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• Quiz Compound is a substance of atoms of two or more elements formed by the chemical combination in a definite proportion by weight. For example, water is a liquid compound of hydrogen and oxygen in which two elements are present in fixed ratio of 1:8 by weight.

#### Molecule

The smallest particle of an element or compound, which is capable of independent existence is called molecule. It retains the properties of its own. It is very small. It is difficult to estimate the size of a molecule or to determine its shape.

#### Atom

The smallest particle of an element, which takes part in chemical change is called anatom.

Structure of the atom

Atom is made up of tiny particles. These tiny particles are called sub-atomic particles or elementary particles. They are electrons, neutrons and protons. The protons and neutrons are located at the center of an atom called nucleus. Nucleus shows the positive charge due to thepresence of protons. It helps to find the atomic weight. The electrons are revolving around the nucleus in their respective orbit or shell.

Atomic number

The number of protons or the number of electrons in an electrically neutral atom of an element is called atomic number. It is denoted by Z.

Mathematically.

Atomic number (Z) = No. pf protons (p+) = No. of electrons (e-)

Atomic weight

The sum of protons and the neutrons in the nucleus of an atom is called its atomic weight. It is also called mass number. It is denoted by A.

Mathematically,

Atomic weight (A) = No. of protons (p+) + No. of neutrons (n0)

Electronic configuration

The systematic distribution of electron in different shells of an atom is called electronic configuration. To explain the arrangement of electrons in different shells, Bohr and Bury purposed a scheme as given below:

1. The maximum number of electrons in each shell is determined by 2n2 rule, where n is the number of the shell.

For example:

For the K shell, the maximum number of electrons will be 2n2 = 2.(1)2 = 2 × 1 = 2

1. The maximum number of electrons in the outermost orbit is 8 and in second last orbit is 18.
2. It is not necessary for an orbit to be completed before another begins.

The energy level or shell nearer to the nucleus s called lower energy level and energy level way from the nucleus is called higher energy level.

• Compound is a substance of atoms of two or more elements formed by the chemical combination in a definite proportion by weight
• The smallest particle of an element or compound, which is capable of independent existence is called molecule.
• The smallest particle of an element, which take part in chemical change is called atom.
• The number of protons or the number of electrons in an electrically neutral atom of an element is called atomic number
• The systematic distribution of electron in different shells of an atom is called electronic configuration.
.

### Very Short Questions

Compound is a chemical substance which is formed by the combination of two or more elements in a fixed proportion by their weight. E.g. water (H2O), Ammonia (NH3) etc.

The systematic distribution of electrons outside the nucleus in different shell is called electronic configuration.

Sodium (Na)
 Shell K L M N No. of e- 2 8 1

Chlorine
 Shell K L M N No. of e- 2 8 7

The fundamental particles which are found inside an atom are called sub-atomic particles. Electrons, protons and neutrons are the sub-atomic particles.

The differences between atom and molecules are as follows:
 Atom Molecules The smallest particle of an element which takes part in chemical reaction without division is called atom. The smallest particle of an element or compound which is capable of independent existence is called molecules. Atom of an element can't be divided into two or more parts. For e.g O, H, Na, N etc. Molcules can be divided into two particles. For e.g O2, H2, N2 etc.

Differences between proton and neutron are as follows:
 Proton Neutron They are positively charged sub-atomic particles which are present inside nucleus of an atom. They are charge less sub atomic particles which are present in the nucleus of an atom. They have equal mass to the hydrogen atom. They have charge equal to hydrogen atom.

Differences between neutron and electron are as follows:
 Neutron Electron They are charge less sub-atomic particles which are present in the nucleus of an atom. They are negatively charged sub-atomic particles which are present outside the nucleus in different shells. Their mass is regarded as 1 (a.m.u) i.e. 1 atomic mass unit. They have mass equal to 1/1837parts of a hydrogen atom.

Differences between proton and electron are as follows:
 Proton Electron They are positively charged sub-atomic particles which are present inside nucleus of an atom. They are negatively charged sub-atomic particles which are present outside the nucleus in different shells. They have mass equal to the hydrogen atom. They have mass equal to 1/1837parts of a hydrogen atom.

The molecules which are made by the combination of two atoms of a same element are called diatomic molecules. E.g.: H2, N2 etc.

Electrons or pair of electrons present at the outermost shell of an atom is called valance electrons. Valance electrons in hydrogen, nitrogen, and chlorine are 1, 5, and 7 respectively.

Potassium is a metal and chlorine is a non-metal. Potassium loses one electron and becomes positively charged and chlorine gains one electron and becomes negatively charged. So, potassium chloride is called ionic compound.

Differences between chlorine atom and chlorine ion are as follows:
 Chlorine atom Chlorine ion It is electrically natural. It is negatively charged. It has equal number of electrons and protons (i.e. 17 each). It has more electrons than protons (i.e. 18 e- and 17 P+). It has 7 electrons in the valence shell. So it is very reactive. It has 8 electrons in the valance shell. So it is non reactive (very stable).

