CHE2: ATOMIC STRUCTURE

Atom is the smallest electrically neutral particle of an element that can take part in a chemical reaction.

An atom consists of a very small and extremely dense region called the nucleus. The nucleus is surrounded by negatively and particles called electrons located in shells

-The biggest part of the atom is empty space

-The nucleus contains two types of particles that is proton and neutrons

The nucleus forms most of the mass of any atom. The mass contributed by electrons is very small.

Atomic number Z (proton number)

Therefore atoms of elements are neutral particles because the total charges due to protons counsels out the total charges due to electrons.

Neutral number

This is a number of neutrons in a nucleus of an atom of an element.

Mass number

This is sum of the neutron and proton number of an atom of an element

Mass number, A = proton number + neutron number

ISOTOPES

For example hydrogen consists of 3 isotopes that is protium (ordinary hydrogen), deuterium and tritium

-Neutral no = mass no – atomic number

Chloride

It consists of two isotopes that is:

Chlorine -35

Chlorine -37

• Isotopes are identified by their mass numbers

Carbon

It has three isotopes that is:

-Carbon -12

-Carbon -13

-Carbon -14

Question

An atom of an element is represented by

⁴⁰₁₂M

State

1. Its mass no

40

1. Its atomic no

12

1. Its electron no

12

• It neutron number ( show working)

10

-12

28

Electronic structure

An electronic structure is a diagrammatic representation of the arrangement of electrons in an atom.

Note

In an atom, electrons are arranged in shells which are at different distances from the center of the atom. The first shell (inner most shell) is called the K shell; it accommodates a maximum of two electrons. The second shell is the L shell and it accommodates a maximum of eight electrons. The next shell is called the M shell and it accommodates a maximum of eight electrons, the fourth shell is the N shell

Examples

Hydrogen [1 electron]

Helium [2 electrons]

Atomic structures and electronic configuration of the following;-

-Metals have a one of the two or three electrons in the outmost shell

-Hydrogen is an expectation because it has one electron in the outmost shell but not metal.

Nonmetals have 4/5/6or seven electrons in the outmost shell.

Examples of metal

-K

-Ca

-Al

-Mg

-Na

-Li

-Boron with configuration of 2:3 has properties of metals and non-metals it is a metalloid

-Elements which have completely filled out most shell are chemically uncreative. They are referred to as noble gas of inactive elements.

Noble gases do not combine with other gases

Valency of combining number

It is the measure of the power of an element to combine with others, it can be determined from electronic structure of atoms. Valency can also be defined as the number of hydrogen atoms. One atom of an element can be combined or displaced.

-Hydrogen is taken as the standard and it has valency ore

Element which have 1or 2 or 3 or 4 out most electrons have a valency which is the same as the number of outer shell electrons

1. State the electronic configuration of
2. U 2:8:5
3. T 2:6

1. State the valency
2. U 8: 5= 3       e ions by losing
3. T 8-6= 2

• L = 2

1. Give reasons for answer in (iii)

Because it is the number of electrons in the outmost shell

Electropositive element

Is one which form positive ions by losing out shell electrons

-Positive ion is called a cation

-Metals and hydrogen are electoral positive because they form positive ions

For example

₁H                                H⁺ + e

O

Hydrogen ion (proton)

                 ₂Li               Li⁺ + e

2

The size of change is the same as the number of electrons removed