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Learning Outcomes

• Why Atoms Are Electrically Neutral
• Why Is An Atom Neutral
• Atoms Are Neutral Because Of The Number Of
• Atoms Are Neutral Because The Number Of Protons Equals The Number Of Neutrons
• Atoms Are Neutral Because The Number Of Regents

Now, in a 'neutral atom', the number of protons must be equal to the number of electrons, otherwise it would not be neutral. Basically, the definition is saying that 'in a neutral atom, the atomic number is equal to both the number of protons, and the number of electrons, because, well, they both are the same.' Compare this with an ion. Atoms are electrically neutral because there are an equal number of protons in the nucleus and In the cloud around the nucleus. A neutral oxygen atom has eight electrons, the same as the atomic number. (The atomic number is the number of protons in an atom, and the number of electrons must be the same as the number of. Notice the number of moles is used from Step 2 to start the calculation from moles to atoms: 0.52 mol of Cu × 6.022 x 10 23 atoms / 1 mol of Cu = 3.13 x 10 23 molecules To answer your example question, there are 3.13 × 10 23 atoms in 32.80 grams of copper.

Why Atoms Are Electrically Neutral

• Define atomic and mass numbers.
• Determine the number of protons, neutrons, and electrons in an atom.
• Identify the charge and relative mass of subatomic particles.
• Label the location of subatomic particles in the atom.
• Determine the mass of an atom based on its subatomic particles.
• Write A/Z and symbol-mass format for an atom.

Atoms are the fundamental building blocks of all matter and are composed of protons, neutrons, and electrons. Because atoms are electrically neutral, the number of positively charged protons must be equal to the number of negatively charged electrons. Since neutrons do not affect the charge, the number of neutrons is not dependent on the number of protons and will vary even among atoms of the same element.

Atomic Number

The atomic number (represented by the letter Z)of an element is the number of protons in the nucleus of each atom of that element. An atom can be classified as a particular element based solely on its atomic number. For example, any atom with an atomic number of 8 (its nucleus contains 8 protons) is an oxygen atom, and any atom with a different number of protons would be a different element. The periodic table (see figure below) displays all of the known elements and is arranged in order of increasing atomic number. In this table, an element's atomic number is indicated above the elemental symbol. Hydrogen, at the upper left of the table, has an atomic number of 1. Every hydrogen atom has one proton in its nucleus. Next on the table is helium, whose atoms have two protons in the nucleus. Lithium atoms have three protons, beryllium atoms have four, and so on.

Why Is An Atom Neutral

Since atoms are neutral, the number of electrons in an atom is equal to the number of protons. Hydrogen atoms all have one electron occupying the space outside of the nucleus. Helium, with two protons, will have two electrons. In the chemical classroom, the proton count will always be equivalent to an atom's atomic number. This value will not change unless the nucleus decays or is bombarded (nuclear physics).

Mass Number

Experimental data showed that the vast majority of the mass of an atom is concentrated in its nucleus, which is composed of protons and neutrons. The mass number (represented by the letter A)is defined as the total number of protons and neutrons in an atom. Consider the table below, which shows data from the first six elements of the periodic table.

Consider the element helium. Its atomic number is 2, so it has two protons in its nucleus. Its nucleus also contains two neutrons. Since (2 + 2 = 4), we know that the mass number of the helium atom is 4. Finally, the helium atom also contains two electrons, since the number of electrons must equal the number of protons. This example may lead you to believe that atoms have the same number of protons and neutrons, but a further examination of the table above will show that this is not the case. Lithium, for example, has three protons and four neutrons, giving it a mass number of 7.

Knowing the mass number and the atomic number of an atom allows you to determine the number of neutrons present in that atom by subtraction.

Atoms Are Neutral Because Of The Number Of

[text{Number of neutrons} = text{ rounded mass number} - text{atomic number}]

Atoms of the element chromium (left( ce{Cr} right)) have an atomic number of 24 and a mass number of 52. How many neutrons are in the nucleus of a chromium atom? To determine this, you would subtract as shown:

[52 - 24 = 28 : text{neutrons in a chromium atom}]

The composition of any atom can be illustrated with a shorthand notation called A/Z format. Both the atomic number and mass are written to the left of the chemical symbol. The 'A' value is written as a superscript while the 'Z' value is written as a subscript. For an example of this notation, look to the chromium atom shown below:

[ce{^{52}_{24}Cr}]

Another way to refer to a specific atom is to write the mass number of the atom after the name, separated by a hyphen. Symbol-mass format for the above atom would be written as Cr-52. In this notation, the atomic number is not included. You will need to refer to a periodic table for proton values.

Example (PageIndex{1})

Calculate each of the three subatomic particles and give specific group or period names for each atom.

1. mercury
2. platinum
3. bromine

Solutions

1. Hg (transition metal)- has 80 electrons, 80 protons, and 121 neutrons
2. Pt (transition metal)- has 78 electrons, 78 protons, and 117 neutrons
3. Br (halogen)- has 35 electrons, 35 protons, and 45 neutrons

Example (PageIndex{2})

Write both A/Z and symbol-mass formats for the atoms in Example (PageIndex{1}).

Solutions

1. (ce{^{201}_{80}Hg}) and Hg-201
2. (ce{^{195}_{78}Pt}) and Pt-195
3. (ce{^{80}_{35}Br}) and Br-80

Example (PageIndex{3})

Identify the elements based on the statements below.

1. Which element has 25 protons?
2. Which element has 0 neutrons?
3. Which element has 83 electrons?

Solutions

a. manganese

b. hydrogen

c. bismuth

Need More Practice?

• Turn to section 3.E of this OER and answer questions #1-#2, #4, and #8.

Contributors and Attributions

• CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.

• Allison Soult, Ph.D. (Department of Chemistry, University of Kentucky)

Atoms vs. Ions

Atoms are neutral; they contain the same number of protons as electrons. By definition,an ion is an electrically charged particle produced by either removing electronsfrom a neutral atom to give a positive ion or adding electrons to a neutralatom to give a negative ion. When an ion is formed, the number of protonsdoes not change.

Neutral atoms can be turned into positively charged ions by removing oneor more electrons. A neutral sodium atom, for example, contains 11 protonsand 11 electrons. By removing an electron from this atom we get a positivelycharged Na⁺ ion that has a net charge of +1.

Atoms Are Neutral Because The Number Of Protons Equals The Number Of Neutrons

Atoms that gain extra electrons become negatively charged. A neutral chlorineatom, for example, contains 17 protons and 17 electrons. By adding one moreelectron we get a negatively charged Cl^- ion with a net charge of -1.

Atoms Are Neutral Because The Number Of Regents

The gain or loss of electrons by an atom to form negative or positive ionshas an enormous impact on the chemical and physical properties of the atom.Sodium metal, for example, which consists of neutral sodium atoms, burstsinto flame when it comes in contact with water. Neutral chlorine atoms instantlycombine to form Cl₂ molecules, which are so reactive that entire communities are evacuatedwhen trains carrying chlorine gas derail. Positively charged Na⁺ and negatively charged Cl^- ions are so unreactive that we can safely take them into our bodies wheneverwe salt our food.