Midterm Review

Topic 1 The Atom

Vocabulary

Dalton’s Atomic Theory

• All elements are composed of identical atom.

• All atoms of a given element are identical.• Atoms of different elements are different;

that is, they have different masses.• Compounds are formed by the combination

of atoms of different elements. (Law of multiple proportions)

Parts of the Atom

• Electron– J.J. Thomson• Smallest unit of the atom• Negativity charged• “Plum Pudding” model

Parts of the Atom

• Nucleus– Ernest Rutherford• Gold Foil Experiment

– Alpha Particles– If the “Plum Pudding” model was correct then the α particles

would pass through the foil with just a few being slightly deflected.

– Some of the α particles were deflected, and some even bounced back.» Rutherford concluded that atoms have a small dense

positively charged central core, and the rest is mostly empty space

Modern Atomic Theory

• The Bohr Model– Proposed a model of the atom that showed

that atom with a dense nucleus with electrons found in surrounding orbits• He proposed that electrons requires a specific

amount of energy to stay in each orbit• He suggested that the first orbital could hold only

2 electrons and that the 2nd orbital could hold 8 e-

Modern Atomic Theory

• The Wave-Mechanical Model– Electrons have properties for both waves and

particles– The major difference between the Bohr model

and the Wave-Mechanical Model is the way that the electrons are pictured. • Instead for moving in fixed orbits, the electrons move

in areas called orbitals.– Orbital is described as a region in which an electron of a

particular amount of energy is most likely to be found.

Modern Atomic Theory

• The Modern model of the atom has been developed over a long period of time

Dalton

Thompson

Rutherford

Bohr

Wave-mechanical

Subatomic Particles

• Because the mass of a proton is so small, it is more convenient to use a different scale whose units are called atomic mass units.

Subatomic Particles

• Each atom of a specific element must contain the same number of protons as each other atom of the same element – The number of protons in the nucleus of the

atoms is the atomic number of that element.• For example : The atomic number of Chlorine is 17 so

therefore each atom of Chlorine contains 17 Protons

Subatomic Particles

• Mass Number– The nucleus of the carbon atoms,

containing 6 protons and 6 neutrons is taken as the standard mass.

– The sum of the number of protons and the number of neutrons is called the mass number

Isotopes

• All atoms of a given elements contain the same number of protons, but the number of neutrons can vary.– When the number of neutrons vary, then the mass

of then atoms will be different.

• These different forms of an atom are called isotopes. – Isotopes are atoms of the same element that have

different numbers of neutrons

Isotope Symbols

Nuclear symbol

Atomic Masses

• This is the number that is found on the periodic table.

100

(%)(mass(mass)(%) )

Avg.AtomicMass

Location of Electrons

• Energy levels – The orbitals in an atom form a series of energy

levels in which electrons may be found.– Each electron in an atom has its own distinct

amount of energy that corresponds to the energy level that it occupies.• Electrons can gain or lose energy and move to different

energy levels• Electrons can absorb only distinct, or fixed about of

energy that allows it to move to a higher energy level.

Excited and Ground State

• When the electrons occupy the lowest available orbitals, the atom is said to be in the ground state.

• When electrons are subjected to stimuli such as heat, light, or electricity, an electron may absorb energy and temporally move to a higher energy level

• This unstable condition is called an excited state.

Excited State

• The electron quickly returns to a lower available energy level, emitting the same amount of energy it absorbed to go to the higher energy level.

• This admission of energy can be seen with an instrument called a spectroscope.

Atomic Spectra• Each element has its unique

atomic spectra. • These spectra can be used

to identify elements

• Although the electrons in an atom contribute little to the mass of an atom, their arrangement determines its chemical properties. – The chemical properties of an element

are based on the number of valance electrons, or outer most electrons.

Electron Arrangements

Orbitals and Electron Capacity of the First Four Principle Energy Levels

Principle energy

level (n)

Type of sublevel

Number of orbitals per

type

Number of orbitals per

level(n2)

Maximum number of electrons (2n2)

1 s 1 1 2

2s 1

4 8p 3

3s 1

9 18p 3d 5

4

s 1

16 32p 3d 5f 7

Electron Configuration

• Follow the following rules to write and electron configuration– Each added electron is placed into the sublevel of lowest

available energy level– No more that two electrons can be placed in any orbital– A single electron must be placed into each orbital of a

given sublevel before any pairing takes place (Empty Bus seat Rule)

– The outermost principle energy level can only contain electrons in the s and p orbitals

Orbital Notation

• While electron configuration notation can be useful, it does not show how the electrons are distributed in each sublevel

HeterogeneousHomogeneous

Types of Matter

• Some matter looks uniform and doesn’t seem to be made up of parts.

• When an material had uniform composition throughout, the sample is said to be homogeneous.– It can contain more that one type of particle, but

particles are evenly mixed

• Other materials are obviously made up of parts.

• Such materials, which have varying compositions, are said to be heterogeneous– They are made up of parts

with different chemical and physical properties.

– These parts are not uniformly mixed or dispersed

HeterogeneousHomogeneous

Types of Mater

Types of Matter

• Matter can be divided into major categories– Pure substances • All pure substances are homogeneous

– Mixture• Can be either homogeneous or heterogeneous

Pure Substance

• A sample is a pure substance id its composition is the same throughout the sample.

• There are two types of pure substances– Element – Compound

Element

• Elements are substance that cannot be broken down or decomposed into simpler substances by chemical means.– There are 91 naturally occurring elements– The elements are arranged on the periodic table

of elements

Compound

• Compounds are composed of two or more elements that are chemically combined in definite proportions by mass.– Although all compounds contain at lease 2

different types of atoms, the composition of the compound is the same throughout.

– The Law Of Definite Proportions

Mixture• A mixture is a combination of two or

more pure substances that can be separated by physical means

Distinguishing between Mixtures and Compounds