Atoms, Ions and Molecules

Dr. M. Abd-Elhakeem

College of Biotechnology

General Chemistry

Lecture 2

Composition of Matter

Matter - Everything in universe is composed of matter Matter is anything

that occupies space or has massMass – quantity of matter an object has

Weight – pull of gravity on an object

Elements Pure substances that cannot

be broken down chemically into simpler kinds of matter

More than 100 elements

Compounds Most

elements do not exist by themselves

Readily combine with other elements in a predictable fashion

A compound is a pure substance made up of atoms of two or more elements The proportion

of atoms are always fixed

Atoms The simplest particle

of an element that retains all the properties of that element

Properties of atoms determine the structure and properties of the matter they compose

Our understanding of the structure of atoms based on scientific models, not observation

The Nucleus Central core Consists of

positive charged protons and neutral neutrons

Positively charged

Contains most of the mass of the atom

The Protons All atoms of an element have the

same number of protons Number of protons balanced by

an equal number of negatively charged electrons

The Neutrons The number varies slightly

among atoms of the same element

Neutrons are neutral charged

The Electrons Negatively charged high

energy particles with little or no mass

Travel at very high speeds at various distances (energy levels) from the nucleus

Story of atom

Dalton’s Atomic Theory (1808)

1. All matter is made of tiny indivisible particles called atoms.

2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element.

3. Atoms of different elements can combine with one another in simple whole number ratios to form compounds.4. Chemical reactions occur when atoms are separated, joined, or rearranged; however, atoms of one element are not changed into atoms of another by a chemical reaction.

J.J. Thomson’s cathode ray tube: discovery of electrons and the e- charge-to-mass ratio

In a vacuum chamber, flow of high voltage (emitted from cathode to anode) is deflected by magnetic & electrical fields

Thomson’s Atomic Model

Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “x” model.

Ernest Rutherford’sGold Foil Experiment - 1911

Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil

Particles that hit on the detecting screen (film) are recorded

Rutherford’s Findings

a)The nucleus is smallb)The nucleus is densec) The nucleus is

positively charged

Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected

Conclusions:

The Rutherford Atomic Model

The atom is mostly empty space All the positive charge, and almost all

the mass is concentrated in a small area in the center. He called this a “nucleus”

The nucleus is composed of protons and neutrons (they make the nucleus!)

The electrons distributed around the nucleus, and occupy most of the volume

1913 Niels Bohr

studied under Rutherford at the

Victoria University in Manchester.

Bohr refined Rutherford's idea by

adding that the electrons were in

orbits. Rather like planets orbiting

the sun. With each orbit only able to

contain a set number of electrons.

Bohr Model According to Bohr’s atomic model, electrons move in definite orbits around the nucleus, much like planets circle the sun.

These orbits, or energy levels, are located at certain distances from the nucleus.

Electronic configuration

Energy Levels Atoms have 7 energy levels The levels are K (closest to the

nucleus), L, M, N, O, P, Q (furthest from the nucleus)

The K level can only hold 2 electrons Levels L can hold 8 electrons

Electrons in the same energy level are approximately the same distance from the nucleus

Outer energy levels have more energy than inner levels

Sub shell (atomic orbital) Each shell is composed of one or more

subshells, which are themselves composed of atomic orbitals.

For example, the first (K) shell has one subshell, called "1s"; the second (L) shell has two subshells, called "2s" and "2p"; the third shell has "3s", "3p", and "3d"; and so on

s orbital has spherical form p orbital is composed of three degenerate

dumbbell-shaped orbitals px, py, pz

d orbital is composed of five degenerate orbitals

f is composed of seven degenerate orbitals Each orbital is occupied with 2 electrons

Maximum number of electrons in each subshell: s=2, p=6, d=10, f=14

Order for filling subshells: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p where the number indicates the energy level and the letter indicates the subshell being filled

Atomic Number This refers to how

many protons an atom of that element has.

No two elements, have the same number of protons.

Atomic Mass Atomic Mass refers to

the “weight” of the atom.

It is derived at by adding the number of protons with the number of neutrons.

H

This is a helium atom. Its atomic mass is 4 (protons plus neutrons).

What is its atomic number?

Isotopes

While most atoms have the same number of protons and neutrons, some don’t.

Some atoms have more or less neutrons than protons. These are called isotopes.

An atomic mass number with a decimal is the total of the number of protons plus the average number of neutrons.

Isotope Protons Electrons Neutrons Nucleus

Hydrogen–1 (protium) 1 1 0

Hydrogen-2(deuterium) 1 1 1

Hydrogen-3(tritium)

1 1 2

Ion• An ion is 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 (cation) or negative (anion) electrical charge.

In this chapter What is atom, ion, molecule, and

isotope. Atomic composition. Electron configuration. Atomic number, atomic weight. Story of the atom.