton), Deuterium (1 proton + 1 neutron), Tritium (1 proton + 2 neutrons). Hydrogen however is the only atom with isotopes given special names.

16Oxygen – 16 (8 P + 8 N), 17Oxygen (8 P + 9 N), 18 Oxygen (8 P + 10 N)

ELECTRON CONFIGURATION OF MONOATOMIC IONS

Important terms

Cation- positively charged ion, having less number of electrons than proton

Anion- a negatively charged ion, having more electrons than protons

Ion- an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge Monoatomic ion- an ion containing only one atom

EC of monoatomic ions can be described by adding or subtracting electrons from those of the parent atoms. For anions, which has more electrons than the atom of the corresponding element, the additional electrons are simply added according to the Aufbau principle

• Write the electronic structure for the neutral atom, and then add (for a negative ion) or subtract electrons (for a positive ion).

Electronic structure of the representative elements (s and p

block)

Example 1:

Write the electronic configuration of the P3-

Solution:The phosphorus atom has a total of 15 electrons. The phosphide ion has 3 more, for a total of 18. Its electronic configuration is the same as that of element 18, Argon. The first 10 electrons go into the 1s, 2s, and 2p orbitals. The last 8 fill the 3s and 3p orbitals. Thus the EC is:

1s22s22p63s23p6

1s22s22p63s23p3

Ex. 2

Write the EC of oxide of O2- ion

1s22s22p4

1s22s22p6

The formation of cations requires the removal of electrons from the parent atom. It involves the removal of a electron with no change in nuclear charge. In general, when electrons are removed from an atom, those electrons in the outermost shell (highest n) are removed first. Thus, a 4s electron should be removed before a 3d electron.

Example 1:

To write the electronic structure of Na+

Na 1s22s22p6s3s1 Na+ has one electron

Na+ 1s22s22p6

Electron Configuration of transition element ions

• The 4s orbitals behaves as the outermost, highest energy orbital.

• Thus, when the transition elements (d-block) form ions, the 4s electrons are lost first

Remember:

To write the electronic structure for Cr3+

Cr 1s22s22p63s23p63d54s1

Cr3+ 1s22s22p63s23p6 4s0 3d3

or

The 4s electron is lost first followed by two of the 3d electrons.

1s22s22p63s23p6 3d3

•Electrons are responsible for many important physical phenomena, such as electricity and light, and for physical and chemical properties of matter. This explains the phenomena behind the production of semiconductors.

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