Differences between atom and ion are as follows:
 Atom Ion Atom is electrically neutral. Ion is (+) or (-) vely charged. It has equal number of protons and electrons. It does not have equal number of protons and electrons. It has usually less than eight electrons in its valence shell. It has usually eight electrons in its valence shell.

Cation: The positively charged ion is called cation. For e.g.: H+, Na+, Mg++ etc. A cation is formed by losing one or more electrons by an atom.
Anion: The negatively charged ion is called anion. For E.g. F-, O--, Cl- etc.

The difference between ionic compounds and covalent compounds are as follows:
 Ionic Compounds Covalent Compounds Ionic compounds are mainly solids. Covalent compounds may be gases, liquids or solids. They consist of ions. They consist of molecules. They have high melting point and boiling point. They have generally low melting and boiling points. They conduct electricity in solution. E.g. NaCl, CaSO4 They do not conduct electricity in solution. E.g. H2O, CH4

• The valency of 'A' is 2, because it needs two electrons in its valence shell to get its octate state.
• D is more reactive than C because it lose 1 electron during chemical reaction and is electropositive, but c is electronegative and receives one electron during chemical reaction to be stable.
• B and C forms AlCl3 and contains ionic bond.

Here, Atomic number (Z) = 11
Atomic weight (A) = 23
Number of protons (P+) = ?
Number of electron (e-) = ?
Number of neutrons (no) = ?
We have,
Atomic number = number of protons = number of electrons.
∴Number of protons (P+) = 11
Number of neutrons (no) = 11
Also,
Atomic weight = Number of protons + Number of neutrons.
Or, A = P+ + no
Or, 23 = 11 + no
Or, no = 12
∴Number of neutrons = 12

The weight of an atom expressed by comparing with hydrogen atom is called relative weight. A hydrogen atom includes only one proton in its nucleus. Therefore, it is considered as 1.
[Note: The weight of proton or neutron is very small. The weight of a proton or neutron or atomic weight cannot be expressed in grams. Therefore, it is expressed in relative weight considering an atom of hydrogen as standard.]

Carbon and oxygen both are non-metals. Carbon needs four electrons to fulfill octet and oxygen need two electrons to fulfill octet. So, both combine together by sharing two electrons. So, it is called co-valent compound.

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Glucose
Gold bar
Wood
Hydrogen gas
• ### Which is false regarding atoms?

Atoms consist of electron, proton and neutron
Atom can be created but cannot be destroyed
Atom cannot be destroyed or created
Atom of lead is different to atom of gold
• ### What does a.m.u stand for?

Atomic molecular unit
Atomic molecular union
Atomic molarity unit
Atomic mass unit

Conductivity
molarity
Polarity
Charge
• ### An atom is different than a molecule in that

Atom consist of electron, proton and neutron but molecules don't
Atoms are bonded to each other by physical bond whereas molecules are joined to each other by chemical bond
A molecule is usually stable to exist by itself but an atom is not stable by itself.
Atom can exist independently whereas molecules can't exist independently
• ### Define a compound

Compound is a chemical substance which is formed by the chemical combination of two or more elements in fixed proportion by their weight
Compound is a chemical substance which is formed by the chemical combination of two or more elements in variable proportion by their weight
Compound is a chemical substance which is formed by the chemical combination of two or more elements in fixed ratio by their weight
Compound is a chemical substance which is formed by the chemical combination of one or more elements in fixed proportion by their weight

62
46
16
30

Hydrogen
Carbon
Helium
Sodium

Neon
Aluminum
Sodium
Magnesium
• ### What is the relation between a.m.u and e.s.u?

1 a.m.u = 1/1837 e.s.u
3.8 X 1029 a.m.u = 1 e.s.u
No relation
1/1837 a.m.u = -1 e.s.u
• ### What is electronic configuration of magnesium (At no:12)?

K=2, L=2,M=8
K=1, L=1,M=10
K=2, L=8,M=2
K=2, L=8,M=8

16
32
8
72
• ### Which is false about electron distribution outside nucleus in various shells?

Electrons are filled from higher to lower energy shells around nucleus
The number of electron in any shell is given by 2n2
No two atoms have the same electronic configuration
The last shell must have 8 electrons

5
4
3
6
• ### In Calcium, how many valence electrons are in which shell (At no: 20)?

2 valence electrons in 4th shell
2 valence electrons in 2nd shell
1 valence electrons in 2nd shell
1 valence electrons in 4th shell

Iron
Potassium
Sulphur
Sodium

Hydrogen
Silicon
Sodium
Boron
• ### One metre is equal to______ wavelength in vacuum of transition of _______ atom.

165078373, Krypton-84
165076373, Krypton-86
165576373, Krpton-82
165075373, Krypton-88

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