Which is the electron configuration for lithium?
1s2
2s3
1s22s1
1s12s2

Electronic configuration of lithium is

Explanation: Lithium is the third element in the periodic table.

Atomic number of lithium = 3

Number of electrons = Number of protons = Atomic number

There are 3 electrons in lithium, so the electronic configuration will be:

Electron configuration:

The arrangement of electrons in an atom by a superscript, in each sublevel is known as electron configuration.  For example the d block elements have (n-1)d1-10 ns1-2   electronic configuration

The atomic number of lithium is 3, which is equal to the number of electron of lithium

Thus the electron configuration of lithium is 1s22s1

1s2 in this configuration the number of electron is 2

2s3 s orbital has maximum 2 electrons  

1s22s1 this is the electron configuration of lithium

1s12s2 first fill lower energy orbital measn 1s than 2s.

it can be observed without changing the identity of the object.

it depends on what substance you're talking about as well as many other factors. in quantum mechanics, the observation of particles affects the observers ability to measure certain properties. for example: you can know the velocity of a particle, but you can't know it's position; or you can know it's position, but not it's velocity. it's like trying to find out where a grain of sand is by throwing other grains of sand at it. once you find out where it is, it's been moved. it's also like trying to find an answer to the question of whether or not a tree falling in a forest makes a sound if nobody is there to hear it. paradoxically, the only way to find out is to be there and hear it.