All About Atoms! Protons, neutrons, electrons, isotopes

To start, remember that:

Protons and neutrons have about the same mass, and both live in the nucleus. Electrons have much less mass and live in orbitals outside of the nucleus.

The "Atomic Number" is the number of protons that atom has. This defines what type of element it is.

For example, Calcium has an atomic number of 20. This means it has 20 protons. ANY atom that has 20 protons is a Calcium atom -- the number of protons an atom has is what makes it that specific element! (If it had 21 protons it would be Sc, not Ca!)

As you could see in the first chart, protons and neutrons (the ones that live in the nucleus) are MUCH bigger than electrons -- so they are what we count when we are measuring the mass of an atom.

The number of protons + the number of neutrons = the mass number


Atoms of the same element can have different numbers of neutrons -- this makes them isotopes!

For example, these are 3 isotopes of Hydrogen. They all have 1 proton (making them all Hydrogen atoms) but they have different numbers of neutrons (making their mass numbers different). In this case, I have Hydrogen-1, Hydrogen-2, and Hydrogen-3. (The number stated after the element is the Mass Number, which is protons+neutrons)

You can always determine the amount of neutrons in an Isotope! You just need to know the mass number and the atomic number (number of protons)

Mass Number - Atomic Number = Number of Neutrons


Neutrons are neutral, so they have no charge -- so that nucleus (with protons and neutrons in it) is positively charged!

Adding or taking away neutrons isn't going to effect the overall charge of an atom (because they are neutral).

However, electrons have a negative charge, so if you have more electrons that protons, the atom will be negatively charged.

For example: If an atom has 8 protons and 9 electrons, it has a charge of -1.. If an atom has 17 protons and 12 electrons, it has a charge of +5

NEUTRAL ATOMS: If an atom is neutral (no charge overall) the negatively charged electrons balance out the positive charge of the nucleus. This means that as Atomic number increases (more protons added), there must be more electrons added as well to keep atoms neutral.

For example: 6 positive protons in the nucleus, balances out with 6 negative electrons outside of the nucleus, making an atom neutral.

How Elements are Written

You need to know this!

When we write out chemical symbols, we arrange the numbers differently than how they are arranged on the periodic table.

Top # is mass number (protons +neutrons), bottom # is atomic number (protons)

On the periodic table, it looks like this:


Protons (positive) and neutrons (neutral) live in the nucleus. They are both about the same mass -- while electrons are much less massive -- making the nucleus very heavy mass, but very small volume at the center of an atom.

Most of an atom is empty space! It's like a grain of sand in a swimming pool.


1. All isotopes of an element contain the same number of ... ?

2. If a Carbon atom has a mass number of 14, how many neutrons does it have?

3. In order for Boron to be neutral, how many electrons does it need to have?


How many protons, neutrons, and electrons does this atom have? (assume it's neutral)

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