Atomic Structure

Inside an Atom• Over time, scientists

have come up with many theories about the inside of atoms.

• Today we believe that an atom has a center nucleus surrounded by one or more electrons.

• The nucleus is the central core of the atom.

nucleus

Subatomic ParticlesAtoms are made up of 3 different kinds of particles:1. Protons- positively charged particles located inside the nucleus of an atom.2. Neutrons- particles located inside the nucleus that are neutral (no charge).3. Electrons- negatively charged particles located outside the nucleus.

Particle Charge LocationProton

NeutronElectron

positiveneutral

negative

insideinside

outside

Subatomic Particle Facts

• Protons are located in the nucleus, have a positive charge, and a mass of 1 amu.

• Neutrons are located in the nucleus, have no charge, and a mass of 1 amu.

• Electrons are located outside the nucleus, have a negative charge and a mass of 1/1836 amu (almost zero).

• Because electrons have basically no mass, 99% of the mass of an atom is found in the protons and neutrons.

The Periodic Table• Different atoms have different numbers of

protons, neutrons and electrons.• The periodic table can you find out how many

p, n and e- an atom has.

Atomic Number• The number of protons in

the nucleus of an atom is called the atomic number.

• Each element has a different number of protons and therefore, a different atomic number.

• The atomic number (or # of protons) is different for all elements.

The atomic number is the large number

at the top of the box.

Positives = Negatives

• In atoms, the number of protons is equal to the number of electrons.

• Therefore, the atomic number tells us the number of protons AND the number of electrons.

protons electrons=

Example…

If the atomic number of potassium is 19, how many protons and how many electrons does a potassium atom have?

Protons= 19Electrons= 19

Every atom has mass.• The mass of an atom is equal to the weight of its

nucleus.• This is because electrons have basically no

weight.• Therefore the number of protons and neutrons

in the nucleus gives us the mass of the atom.

Mass Number

• An atom’s atomic weight is listed on the Periodic Table.

• If you round the atomic weight to the nearest whole number you get the atom’s mass number.

Round 4.003 to 4

Number of neutronsSince the mass number is equal to the number of protons plus neutrons, we can use it to find out how many neutrons there are.

Round the atomic weight to a whole number= 20

This means that protons + neutrons in Neon= 20

The atomic number of neon is 10 which means neon has 10 protons.

How many neutrons does it have?20-10= 10Atomic weight of neon is 20.1797

To find the number of neutrons, you can…

Mass number – atomic number

Try this…

How many protons?How many electrons?How many neutrons?

(Remember: 96-42)

424254

Number of neutrons can vary

• All atoms of the same element will have the same atomic number and therefore the same number of protons.

• Sometimes, however, an atom of an element can have a different number of neutrons.

• This would still be the same element, it would just have a different atomic weight.

Isotopes• An isotope is an atom with the same number of

protons but a different number of neutrons from other atoms of the same element.

• The atomic weight of an atom is an average of the weights of the different isotopes of that element.