Isotopes are forms of a chemical element that have the same atomic number but differ in mass. 16O → 8 protons + 8 neutrons; a “light” oxygen); The relative amounts are expressed as either 18O/16O or δ 18O Oxygen – 18 (aka 18O → 8 protons + 10 neutrons; a “heavy” oxygen).

The atomic number of oxygen is 8. The mass of an atom is given by the sum of the masses of protons and neutrons present in the nucleus. Also, atomic number is the number of protons present in the nucleus. Since, the number of neutrons present in oxygen is 8 and given mass is 16.

Because protons and neutrons are roughly equal in mass, an isotope’s number is equal to the sum of its protons and neutrons. Therefore, oxygen 16 has 8 protons and 8 neutrons, oxygen 17 has 8 protons and 9 neutrons, and oxygen 18 has 8 protons and 10 neutrons.

For example: O2- indicates an ion of oxygen having two extra electrons. I.e., since an oxygen atom normally has 8 protons and 8 electrons, this ion has 10 electrons (-10 charge) and 8 protons (+8 charge) giving it a charge of -2 (-10 + 8 = -2).

2, 6

8

Explanation: For oxygen, Z , the atomic number =8 . There are 8 protons in its nucleus (and protons are positively charged particles). To balance this charge (the atom is electrically neutral after all), the oxygen nucleus is surrounded by 8 negatively charged electrons.

8 electrons

15.994914

16

2 electrons

6 electrons

The number of electrons in a neutral atom is equal to the number of protons. The mass number of the atom (M) is equal to the sum of the number of protons and neutrons in the nucleus. The number of neutrons is equal to the difference between the mass number of the atom (M) and the atomic number (Z).

2,1

So… for the element of LITHIUM, you already know that the atomic number tells you the number of electrons. That means there are 3 electrons in a lithium atom. Looking at the picture, you can see there are two electrons in shell one and only one in shell two.